Pieces of decomposing foam constantly blowing out of AC vents

[bikes!]

SteveZ - this is great! Thanks for taking the time to document your experience. This has encouraged me to go ahead and dive into the task.

Can you provide a link, or the source, for the foam you used?

One of the details I haven't seen mentioned much in the postings is why VW chose to use foam for the blender boxes on almost all car & van models for years rather than a solid door. I believe that they used foam with the openings in the doors, not just for reduced weight, but to provide less abrupt changes in air flow. If there is some bleed through of air, it will prevent a flap from being sealed against the housing, and this should reduce the force required at the blend controls to switch between modes.

[SteveZ]

The foam I used was suggested and used by David in his post here:

https://1705.net/dash-removal-and-foam-repair/

I did a quick measurement of my needs after I removed the blender box, since there might be differences in the amount of foam needed for various model years, before ordering what David ordered. I have some left over, but that's okay.

Like him, I got it from McCaster-Carr. I considered using cheaper foams - ones without as high a heat rating - but decided that I would do this job only once, and using foam and adhesive that might soften in heat was not worth the risk, although I know many owners have used craft foam and tapes with low heat adhesives. Whether there have been problems or not, I could not ascertain from forums and postings, so I took the no risk path.

As to VW's motives, like you, I can only speculate and have not read any rationale from VW. It makes sense to me to use products and methods as similar as I can to OEM when I have the option, unless cost is unacceptable.

[VanGeek]

So... I'm in the process of rebuilding my HVAC blender box. I've been researching why VW used metal doors with large holes covered by foam. Most folks who rebuild these either recover the doors with metal tape or high temp foam. But both solutions are different from the lightweight, low density, porous foam used by VW -- which unfortunately decomposes after 15-20 years.

The explanation that seems to make the most sense, is that VW chose metal doors with holes covered by foam because they are lightweight, quiet, and redirect airflow without completely sealing a passage when closed. If the holes and sides of the doors are covered with a non-porous material, the doors could become stuck closed if the blower motor is running at high speed. See scenario below where fan blower is on high, blender door redirects all air across heater core (so the path to bypass core is closed), other door is closed to defrost and is redirecting air to plenum and dash vents, and assume (worst case scenario) some dash vents are closed or restricted. In this case, the two doors will have a lot of air pressure forcing them to stay closed. If foam or tape seals holes and some of the door edges, they may be difficult to open or cause excessive wear or breakage on cable assemblies and mechanisms. Again, I think this is why VW used low density foam.

While most folks who rebuild with high temp, non porous foam or foil tape seem satisfied with the result, I'd like to hear feedback from those forum members that used this approach. Any problems with the mechanisms or doors slamming into position or difficult to move on high blower speeds?

[TheOneTrueQuux]

[Saturn]

[VanGeek]

Finished up the new foam on my blender box. I ended up using some high temp silicone foam with PSA rated at 300 degrees that I sourced from a vendor on Amazon. The McMaster Carr prices for silicon foam are higher now and 3 sheets would have cost over $140 shipped! The Amazon vendor was half of that. Was spec'd as 1/16 thick but arrived as closer to 1/8. The adhesive was strong but the foam must extend beyond the edge of the flap and I didn't want the two sides to pull apart or become damaged so I applied a thin layer or high temp silicon to the exposed ends. Probably overkill but makes me feel better.

I considered using HVAC tape but thought the silicon foam might be a more robust and potentially quiet solution and more like the original approach.

Flaps with foam installed and silicon edge treatments curing.

Flaps test-fit into one-half before assembling the other half.

Completed housing ready for new core and reinstallation

Not much else to add to what was already shared here. I did separate the halves, removed the flaps, and used them as quasi templates to cut the foam before applying it. I am satisfied with the result and hope/expect it will function well for many years...

[VanGeek]

So.... I noticed the mating surface of my plenum was deformed and making poor contact with the blender box. Not something I examined during disassembly from the dash but it's obvious after inspecting the plenum and blender box mating surfaces. I'm certain the gap wasn't very well sealed given the warped surfaces and gasket foam that was 50% decomposed or missing.

I carefully applied heat with a heat gun and used my silicon "meat gloves" (I use them for handling smoked brisket etc on my pellet smoker they are well insulated and rubberized) to un-warp the upper plenum mating surface. All good but that portion is also very fragile, thin, and likely prone to warp again so I adhered a small thin metal plate to the surface with jb-weld. It's more rigid now.

However, when I applied the new neoprene gasket tape (1/2 x 1/4) I found the surfaces still had a few air gaps because (a) the neoprene foam is rather firm and (b) the mating surfaces are now flat but there is still a small amt of warpage between the two the difference between the widest and narrowest gap is nearly 1/8". I didn't want to heat the entire plenum and attempt to twist-out the warpage for concern that I might deform it. I found a thicker and softer gasket tape (1/2 x 3/8 ) that is also heat resistant and that seems to do the job. https://www.amazon.com/dp/B07W5GGVFT. Fills the gaps, compresses well, and seems to make a good seal.

For anyone removing and rebuilding the blender, check the fit of your plenum. Pehaps mine was an anomoly -- but the plenum plastic is thin, it only mounts to the blender in two locations, and seems to easily warp.

View of plenum warped area along the top mating surface to the blender

Metal plate jb-welded to the top surface after straightening it

View of the prior warped surface after repair

Side view of new gasket foam showing thickness and compression

New foam installed on the plenum itself (instead of the blender box

Plenum and blender xface showing thick and thin gaps filled by new foam

[vwwestyman]

Thought I'd link this other thread here in case someone finds this one first, and doesn't want to take the whole dash apart.

https://www.thesamba.com/vw/forum/viewtopic.php?t=788930
_________________
Dave Cook

President, Wild Westerner Club

1978 Champagne Edition Westy, repowered to '97 Jetta TDI
1973 Wild Westerner
My Thing

[VanGeek]

So I'm fairly certain that the intake flap that directs fresh vs recirculated air is not a big source of decomposed foam bits blowing out of vents.

However, the flap was originally built with a thin foam and rubberized plastic surface covering both sides of the flap. The foam and covering decomposes at about the same rate as the blender box coverings. Mine was gone from the top of the flap and about 50% gone from the bottom. The flap works well without the foam covering (there are no holes in the flap), but the old double-sided adhesive mat remains and it's somewhat rough and porous and prone to attract crud.

I resurfaced mine with a thin silicone covering which I hope may improve how well it seals, repel dust and dirt better, and possibly be quieter than the hard surface of the uncovered flap.

It's a tight space, but with the dash and blower motor removed you can reach inside the duct and clean debris off the flap. I used soap and water, followed by one of the natural/orange adhesive cleaners, followed by an alcohol wipe -- allowing the flap several hours to dry inbetween cleanings.

I made a paper template with the flap dimensions on it and cut a 1/25" thick silicon sheet with adhesive backing. Adhered a sheet to each side of the flap and pressed it firmly into position.

Perhaps not entirely necessary, but why not do it as long as the blower motor is out. This is the material I used https://www.amazon.com/dp/B0C9Y7DVDN

Condition of topside flap with foam gone. Yours probably also looks like this...

Condition of bottomside flap with half the foam gone.

Condition of topside flap with silicone sheedtaffixed. Sorry the pic is horrible and it seems my flash didn't go off well. Blower motor is reinstalled so I can't take a new pic - but hopefully you get the idea: