Firewall Rust

[scenic760]

Hey All!

So I've rebuilt the engine, trans and transfer case in my 1981 GMC K1500 Suburban and getting ready to install. I was going to flat black the engine compartment and was cleaning it up a bit and noticed an area of rust on the firewall. I have never had the truck running/driving so I have no idea what would have caused it.

Does anyone recognize what might be causing this? After reading a few threads it seems like it may be the seam sealer failing at the cowl?

Thanks in advance!

 

[Grit dog]

Looks likely but more like the cowl drain was plugged so water pooled there.
There is a drain hole one each side.
Fwiw, the seam sealer in my cowl was all cracked and impossible to get to to re seam seal it.
I costed the whole inside of the cowl with spray bomb bed liner stuff and thick in the big seam. Should keep water from leaking into cab forever.
(wasn’t leaking yet but could have). And drain holes clear.

 

[Octane]

Looks likely but more like the cowl drain was plugged so water pooled there.
There is a drain hole one each side.
Fwiw, the seam sealer in my cowl was all cracked and impossible to get to to re seam seal it.
I costed the whole inside of the cowl with spray bomb bed liner stuff and thick in the big seam. Should keep water from leaking into cab forever.
(wasn’t leaking yet but could have). And drain holes clear.

I have a 77k10,where exactly are the cowl drain holes?

 

[scenic760]

Looks likely but more like the cowl drain was plugged so water pooled there.
There is a drain hole one each side.
Fwiw, the seam sealer in my cowl was all cracked and impossible to get to to re seam seal it.
I costed the whole inside of the cowl with spray bomb bed liner stuff and thick in the big seam. Should keep water from leaking into cab forever.
(wasn’t leaking yet but could have). And drain holes clear.

Thanks Grit,

I was looking for the drain holes but cant seem to find them?? I saw another post that said they are behind the fender shield but I didn't see anything that looked like a drain hole....so does water drain into the cowl and run through that channel pop out at the firewall out to drains? And the cab is spot welded in that channel so once it starts rusting it looks like it runs into the cab?

 

[Grit dog]

@scenic760 yes I believe everything you said is correct.
I’ll see if I can find or take a pic of the drain holes today. My truck is tore apart so might be visible.
If you fill the coal with water though you should see a steady stream at each side draining out behind the fender wells.

 

[Grit dog]

Nope, can’t see them. But they’re there.

 

[Rickf]

I thought the drains could be seen with the fender off.

 

[scenic760]

So I cleaned it up and went looking for an outlet but still couldn't find one..? I may take a piece of wire and try poking it through to see where it ends up..

However, I did poke a pencil through the hole and it ended up inside the cab on top of the carpet! Needless to say I think I need to at least fix that part. I have read everything from put some flex seal on it to replace the cab...anyone have any favorite fixes on these?!

 

[scenic760]

So I found some pics online and it almost looks like the outlet is behind the fenders? Can anyone confirm before I start dismantling them?

 

[DanMcG]

Yes that's it. Put a hose in the cowl and watch it pour out the drain behind the fender. also check the floor for wet spots

 

[86 SCOTTSDALE]

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[scenic760]

Yes that's it. Put a hose in the cowl and watch it pour out the drain behind the fender. also check the floor for wet spots

I think in my case if I put a hose in the cowl it's going to start pouring out that hole I'm my firewall!?

I have a little and held inspection camera and ran it on the inside of the fender, down to about where that outlet should be. It looked to me like an old mud wasp nest was right where the drain was. The truck sat for a few years so probably all the rain/melted snow just sat in that rusted spot... I'm going to Naval Jelly it, inside and out, and try welding a patch over it on the firewall...on the inside I may try sheet metal/polyurethane calling and rivets ...

Anyone have any better solutions?

 

[scenic760]

Finally got around to looking at this a little more in depth..I poured water in the cowl and it washed a **** ton of dirt out of the fenders on both sides..I didn't see any water pouring through the seam but what I did see was a trickle of water coming down the outside of the firewall seam right above where the rust was happening. Does this seem like the culprit and if so do I just put some urethane in that seam?

 

[Ken B]

I just did this hose test the other day. 1st time trying the drain holes behind the fenders....I was pleasantly surprized. I will wipe out the inside of the cowl so I can look at seam sealer and maybe use a fresh coat of seam sealer to cover my bets.
I didnt see any moisture inside the cab That old insulation has been wet but might have been from wet, feet or who knows over the years, the insulation on the inside of the firewall might be coming off so i can confirm everything is sealed. How hard is it to remove that insulation?

 

[scenic760]

Okily dokily...so I ground down the firewall rust, naval jellied it and then put 3 layers of 6oz epoxy fiberglass down-surfboard ding repair stuff...I might sand it a bit to smooth it or maybe overcoat with some Bondo, paint it and move on...however I did take the cowl off and saw the ever so small gap in the seamsealer/firewall. Could this really have been the cause of that rust? Should I use actual seam sealer or do you think any elastomeric type sealant would work here?

That and what is the black thing doing on the passenger side of the cowl besides trapping dirt and leaves?? I see that it may be channeling whatever drops directly on it past something behind the fender but it looks like more of a dam for things draining off the cowl?!