Cowl / Heater Box Leak

Maverick's_Monte

Greasemonkey
Sep 15, 2011
113
28
Rhode Island
Hey now,

Washed the Corona virus off my ride yesterday (first wash since 2020) and I was reminded that I still need to fix the leak coming from somewhere in the cowl/heater box. I understand this is a frustrating and rather typical issue with G-Bodies. I searched the forum for existing threads and any how-tos and found the following...


Before I go off and try to find the source of the leak and fix it, does anyone have any final suggestions, any other threads I might read before opening up a toolbox, or any "make sure you do this, or don't do this" or "while you're in there, you might..." suggestions I would greatly appreciate it.

Thank you!
 

ssn696

Living in the Past
Supporting Member
Jul 19, 2009
5,011
113
Permanent Temporary
I picked up a new gasket from Gbody Parts. I was lucky, the firewall was in good shape underneath. Where it is hidden, I lathered on the POR-15. When you get to the point of putting the top and bottom together, PM me. I needed 3 feet of the butyl dum-dum tape - and had to buy a $36 box of the 3M stuff, so I have a bunch left over. (Have to find the box again if you take me up on it…🙄).

Hope you do this whole unit on the bench. I did a heater core once in the car. Be careful when you take the divider plate out - held in with a couple screws. Hard as hell to get them back in without dropping them. And true to form, GM used a mix of SAE and metric screws to put the housing together. If you will be going with AC, put all new stuff in there now.
 
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Ribbedroof

"That wagon guy"
Supporting Member
Jan 4, 2009
4,149
113
Wellston, OK
Also suspect is the seam sealer at the top of the cowl. It dries out and cracks off, and the resulting leak appears to come from the HVAC case, but is actually coming in the cowl/firewall.

Unfortunately, the fenders really need to come off to repair this.
 
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-dw-

Apprentice
Mar 15, 2021
77
33
Central MN
My 84 olds bout 20yrs ago leaked first on the passenger side. Smeared rtv across the top of the heater box. The next rain found it wasn't fixed but started leaking on both sides. Found seams on the corners under the lower windshield chrome moulding that the sealer was all cracked up. I picked it out and filled with rtv, haven't leaked since.
 
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Texas82GP

Just-a-worm
Supporting Member
Apr 3, 2015
7,040
113
Spring, Texas
There is a rubber flap at the bottom of the HVAC case, on the engine compartment side. The rubber flap covers the drain for the case. Make sure it isn't packed with leaves, pine needles, etc. or the water that enters the case from the cowl at the fresh air inlet can't drain, builds up and just overflows into the car. Check out the pics in post 155 of my thread here....
 
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Maverick's_Monte

Greasemonkey
Thread starter
Sep 15, 2011
113
28
Rhode Island
This is great everyone, thank you for the replies. Gonna have to spend some quality time troubleshooting inside the engine bay. Luckily, no AC in the car and no plans to add it (much to my wifes chagrin)
 
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