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Cowl Plenum Seam Sealer Failure - Investigation and Repair

3789 Views 27 Replies 7 Participants Last post by  hit_4_show
ETA 1-9-2023

Subject vehicle: 1996 Sonoma SLS RWD 2.2L MT Reg cab, 70k miles.

Over ~18 months, as I infrequently worked on this problem, this thread morphed into instructions and a collection of reference photos. Some of my comments and photos in this 1st post did not well support that, so I deleted and/or replaced them. Replies to this post may seem out of place as a result.

This thread contains photos and instructions for investigation and basic repairs related to seam sealer failure in the Cowl Plenum; in and around the Cowl Plenum Drains; and at the vertical hem seams forward of the front door hinges. Removal of all front end sheet metal, excepting the Radiator Support, is addressed.

This was prompted by discovery of rust at the inside, R, front, cab corner, near the bulkhead connector on the subject vehicle. Subsequent investigation determined debris had accumulated in the Cowl Plenum, below the Air Inlet Assembly (behind Glove Box). Possibly 20+ years of moist environment there caused seam sealer failure and rust; allowing a pathway for water from the Cowl Plenum into the Cab interior. Investigation indicates similar may happen at the extreme L side of the Cowl Plenum.

Thread Table of Contents (with hyperlinks)
The original photos for this 1st post were removed from the body of the post, but attachments remain below. They are supplementary and not likely as useful as the photos provided as the thread developed.


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i dont see any rust in your pics?
2nd & 4th photos. This thread is intended primarily as an aid to others, primarily by way of photos. I would like this forum not to die. Traffic, and helpful user posts on auto forums for 90’s & 2000’s vehicles is far below what it was 10 years ago. This thread will not reach maximum usefulness unless/until I remove the fender (and everything else required to do that) and remove seam sealer on the inside and outside to expose the extent of the rust. My guess is the rust started on the outside and worked its way through the seam(s) and/or spot welds, unless water was directly intruding into the interior, which I have thus far been unable to replicate. (ETA long after I made this post - My guess was later shown to be wrong. Cowl Plenum seam sealer failed.)

My truck has the same thing and it's from AZ. The factory seam sealed both sides of the panels and this didn't allow water to escape and it never dries out and you get rust. See it all the time. I'm pretty sure all second gens have this just most people don't realize it.
Agree with that, or at least with it being super common, but not recognized. I know I get a little water through the fresh air damper assembly on my 02 daily driver, but it is not worth fixing given the condition of the vehicle and difficulty in repair.
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Accessing the R Cowl Plenum Drain (from exterior)

ETA 6-4-2022. Select front end disassembly documentation appended to end of post.

Removed the R Fender to allow for inspection. R side of Cowl Drain was full of damp debris. Now that I can visually inspect the area behind the Fender, it is clear the R Cowl Drain Outlet is directly above the worst rust I currently observe inside the Cab at the top of what is often called the “Kick Panel” area. The Air Inlet Assembly, which is rear of the Blower Motor is located in the upper portion of the sheet metal enclosure that is the R end of the Cowl Drain. I will likely try to remove the Dash and Air Inlet Assembly later today. I hope I do not break anything, or at least nothing difficult to replace or repair. Intend to update this thread as I disassemble and get a better view of the rust.

So, why do I care about this? The relevant truck is intended to be my next daily driver. I would like to get 200k more miles out of it, and do not want the cab rotting away until I get closer to that goal.

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Wide angle view of R front Cab corner with sheet metal removed.

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R Cowl Drain Outlet. The black, plastic Cowl Ventilator Tube (water deflector), has been removed for better access.

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Partially obstructed view of interior Cab rust below and opposite the R Cowl Drain Outlet. Some seam sealer removed for inspection.


Following information appended 6-4-2022.

R Wheelhouse and Fender Removal.

*Unfortunately, the following must be removed before either Fender can be removed: Radiator Grille; Front Bumper Assembly (complete); and Hood.

I did not document this early parts removal well at all, especially with photos. However, I will include notes and a few photos for the process on my 1996 Sonoma SLS. Some photos are of parts after removal, showing fastener locations. Later portions of the inspection and repair process are better documented in subsequent posts.

Remove Composite Headlamps (x2). Remove rubber or PVC caps for the 2 – long, mounting screws. Remove the 2 – screws (5mm). Disconnect all 3 – lamp holders from Headlamp. Headlamp is now free for removal.

Remove Radiator Grille. Remove Grille to Radiator Support screws: 6 - M4.2-1.41 x 20mm 12mm washer SEM screw (7mm). Carefully pull Grille straight out from vehicle to release the 4 – metal Retainers. The Retainers are mounted on very thin extensions of the Grille that will not withstand much lateral force before breaking.

Remove Front Bumper Assembly (complete). The following can be removed as 1 – assembly: Front Bumper Impact Bar; Bumper Fascia; Park/Turn Lenses; and Radiator Air Deflector. From below vehicle, remove 2 - M10-1.5 x 30mm flanged hex bolt (15mm) connecting outer portion of Bumper to Bumper Brace. While still down there, disconnect lamp holder from each Park/Turn Lamp. From above Bumper remove 4 - M10-1.5 x 35mm button head dog point bolts (T50). Pull Bumper Assembly forward to disengage from locating pins projecting out from Fenders.

Remove Hood. First, remove R & L Cowl Grille Panels. Each is held in place with 1 - M4.2-0.79 x 17mm 12mm washer SEM screw (7mm); metal retainer; and tab interlocking it with the Center Cowl Grille Panel. Remove screw. Carefully lift outer edge to free Retainer, understanding interlocking tab is interlocked with Center Cowl Grille Panel. Pull up and outward to remove. Be careful. It is easy to damage the Center Panel interlocking loop. Disconnect Windshield Washer Hoses from Windshield Washer Nozzles. Be careful. The original Nozzles are NLA and OEM/aftermarket replacements do not fit the original hoses. Disconnect Vacuum Hose from Vacuum Reservoir. Disconnect any Underhood Light wire harness. With Hood propped in the upright position, remove 2 – M8-1.25 hex nut w/ free-spinning 20mm washer (13mm) for Hood Hinges. With a helper, keep Hood upright while lowering Prop Rod; remove 2 – M8-1.25 x 22mm flanged hex shoulder bolt (13mm) for Hood Hinges; and lift Hood off vehicle. I set mine, upside down, on top of the Cab & Cap with towels in between.

Remove R Wheelhouse. First, if battery not already removed, do so now. Disconnect the Battery Cables by loosening the Studs (8mm). Remove 1 - M8-1.25 flanged hex nut (13mm) for the Battery Hold Down and 1 – Battery Hold Down Retainer (rubber). Remove Battery. Remove the 2 - M8-1.25 x 30mm 24mm washer SEM bolts (13mm) for Battery Tray. Remove Battery Tray. Disconnect both Hoses at the Radiator Coolant Recovery Reservoir. Keep the Hose from the Radiator held up or it will dribble Coolant. Remove 3 - M6-1.0 x 16mm 17mm washer SEM screws (10mm) for ECM and support the ECM safely out of the way. Remove 3 - M6-1.0 hex nut w/ free-spinning 18mm washer (10mm) for Engine Coolant Recovery Reservoir and remove Reservoir. From below Wheelhouse, remove Reinforcement at front, bottom of Wheelhouse: 2 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) and 1 – M8-1.25 x 17mm 24mm washer SEM screws (13mm) (closest to Fender). Remove 2 – M8-1.25 x 22mm 32mm washer SEM screws (13mm) for Firewall tabs. Remove final 6 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) for Fender connection. Wheelhouse will wiggle loose and out.

Remove R Fender. First, separate the coaxial Antenna Cable at the connection approximately 6” from the Antenna Base mounted in the Fender. Then, remove 8 – bolts; all M8-1.25 x 30mm 24mm washer SEM (13mm). 2 – bolts at hood hinge location. 2 – bolts at rear edge of Fender, accessible from Cab area through gap between Door and Cab. 1 – bolt at rear, bottom of Fender. 2 – bolts at front, bottom of Fender. 1 – bolt at front, top of Fender. Slip Fender forward, away from Door and remove. 1 – bolt from back of the Radiator Support was previously removed as it was 1 of 2 - bolts that mounted the Battery Tray.

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Headlamp mounting screws. Headlamp was temporarily reinstalled to obtain reference photo.

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Radiator Grille (front view). Yellow circles show all locations where screws attach Grille to Radiator Support.

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Radiator Grille (rear view). Yellow circles show locations of all Retainers.

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Bumper (top, rear view). Yellow circles show locations of all mounting bolts.

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L Cowl Grille Panel (rear view). Yellow circle shows location of mounting screw. Green circle shows location of Retainer. Red circle shows location of interlocking tab. R Panel is mirror image.
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Beginning Instrument Panel (I/P), AKA Dash, Removal.

I am using the Factory Service Manual and the following thread to access and remove the Air (fresh) Inlet Assembly.

Front is front. Rear is rear. Left is driver side. Right is passenger side.

Started working on the dash; termed Instrument Panel (I/P) in factory literature. Provided are photos of the Speaker Covers; center Defrost Cover; and top of I/P. The Speaker Covers are polycarbonate; likely very brittle; and easy to break. The Defrost Cover is a 2-piece assembly, with the top, interior-colored portion being metal and the lower, black portion being Polyphenylene Ether (PPE). It is much more durable, but excellent condition replacements are difficult to source. Try not to apply any more pressure to the I/P than is necessary.

When removing the Defrost Cover, some recommend trying to insert a thin blade of metal or alternative beneath the edge of the Cover to push on the spring leg of a retaining clip as you pull on that clip with something like a hook-ended pick. I removed the cover 1 – clip at a time, using a hook pick, starting at 1 – side and working my way around the rear till I reached the other side.

The Speaker Covers are easily removed by first removing the 2 – M4.2-1.41 x 40mm 8mm washer SEM screws (T15) for each Cover. Then, use the screw threads to grab the rear of the Cover and lift up just enough to clear the I/P and pull back. There are fingers on the bottom of the cover near the front that hold the front down. If you apply much upward force to the Cover before those fingers are slid rear, out of the mating slots, the fingers will break off.

All plastics get cleaned and then treated with 303 Aerospace Protectant. I treat top and bottom sides when accessible in the hope of extending the life of the part.

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Defrost Cover removal.

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Defrost Vent with Cover removed.

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1 - Defrost Vent mounting clip recess at rear of opening.

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1 - Defrost Vent mounting clip recess at side of opening.

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Defrost Cover removed – top view.

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Defrost Cover removed – bottom view.

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Defrost Cover mounting clip close-up.

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R Speaker cover removal.

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R Speaker in top of I/P. Note the slots in front for the fingers on the bottom of the Cover.

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R & L speaker covers – bottom view.
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Continuing I/P Removal

Disable Supplemental Inflatable Restraint (SIR / Air Bag). Disconnect negative Battery terminal if battery not already removed. Steering Wheel straight w/ no key in ignition. Remove Fuse 22 from Fuse Block (R side of I/P).

Remove L I/P Sound Insulator. 1 – M6-1.0 hex nut (10mm) by Accelerator. 1 – M4.2-1.79 x 14mm 12mm washer SEM screw (7mm) L of Clutch Pedal. 2 – M4.2-1.79 x 18mm 12mm washer SEM screws (7mm) for DLC connector. 3 - M4.2-1.79 x 28mm 17mm washer SEM screws (7mm) at top of Insulator. 1 – electrical connector for Interior Lamp Control Module Assembly mounted to “back” side of Insulator.

Remove R Door Sill Trim. You can pry it straight up. Safer is to compress the individual clips from below the Cab and push each clip up through the floor a bit till the trim pops free.

Remove R Kick Panel Trim. Reference photo to see where you need to pull/pry to unclip it. When unclipped, rotate the bottom down and rearward to un-mate it from the trim piece above it. The SIR Control Module can be left attached to the Panel. Just disengage the connector lock and disconnect from harness.

Remove R I/P Sound Insulator. 4 - M4.2-1.79 x 23mm 17mm washer SEM screws (7mm).

Remove Center Console (manual transmission). Review photos. Wiggle and pull straight up. Remove 2 - M6-1.0 free-spinning washer hex nuts (10mm) to remove base.

Remove Center I/P Sound Insulator. 1 - M4.2-1.79 x 14mm 12mm washer SEM DST screw (7mm) at R side. 3 - M4.2-1.79 x 19mm 12mm washer SEM screws (7mm) at top. 1 - M4.2-1.79 x 14mm 12mm washer SEM screw (7mm) at bottom L side.

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L I/P Sound Insulator.

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R Door Sill Trim.

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R Kick Panel Trim – back side. Notice location of mounting clip near top, and the post near the bottom which never received a mounting clip.

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R I/P Sound Insulator

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Center Console (manual transmission) and Center I/P Sound Insulator.

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Center Console (Manual Transmission) – back side mounting clips.

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Center Console (Manual Transmission) base.
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Continuing IP Removal - Knee Bolster.

Remove Ash Tray by pressing up on the bright silver, spring metal retainer and pulling the Ash Tray out.

Disconnect Parking Brake Release Handle cable from lever. Remove the slug at end of cable from holder. Also slide cable out of bracket.

Remove Knee Bolster fasteners. 2 – M4.2-1.79 x 18mm 12mm washer SEM screws (7mm) below Steering Column for HVAC duct and courtesy lamp. 1 – M4.2-1.79 x 18mm 12mm washer SEM screw (7mm) at upper, L corner of Ash Tray opening. 2 - M4.2-1.79 x 28mm 17mm washer SEM screws (7mm) at the bottom near where the Center Console was. 4 - M6-1.0 x 25mm flanged button screws (T30).

Lower bolster carefully to avoid damaging HVAC duct and 12V Accessory Port Wires. The Parking Brake Release Handle cable may have to be guided out as it tends to snag on stuff. Remove 4 – individual wires and the 1- connector with the 2 – wires for the main port.

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Knee Bolster.

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Ash Tray retainer.

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Parking Brake Release Handle cable connected.

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Parking Brake Release Handle cable disconnected.

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HVAC duct and courtesy lamp that were connected to bottom of Knee Bolster.

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12V Accessory Port Wires.
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Your right side speaker cover has the front hooks broken off 😟
Yes. That is why I explicitly explained how it attaches to the dash and 1 - simple method of removing w/o breaking the fingers.
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The holes and the hooks are fragile. I'd say 80% of the covers I find in the yard have one of the two broken.
If I broke them, I likely did the damage 1st time I pulled them when detailing the filthy interior after I bought it 2020. I was gentle this time. Maybe the photos will prevent someone from making the same mistake. I hate removing automotive interior trim components when I do not understand how they are attached.
Finishing I/P Removal

Further work on I/P removal. Much of this post is just recreating some of BravadaX’s previously linked thread, but with my photos hosted on this site, as opposed to BravadaX’s which are hosted on Photobucket.

At the point of tilting the I/P away from the Firewall, electrical disconnection, and removal from vehicle, I suggest using 2 – people if I/P and interior trim components are in good or better condition. Be careful.

Removing the Steering Column. Unbolt (7mm) and separate the large connector above and left of the column. Disconnect SIR (yellow). Also, detach the Firewall-side SIR connector from the push nail mount on the I/P. Otherwise, the SIR harness will still be attached to both Firewall and I/P. Support Column and remove 4 – M10-1.5 flanged hex nuts (15mm). Lower Column to prevent attached bracket from damaging I/P and pull out, through firewall, wiggling as necessary.

Eventually, the antenna cable must be addressed as it is routed from the R Fender to the Radio. It must either be pulled into the Cab through the grommet in the Firewall, or disconnected from the Radio and pulled out of the I/P. I chose to disconnect at the Radio because I had to disconnect a microphone from the aftermarket Radio in this vehicle. If disconnecting at Radio, you must remove the I/P Accessory Trim Plate Assembly from the I/P. That assembly is attached to the I/P with 2 - M4.2-1.41 x ?mm 12mm washer SEMS (7mm) directly above the Instrument Cluster; 6 – spring-type mounting clips; and a number of electrical connectors dependent on vehicle options. After Trim Plate Assembly is detached, the mounting screws for the Radio/Trim and HVAC Control Panel are accessible.

Remove fasteners attaching I/P to Cab. 3 – M6-1.0 x 30mm 17mm washer SEMS dog point (10mm) along bottom of I/P at L & R pivots and approximately center. 4 – M4.2-1.41 x 20mm 12mm washer SEMS (7mm) at top: 1 - each by L & R Speakers and 2 – by Defrost Vent. After removing the previous 7 – fasteners, the I/P is able to be tilted back on the pivots at the L & R Kick Panel areas. It may be a bit stuck at first.

While supporting the I/P or resting against a padded Manual Transmission Shift Lever, disconnect/reroute everything still attaching I/P to Cab. Electrical connection above Steering Column location. Antenna cable (if necessary). HVAC vacuum connection near center. 4 – electrical connections at right side, including the large connector with the 7mm bolt head. The I/P is now free to be removed from the vehicle.

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Main electrical connector under Steering Column with red bolt head.

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SIR (yellow) connector under Steering Column and Lower Column mounting nuts.

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Upper mounting nuts for Steering Column.

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I/P Accessory Trim Plate Assembly showing locations of fasteners and clips. Yellow circles show all locations where screws connect Assembly to I/P. Green circles show all locations where clips connect Assembly to I/P.

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I/P mounting bolt at L pivot.

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I/P mounting bolt at bottom, center.

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I/P mounting bolt at R pivot.

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Cab with I/P removed showing locations of mounting fasteners and other connections. Yellow circles show all locations where screws or bolts connect I/P to Cab. Green circles show all locations of electrical, antenna, or vacuum connections.
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R Cowl Plenum Access (from Cab interior).

The Air (Fresh) Inlet Assembly is now accessible. It and a few other items in the area need to be removed to create work access to this R Cowl Plenum drainage area.

Air Inlet Assembly removal. Gently pry the plastic retainer up on the Vacuum Actuator to allow Actuator to be slid rear off Assembly. Disconnect Actuator arm from damper. Remove the 1 – M6 plastic retaining cap, and 3 – M6-1.0 hex nuts w/ free-spinning 19mm washer (10mm) from the studs. The Actuator and wire harnesses can now be temporarily relocated to improve work access. Remove the 4 – M6-1.0 x 45mm 24mm washer studs (10mm) mounting the Assembly. The Insulating Pad attached to the firewall likely overlaps the Assembly Seal. The Pad and Seal can be carefully separated using 70%+ isopropyl alcohol. Once the Pad is free of the Seal, the Seal also can also be carefully separated from the Firewall using 70%+ isopropyl alcohol. There is no adhesive on the Firewall side of the Seal, but years of contact and heat have adhered the two. The Seal is NLA. Be patient. Once free, the Air Inlet Assembly will easily pull out of the Firewall opening.

While here, the Plenum Drain Valve Assembly can be removed. This is the weighted, rubber flap covering the Drain Outlet. These may also be NLA, so be careful. A small slotted screw driver or alternative can be used to compress the rubber mounting nipples inside the plenum, and carefully work them out through the holes in Cab.

Remove wire harness attached to side of cab below Drain Outlet. 2 – M4.2-0.79 x 25mm 12mm washer SEMS (7mm). The entire harness and Bulkhead Connector can be pulled out of the Cab and set out of the way in the engine bay. While here remove the 1 – M6-1.0 x 30mm 20mm washer SEMS dog point (10mm) mounting the Evaporator and Blower Module and protruding into the bottom of the Cowl Plenum.

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Air (fresh) Inlet Assembly installed. Yellow circles show all locations of plastic retaining caps; nuts; and studs. Green circles show Vacuum Actuator retainer and connection between arm and damper.

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Air Inlet Assembly removed.

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R side of Cowl Plenum showing HVAC Blower Motor; R Drain Outlet; Evaporator and Blower Module mounting screw (yellow circle); and mounting nipples for the Plenum Drain Valve Assembly (green circles).

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Wire Harness and Bulkhead Connector below Drain Outlet removed.
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Accessing the L Cowl Plenum Drain (from exterior).

Removing the L Wheelhouse.

Support Electronic Brake Control (EBC) module if you are concerned about stressing brake lines.

Separate EBC Brackets from Wheelhouse and Fender. Remove 3 - bolts from Brackets. 1 - M8-1.25 x 30mm flanged hex bolt (10mm) at fender and 2 – M8-1.25 x 27mm 17mm washer SEM screws (13mm) from below Wheelhouse for Lower Bracket on Engine side. Upper and Lower EBC Brackets can remain bolted together.

Remove rest of M8-1.5 x 25mm bolts. 3 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) for Reinforcement at front, bottom of Wheelhouse. 2 – M8-1.25 x 22mm 32mm washer SEM screws (13mm) for Firewall tabs. 6 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) for Fender connection.

Wheelhouse will wiggle loose and out. The Windshield Washer Solvent Container is still bolted to the Fender. Lower the Wheelhouse to separate from Container before pulling outwards.

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Supporting EBC module.

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EBC bolt hole just below Fender lip. Bolt already removed.

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Note routing of Hood Latch Release Cable through EBC Lower Bracket legs for reassembly.
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Accessing the L Cowl Plenum Drain (from exterior) Continued.

See Post 6 if Radiator Grille, Front Bumper, and Hood not yet removed.

Removing the L Fender.

Windshield Washer Solvent Container and immediate area. Remove 1 - M6.3-1.81 x 20mm 17mm washer SEM screw connecting top of Container to Fender. Support (simpler) in place or remove (more work). Disconnect horn inside fender from wire harness. Open retainer for Hood Latch Release Cable. Open 4 – split wire loom retainers.

Battery (+12Vdc) Junction Block and immediate area. Remove 2 – plastic nuts (13mm) from Junction Block Cover and remove Cover. Remove 2 – M6-1.0 studs and pull Block away from Fender. Open 1 – split wire loom retainer. Free 4 – electrical connectors from push nail (Christmas tree) mounts.

Unbolt Fender. Remove 9 – bolts; all M8-1.25 x 30mm 24mm washer SEM (13mm). 2 – bolts at hood hinge location. 2 – bolts at rear edge of Fender, accessible from Cab area through gap between Door and Cab. 1 – bolt at rear, bottom of Fender. 1 – bolt from back of Radiator Support; somewhat hidden by Washer Solvent Container. 2 – bolts at front, bottom of Fender. 1 – bolt at front, top of Fender. Slip Fender forward, away from door and remove.

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Windshield Washer Solvent Container mounting bolt.

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Battery (+12Vdc) Junction Block.

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View of Battery (+12Vdc) Junction Block area from below.

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L Wheelhouse and Fender Removed.

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Front, L edge of cab.

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L Cowl Drain Outlet. Just below Hood Hinge area. I cleaned this section of the Cowl before I had the windshield replaced last year, but did not notice the Outlet at that time.

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Failing seam sealer in Hood L Hinge area just below windshield.

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More crumbling seam sealer at front, L edge of cab.
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Accessing Cowl Plenum (L and Central Portion)

There has been a significant pause in this project, including documentation of last work completed. I will try to accurately recount what I did last.

Removing remaining items to create access to Cowl Plenum.

Removing Windshield Wiper Arms. Remove the semi-flexible, black Wiper Arm Covers. If necessary, review the photo to understand how they are attached. Remove the M8-1.25 hex nut (13mm) for each arm, and carefully pry the arm off the tapered, knurled, Windshield Wiper Transmission shafts (spindle). Sometimes rocking back and forth is necessary to break them free. In rare instances, you may need a puller tool.

Removing Air Inlet Grille Panel (Center Cowl Panel trim). Remove 2 – M4.2-0.79 x 17mm 12mm washer SEMS (7mm) and 5 - push-type retainers. Note: the 5 - push-type retainers utilize a plastic screw with a Phillips head for the “nail”. Just pry them partially out of the retainer to collapse the retainer and allow it to be removed. Remove the Panel.

Removing Wiper Transmission. Remove 3 – M6.3-1.69 x 20mm 17mm washer SEMS DST (10mm) at both Wiper Transmission shafts. Disconnect Transmission from Wiper Motor. The Transmission Linkage Bushing will “pop” off the ball on the Motor actuating shaft, possibly with prying. Be careful; AFAIK, replacement bushings of good quality are not available apart from purchase of a complete transmission. Go ahead and remove the 3 – M6-1.0 x 29mm countersunk hex bolts (10mm) mounting the Wiper Motor. This allows the Motor actuating shaft to be shoved out of the way for Transmission removal; same with the bolts while working on the sealer later. Work the Transmission out of the Cowl Plenum. While removed, check each spindle housing for free rotation. They sometimes bind, causing the Wiper system to fail. Unfortunately, I do not believe the spindle housings can be reliably or easily disassembled without damage for cleaning, rust removal, and lubrication. I disassembled and reassemble one. It was not “pretty”. However, there is plenty of free volume around the middle of the spindle shaft inside the plastic housing for injection of grease with a needle tip if you are inclined to drill a small hole through the housing. There are bushings on both the “top” and “bottom” of the housing.

Removing remaining obstructions to Cowl sealer access. There are a variety of fasteners that likely still protrude into the Cowl Plenum. I am not listing those additional I removed to improve access and decrease scratches/cuts to hands and arms. Remove what you deem appropriate.

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Windshield Wiper Arm Cover removed. Yellow circles show small, retaining tabs.

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Push-type retainer removed from Air Inlet Grille Panel.

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Remove these 3 – Dual Spaced Thread (DST) SEM screws at both Windshield Wiper Transmission shafts.

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Windshield Wiper Transmission pried free of Motor.

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Windshield Wiper Transmission removed from Cowl.

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L side of Cowl Plenum. Surface rust on left (in photo) is forward of the Instrument Cluster in the Cab. Sealer to be replaced is bounded by yellow lines. Red circle is Cowl Plenum Drain L Outlet. Blue circle is mounting location for Windshield Wiper Transmission L Spindle.

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Some of the seam sealer (in green) between the Cowl and Firewall. Red circle is a work light inserted from the cab interior, through the opening for the Air Intake Assembly.
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Old sealer removal.

I am removing original, failing or failed sealer for rust treatment and sealer reapplication in 3 – general areas. The first 2 – areas are the external, vertical hem seams near the L and R, front, vertical, Cab corners. These span the vertical distance from the bottom corners of the windshield to the bottoms of the Cab on both sides. I am not aware these were allowing water into the cab, but the sealer is failing. These are preventative measures. I am not concerning myself much with any transverse seam(s) at the rear surface of the Cowl Plenum. The third general area is the outer, bottom of the Cowl Plenum and some adjacent or intersecting seam(s). I will explain in words and in photos below. We will start at the extreme L, back, upper corner inside the Plenum. This seam drops down to the “floor” of the Plenum; travels forward, through the bottom of the L Drain Outlet, to the front of the Plenum; there intersecting a short, vertical seam. Next, it travels transversely from L to R till it dips down to the Air Inlet Assembly area. After passing through the front, bottom of the Air Inlet Assembly area it intersects a vertical seam at the extreme R. Then, it continues rearward, through the bottom of the R Drain Outlet; to the rear of the Air Inlet Assembly area. There it angles upward, along the back, R edge of the Air Inlet Assembly area, heading towards the bottom of the R A-pillar. On my truck, almost this entire section of seam(s) exhibited sealer detachment from the “interior” side of the seam and rust at the detached area. Somewhat strangely, the sealer at the “exterior” was mostly in good shape, remaining firmly attached, at “exterior” side of the seam.

I believe the most likely places to leak are the extreme R & L seams of the third general area I described. Debris tends to accumulate in these areas, promoting a continuously moist environment that leads to seam sealer failure, rust, and water intrusion to the cab interior. In my truck, water was leaking through the seam in the R, bottom of the Air Inlet Assembly area. Excluding rust perforation of the sheet metal, water cannot leak into the cab through the transverse (R-L) section at the front, bottom of the Cowl Plenum. The sheet metal on the interior surface of the front of the cab behind this seam is continuous. Barring rust perforation of the interior sheet metal, sealer failure will only lead to water dripping down the exterior surface of the firewall, inside the engine bay.

I performed an initial cleaning of work areas many months previously with degreaser, scrub brush(es), and garden hose.

General recommendation for sealer removal is using heat to soften and loosen it; generally with a torch. My engine bay was mostly intact, as was much of the cab interior. Since I did not want to damage with heat the sealer, paint, and etc on the “other side” of the seams & sheet metal, I used a variety of hand tools and abrasives to remove the old sealer. I tried to pry out the sealer inside the Cowl Plenum in as large of sections as possible, but it was slow work. Access is very limited in places, and work was quite tedious. I only used power tools + attachments on the 2 – external, vertical, hem seams. I spent considerable time hand sanding with just paper and simple sanding blocks to remove residual sealer and rust from the seams and other rusted areas in the Cowl Plenum. I did not take many photos of the seams after sealer removal and prior to encapsulation.

Tints and shades Gas Font Circle Automotive exterior

Critical seams (in yellow) inside Cowl Plenum at extreme L. Notice the rust at the back of the Plenum from long term contact with moist debris I previously removed. The seam in the direction of the green arrow is continuous to the green arrow in the next photo.

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Critical seams (in yellow) inside Cowl Plenum at extreme R at Air Inlet Assembly area. This area was packed with moist debris I previously removed. The seam in the direction of the green arrow is continuous to the green arrow in the previous photo.

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Some of the hand tools used for sealer removal

Body jewelry Cosmetics Wood Finger Nail

I was most efficient when I focused on prying sealer out of Cowl Plenum seam in large chunks. Where possible, I used a large, flat screwdriver and mallet to break/cut the sealer “wedge” free from the side there was still good adhesion.

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Beginning sealer removal at the external, vertical hem seams near R, front Cab corner.

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Sealer removed from some of the transverse (R-L) section at the front, bottom of the Cowl Plenum. No additional clean up performed yet.

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View through opening of extreme L of Cowl Plenum, from engine bay. Limited access lead to slow work prepping the rust at the back for treatment and removing the sealer along the side and front in this area.
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Rust and encapsulation

I am not skilled in body work. I will explain what I did.

Materials (excluding some tools)
  • Various grit sandpapers
  • Scotch-Brite™ Hand Pad 7447 Maroon (coarser would be better)
  • Mineral spirits
  • Tack cloths
  • Klean-Strip® Concrete & Metal Prep (liquid phosphoric acid)
  • Henkel Naval Jelly (gelled phosphoric acid)
  • Acid, chip, and foam brushes.
  • PM Industries Mastercoat® Ultimate Permanent Rust Sealer (moisture-cured urethane w/ aluminum pigment)
I welded in a patch at the front, bottom, R of the Air Inlet Assembly area to provide a sealer backer in a rusted out area. I also continuously welded the seam at the bottom of the R Drain Outlet. There is no good way to seal the bottom of the R Drain Outlet (with sealer); as opposed to the L Drain Outlet that has a trough for sealer fill. At the factory, a thin layer of sealer was simply “smeared” over the seam at the bottom of the R Drain Outlet.

Some constraints not relevant to other’s projects supported only limited use of power tools and attachments. I mechanically removed rust by hand with sandpaper anywhere it existed in the work area till it was gone, smooth, or I was sick of sanding. I only did a small amount of removal w/ power tools at the exterior of the R Drain Outlet. All surfaces to be coated not previously abraded were scuffed w/ 3M maroon hand pad or 220 grit sandpaper. I suggest a more aggressive 3M hand pad. I used what I had “on hand”. I brushed, blew, and/or vacuumed all surfaces to be coated. I wiped everything down with mineral spirits and let flash. I wiped with tack cloth.

Phosphoric acid converts rust (iron oxide) into inert iron phosphate and water. Iron phosphate is black in color. Light surface rust “seems” to disappear. I applied to any rusty areas, depending on severity, one or more coatings of liquid or gelled phosphoric acid. After conversion, do not leave any powdery or gummy deposits! A slightly tacky, but clean surface is acceptable. I removed any gummy or powdery acid residue with fresh, liquid acid, scrubbing as necessary. Any acid remaining on the sheet metal, not “used up” in the conversion process, was towel removed. I took no photos of the rust conversion process.

Protect any threaded holes as you see fit. I applied 2 – coats of Mastercoat Rust Sealer adhering to manufacturer’s instructions. Exception is substituting Klean-Strip® Concrete & Metal Prep as detailed above for the specified PM Industries Mastercoat® Metal Prep & Rust Remover. Tip: bend an acid brush at the crimp to reach seams in low-access spots. Application is done “blind” in many locations. Just patiently and thoroughly work through the process. You probably do not want to repeat this in a couple years. A telescoping, swiveling inspection mirror is helpful in verifying you coated what you intended.


Encapsulated L front, cab exterior (vertical hem seam & Drain Outlet).


Encapsulated L Cowl Plenum.


Encapsulated L central Cowl Plenum.


Encapsulated central Cowl Plenum.


Encapsulated R front, cab exterior, including vertical hem seam at left in photo & Drain Outlet at right in photo. Sloppy, continuous FCAW weld at bottom of Outlet (yellow oval).


Encapsulated area below where Air Inlet Assembly installs. Sloppy FCAW welded patch at the radius (yellow circle). Area was rusted out to an extent I did not want to attempt floating over with sealer.


Section of Encapsulated interior of Air Inlet Assembly area.
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Sealer application

Materials (excluding some tools)
  • Scotch-Brite™ Hand Pad 7447 (maroon)
  • Tack cloths
  • JES 200ml 1:1/2:1 26:1 Dual Cartridge Gun
  • SEM 39777 Dual-Mix™ High-Build Self Leveling Seam Sealer (2K grey epoxy)
  • SEM 39477 Dual-Mix™ Medium-Bodied Seam Sealer (2K beige epoxy)
  • SEM Integral Nut Square Static Mixers Part #70012 (qty 50)
  • 3/16" ID x 5/16" OD Clear Vinyl Tubing
  • Vinyl electrical tape
Areas for sealer application were scuffed with maroon pad. Tack cloth again. Following manufacturer’s instructions (somewhat), I applied medium bodied sealer where I wanted it to stay and self-leveling where I wanted it to run. 3/16” ID vinyl tubing can be slipped over 3 – ribs of the mixer tips, and taped in place for extensions. The SHFT™ SEM Hem Flange Tip packaged with the medium-bodied sealer was helpful for the vertical hem seams. With more practice I think I could have made a nice application there. Overall, I was disappointed with the sloppiness of my work, especially where using the tubing extensions. Some type of extension is necessary, but my implementation was not as successful as anticipated. I encountered some problems and uttered many cuss words.

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Interior of Cowl Plenum near Air Inlet Assembly area sealed. There is more critical seam rear of the Drain Outlet, off photo, that was also sealed.

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Kick panel area (below Air Inlet Assembly) sealed. The re-applied sealer here is likely much, much less important than that inside the Cowl Plenum in previous photo.

Wood Watch Trunk Tints and shades Bumper

L Central Cowl Plenum sealed.

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L exterior, vertical hem seam sealed.
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Optional top-coat.

Materials (abbreviated)
  • PM Industries Mastercoat® Ultimate Permanent Rust Sealer (moisture-cured urethane w/ aluminum pigment)
  • Rust-oleum Farm & Implement Low Gloss Black (280107 - 1 Quart)
  • Krylon Farm & Implement Catalyst Hardener K02046000 (8oz)
Scuffed sealer with maroon pad and/or wire brush. Another application of Mastercoat according to instructions. 2 – coats catalyzed Rust-oleum enamel not according to instructions. These 3 – applications can likely be omitted.

IIRC, I have a few issues to sort out in the cab before beginning cab interior reassembly. Also, I want to ponder how I will seal the connector passing through the cab sheet metal below the R Drain Outlet. That originally had a circumferential foam gasket augmented by some butyl sealant and a water deflector at the top. I do not know where the deflector is. Additionally, I prefer to let the Rust-oleum harden before mounting anything to it or banging stuff into it. Even catalyzed, my experience is it hardens slowly.

Omitting photos as there is little to be gained from them. They are just a black version of previous photos, with some details lost in a black void.
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Beginning Reassembly - Sealing the R Bulkhead Pass-Through.

I am now beginning reassembly. Unless I encounter something else unexpected and noteworthy, this will likely be my last post of substance. Reassembly is mostly the reverse of disassembly.

On my 1996, there is a Bulkhead Pass-through below the R Cowl Drain Outlet. As installed at the factory, the Pass-through utilizes what appears to be a ¼” or 6mm thick, neoprene foam (sponge) gasket. The top of the 2 –sheet metal screws for the Pass-through also loosely mounts a plastic, Water Deflector. There was a small amount of butyl sealant between the Pass-through and Cab at the top screw only.

Even though I have been informed the Pass-through rarely leaks, if ever, I attempted to improve the water exclusion system in this area. I replaced the original, slightly compressed gasket with one I cut from ¼” EPDM foam. I applied butyl tape to both the top and bottom screw holes; the entire perimeter of the Pass-through; and between the Deflector and Cab and Pass-through. I did not place any between the EPDM gasket and Cab or Pass-through. I carefully molded the butyl near the Drain Outlet to prevent butyl contact with the rubber, Plenum Drain Valve Assembly.

This addition of butyl is ugly, and will be messy if the harness needs to be pulled out of the Cab in the future. However, I was not comfortable with the factory installation given how much water flows past this Pass-through.

The threads of any other screws passing into the bottom of the Cowl Plenum or below the L Cowl Drain were coated with RTV during installation. Pouring water into the Cowl Plenum yielded no discernable leaks.

In a subsequent model year, the R Bulkhead Pass-through was moved to the firewall.

Rectangle Automotive exterior Bumper Gas Font

New EPDM foam gasket for R Bulkhead Pass-through.

Font CD Circle Metal Packing materials

The 3/32" x 3/8" Butyl Tape I used to augment the R Bulkhead Pass-Through seal.

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Butyl tape added to top and bottom of pass through.

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Top view of mounted Pass-through, Deflector, and Plenum Drain Valve Assembly. I better molded the butyl tape after photographing to eliminate contact between it and the Drain Valve.

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Side view of mounted Pass-through and Deflector.
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You scraped all of that seam sealer out by hand? I just torch that stuff, it turns soft and peels right off.
Mostly. Limited use of die and angle grinders + attachments outside the Cowl Plenum for residual sealer removal. With torch assisted removal, does sealer and paint on the other side of the sheet metal survive?
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