help me diagnose this moisture problem?

[3X00-Modified]

No reason to do anything from the inside of the vehicle that would just seal water between the two causing more rust issues, just seal the seams in the engine bay for the firewall to the fender well and around the bulkhead connection, and stop wasting your time with whatever is under the cowl cover... I've now given the info that has fixed my leak and many others so I'm out.

[Rettax3]

I will be priming mine from the inside to prevent rust, but that is more critical for my car because my paint was toasted-off when I welded the seam. If you look at the factory sealant, they apply it to both sides, so if the seam is leaking, it makes sense to me to remove the old failed sealant, and redo the whole area from both sides, as they originally did at the factory (unless they just pushed one bead of sealant through the seam making it look like it was separately applied to both sides, but that would be a difficult process for us to replicate). However, since the factory seam leaked in the first place, I also recommend using a better material. I still like butyl-tape for this because it remains flexible practically forever and has adhesion qualities out the wazoo, but there are many new sealants on the market that are better than what was around for these cars 20 years ago, especially when you consider the lengths to which GM would go to save a buck on constructing a car.

Personally, if I put fresh carpet in my car, I am going to do everything I reasonably can to keep water away from it -ruined carpet is NOT an acceptable gauge to show a failing body-seam. Hopefully, if the outer layer of protection fails, there won't be substantial rusting before it is caught and corrected.

[themadness]

we cant get the tape locally(under 50 miles) but my uncle runs a roofing business so i might ask him. will it hold up to engine bay temps though? it gets pretty dag on hot a roof but i'm sure the engine is a bit hotter. i dont have the resources to snatch the engine to do a lot of body work and rust removal. all i can do is the best i can. apparently, the leak is a problem with a lot of cars from GM and not just the older ones. vehicles as new as the colorado truck and pontiac vibe have issues too. my old man has a vette and 2 el caminos which also have small leak issues.

[3X00-Modified]

Don't use tape use seam sealer


PN MS 4313 "Netural"
PN MS 4201 "Gray"
PN MS 4323 "Black"
PN MS 4301 "Firm White"

The main reason why you see sealant on both sides is due to it being squished between the seam itself so they have excess on the inside and outside they just brush down.

[Rettax3]

Butyl-tape is, if nothing else, an automotive product -it is used to hold windshields and other glass on older cars, and it used to be used to hold in tail-lights and many other components that needed to be sealed to the body of the car to prevent leaks. The question about the engine-bay heat is a very good one, but yes, it should hold up just fine -I believe it retains shape at over 200 degrees, but that is guestimate based on what I was told by a body-guy way back, so it could do with more research.

The sealant 3x00-Modified is proposing should do fine, maybe even better, since it applies in liquid form and may seal into the cracks better. But, I still suggest spreading the sealer in from the inside as well (maybe even first, and try to push it all the way through from the inside to the engine bay to make the sealant form one solid piece and prevent any air-gaps from forming between the multiple applications). There is better access from the inside, and the sheet-metal pieces meeting at this seam diverge on the inside, so it would be almost impossible to fill the entire gap and have it factory-smooth inside without applying it from under the dash.

x-sec.png (9.84 KiB) Viewed 2734 times

Okay, look at it this way, if this sketch represents the cross-section of the firewall where the two pieces meet, applying sealant only to the outside (left side, represented in red) is a pretty flimsy seal as compared to pushing the sealant from the divergent inside joint under the dash (as represented by the teal color on the right).

Either way, good luck with it. I'm still going for the butyl-tape on the inside, but I think I'll apply a thin, deep layer of a sealant more like Jon suggests first just to get the gap filled better, and cap it off with the butyl-tape to make sure water never gets through it again.

[3X00-Modified]

thats not how the seems look... but ok.