Cowl Plenum Seam Sealer Failure - Investigation and Repair

bk2life said:

i dont see any rust in your pics?

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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.)

KJF95S-10 said:

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.

Click to expand...

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.

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.


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



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



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.




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




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




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




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




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.

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.


Defrost Cover removal.



Defrost Vent with Cover removed.



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



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



Defrost Cover removed – top view.



Defrost Cover removed – bottom view.



Defrost Cover mounting clip close-up.



R Speaker cover removal.



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



R & L speaker covers – bottom view.

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.



L I/P Sound Insulator.



R Door Sill Trim.




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.



R I/P Sound Insulator



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



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



Center Console (Manual Transmission) base.

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.



Knee Bolster.



Ash Tray retainer.



Parking Brake Release Handle cable connected.



Parking Brake Release Handle cable disconnected.



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



12V Accessory Port Wires.

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.



Main electrical connector under Steering Column with red bolt head.



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




Upper mounting nuts for Steering Column.



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.



I/P mounting bolt at L pivot.



I/P mounting bolt at bottom, center.



I/P mounting bolt at R pivot.




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.

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.



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.



Air Inlet Assembly removed.



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).



Wire Harness and Bulkhead Connector below Drain Outlet removed.

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.



Supporting EBC module.



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



Note routing of Hood Latch Release Cable through EBC Lower Bracket legs for reassembly.

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.



Windshield Washer Solvent Container mounting bolt.



Horn.



Battery (+12Vdc) Junction Block.



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



L Wheelhouse and Fender Removed.



Front, L edge of cab.



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.



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



More crumbling seam sealer at front, L edge of cab.