Stopped by last night,not much going on so I checked out other forums I'am active in, read this post about interior heat coming throught the ducts when AC is off,like most of you I've tryed to fix the problem, tryed most everything,read everything, just can't get it handled, read this post and it sounds like it may be the answer,haven't heard this one before and it sounds right,because I don't have a printer and the post is lenghtey I'am going to tell you where to find it, go to, 1977 C3 Corvette Forum, go to, Performanc Tech, go to, Interor Compartment Heat by Fredtag, go to reply by, Chuck63-Aministrator, click on link, there you have it, it tells how this person tryed everything and found exhaust heat was getting in the plumn through seperation in plumn and firewall and how to fix it

|UPDATED|7/8/2004 9:17:27 PM (AZT)|/UPDATED|

Which corvette forum?? How about a URL.

whats a plumn

vho69 said: whats a plumn

Think he means plenum. Not sure if this is the one he found or not, but it sounds the same.From CorvetteForum.com
c3 tech tips

Eliminating Cockpit Heat in a C3 -- 1 of 1
Date Published: 2003-09-23

Submitter's Name: Al Sindoni
Email Address: vettfixr@comcast.net
Corvetteforum.com Member Alias: vettfixr

Like most of you I've battled the problem of cockpit heat. I think I've finally found a problem in my 74 that may be common to a lot of C3s and which may cause heat to come in to the cockpit no matter how much insulation or sealing is done to the heat/AC system. Over time I've noticed that I always got an "engine" smell in the cockpit. I figured it was air coming out of the hood and being drawn into the heating system through the air intake in the wiper trough or air leaking into the system through the heater box. I put a wiper cover on earlier this year and I could see that the hood sealed really well in this area so this shouldn't cause a problem with air leakage past the gasket. Since I pulled my heat/AC system out last winter and resealed everything I discounted this as a cause for heat leakage also. When doing the heating system I also installed a shutoff valve for the coolant. I also insulated my entire cabin so I'm not getting heat through the floorboards or the body. This left the air intake on the system as a possible source. I removed the right hand dash pad and kickpanel which exposed the flapper mechanism for the inside/outside air control. I removed the vacuum motor and flapper mechanism by unbolting the two bolts that hold the motor, moving it out of the way, and unhinging the flapper. This is done by pulling down the top spring loaded pin, pushing the door into the fenderwell and twisting it so that it can be removed from the inside of the car. I then started the engine with the hood closed and the heat/AC off. I felt hot air coming into the plenum from the engine compartment. This plenum is secured with the same adhesive that holds the rest of our body panels in place and like those the adhesive will dry and crack over time. Anyone who has dealt with stress cracks on the fenders can attest to this. Remember also that air leaking into this plenum will be coming directly off the exhaust system and with headers the problem is exaggerated. To seal this plenum I used windshield sealing caulk. You can get this at your local parts store for around $5 a roll. I used the caulk to seal around the entire perimeter of the plenum, or as much as my arm would reach since you're working through the flapper hole. The most important areas to seal are the forward and lower area since that's where the majority of the heat will come from. The rear area is sealed against the forward door jam and really shouldn't leak hot air. After doing this I repeated the test with the engine running and felt no heat entering the cockpit. I reinstalled everything to complete the job. I've only driven the car twice since then but I can tell you that there was a vast improvement. I took the car to work on a day when the temperature hit about 85 degrees. Before the fix I would have had to have the AC running or the heat would have been unbearable in the footwells and even worse with the t-tops off. I ran the car home that day with the t-tops off and the footwells were no hotter than the rest of the car. I've also noticed that the AC tends to run a little cooler by not having to fight the heat coming into the system. So far I also haven't had that "engine" smell anymore. I'm hoping that as I use the car more, in different situations, I'll be able to report more benefits from this fix but until then I thought I'd give a heads up to everyone. For details on the cockpit insulation go to my website. You'll see the article on insulation and also what you have to remove to reseal the heater box. UPDATE 9/12/03: Since posting this topic in the C3 forum over a year ago I've had time to live with this fix and I can tell you it's completely eliminated the footwell heat common to C3s. I've also recommended this fix to others who have had similar success. One tip that I left out in the original post was to put a soda straw through the bottom of the plenum until it sticks out below the rocker panel. Seal the plenum all around the straw and when you've completed the sealing pull the straw out through the bottom of the rocker panel. This leaves a drain hole that will channel water in the event that it enters the plenum. Other than that it's an afternoon fix that really pays off. Good luck.

The Plenum is the area behind the firewall or cowl where the wind shield wipers are, on the passenger side between the fender and cowl there is a heater controlled door that opens and closes to let outside air in,....Just checked the tread, Snoopy just provided the post of which I'am speaking of, Thanks Dog

|UPDATED|7/9/2004 11:33:09 AM (AZT)|/UPDATED|

Owen said: Which corvette forum?? How about a URL. ...

The forum first mentioned above, '1977 C3 Corvette Forum', appears to be for members only. I didn't find a 'guest' login like we have here on C3VR.

Hey Jim, my recent post "answer, cockpit heat" has somehow ended up in "Forums non-vette for sale/wanted",probley my fault, I believe it belongs in the heating-air conditioning forum, how about putting it where ever you think it belongs. Thanks

Consider it moved!
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Jim O.
Save the Wave...!!!