Chasing Interior Water Leak

[HipsterDriver]

Hey Everyone,

I've been chasing an interior leak near the firewall that I can't seem to track down. I've read a lot of other posts on here about possible leak points (I.e., The dreaded cowl attach screw). The leak seems to be coming from the top corner joint on the passenger side of the firewall. Yesterday I cleaned all the old seam sealer from the top firewall joint, AC intake cover on the cowl, and cowl drains. I cleaned everything and reapplied new seam sealer to all these areas. I'm getting some rain today and still see water coming in. You can see in the image below the puddle in the a pillar and the little stream running down the seam joint of the floor pan.

What are some other areas to check? I'm strongly considering pulling the front quarter off to check more of the cab/firewall joint seams that are hidden from the outside. There is also a leak into the lower a pillar, but it's hard for me to figure out exactly where this is on the outside of body to know where to seam seal. Any help would be greatly appreciated. I don't have any carpets in now, until I fix these leaks.

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[CumminsFever]

I believe there are slight differences across the gmt400 platform for cab leakage, depending on year and model of vehicle. Can you tell us what vehicle this is? That can help us help you!

 

[east302]

I had a leak at the rubber door boot (where the wiring goes through) that puddled similar to your last photo. That may be something to check.

 

[Scooterwrench]

I chased that problem when I got my truck and finally discovered it was way up in the corners where the vent cowl meets the firewall at the extreme top corners. I removed the vent grille and had my brother run a water hose all around the inside of the vent cowl. He finally hit the spots and the water poured in. Once I moved some wiring out of the way I could actually see daylight coming through a hole you could stick a pencil in. It wasn't rusted,it got missed during assembly. I had to put globs of sealer on my fingertip and shove it up in the hole then smear it down across those seams that run along the sides. I pulled both drains and that insert that covers the air intake to get to all the seams in the cowl and did around the intake too. Only sealer I had on hand was Permatex ultra grey silicone. I precleaned all the seams with a carb cleaner soaked rag before applying the sealer. No more leaks and I figure it will be there till the next ice age.

 

[HipsterDriver]

I believe there are slight differences across the gmt400 platform for cab leakage, depending on year and model of vehicle. Can you tell us what vehicle this is? That can help us help you!

Definitely didn't even think of that...Good call. I have a 1994 GMC C1500 extended cab that's been in my family since new minus a brief stint with someone else for a few years until I bought it back.

 

[LayLow21]

I chased that problem when I got my truck and finally discovered it was way up in the corners where the vent cowl meets the firewall at the extreme top corners. I removed the vent grille and had my brother run a water hose all around the inside of the vent cowl. He finally hit the spots and the water poured in. Once I moved some wiring out of the way I could actually see daylight coming through a hole you could stick a pencil in. It wasn't rusted,it got missed during assembly. I had to put globs of sealer on my fingertip and shove it up in the hole then smear it down across those seams that run along the sides. I pulled both drains and that insert that covers the air intake to get to all the seams in the cowl and did around the intake too. Only sealer I had on hand was Permatex ultra grey silicone. I precleaned all the seams with a carb cleaner soaked rag before applying the sealer. No more leaks and I figure it will be there till the next ice age.

You wouldn't happen to have any pictures of the area you sealed up ? I have a 99 C2500, and I'm chasing a leak as well it's not coming from the cowl screws or the drain but if I poor water on the passenger side cowl, it comes into the cab through the blower motor area I'm gonna pull it out when I get the chance.

 

[Scooterwrench]

Sorry man,I didn't take any pics.
Sounds like you may have a leak around the vent stack where it pops up through the vent cowl. Not sure if 99 has the same glued in plate over the vent stack but if it does you may have to cut the caulking around it and carefully pry it up and off to gain access to that area.

 

[monsterwake]

I am still chasing mine too. I did find that it was leaking at the gromet for the emergency brake. That was odd because of the placement and direction of that gromet. I got that sealed up but I still have a leak somewhere.

 

[LayLow21]

Sorry man,I didn't take any pics.
Sounds like you may have a leak around the vent stack where it pops up through the vent cowl. Not sure if 99 has the same glued in plate over the vent stack but if it does you may have to cut the caulking around it and carefully pry it up and off to gain access to that area.

Appreciate the response man but luckily I have the plate on mine, I'll give that a look when I get the chance, that sounds about right

 

[evilunclegrimace]

Just heat up around the edge of the plate with a propane torch and the sealer will soften up enough to remove with out bending it out of shape. BTDT