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590 Posts
Discussion Starter · #21 · (Edited)
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.

Office supplies Tool Auto part Composite material Stationery

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

Gas Auto part Metal Tints and shades Titanium

Push-type retainer removed from Air Inlet Grille Panel.

Motor vehicle Automotive exterior Vehicle door Vehicle Fixture

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

Wood Gas Bumper Metal Auto part

Windshield Wiper Transmission pried free of Motor.

Composite material Natural material Bicycle part Metal Auto part

Windshield Wiper Transmission removed from Cowl.

Asphalt Road surface Gas Wire Automotive lighting

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.

Automotive lighting Motor vehicle Gas Engineering Auto part

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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590 Posts
Discussion Starter · #22 · (Edited)
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 simply 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.

Hood Automotive lighting Eyelash Motor vehicle Material property

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.

Hand tool Office supplies Tool Metalworking hand tool Household hardware

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.

Product Automotive tire Tire Tread Fender

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

Hood Automotive lighting Rectangle Automotive exterior Bumper

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

Automotive parking light Hood Automotive lighting Wood Vehicle door

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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590 Posts
Discussion Starter · #23 · (Edited)
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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590 Posts
Discussion Starter · #24 · (Edited)
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.

Automotive tire Hood Motor vehicle Automotive lighting Tread

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.

Automotive tail & brake light Automotive lighting Hood Trunk Motor vehicle

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.

Automotive lighting Motor vehicle Hood Automotive tire Automotive exterior

L exterior, vertical hem seam sealed.

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590 Posts
Discussion Starter · #25 · (Edited)
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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590 Posts
Discussion Starter · #26 · (Edited)
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.

Hood Automotive lighting Automotive tire Fender Bumper

Butyl tape added to top and bottom of pass through.

Automotive tire Hood Motor vehicle Automotive lighting Automotive exterior

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.

Hood Automotive lighting Motor vehicle Automotive tire Automotive design

Side view of mounted Pass-through and Deflector.
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