Eliminating Cockpit Heat in a C3

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.