I expect this is a common G-body problem since both my TTypes and a buddy's El Camino do it, but has anyone found the exact cause of water leaking into the front footwells after a good rain? Curoiusly, the 87 leaks on the right side, while the 86 leaks on the left... Both cars are well maintained. Any ideas are welcome.
First, I would make sure that what you are experiencing is indeed rain water and not a heater core leak.
That being said, the top of the housing for the heater core and A/C evaporator is often not sealed well to the firewall. Especially if its ever been removed for heater core replacement. This can be a cause for the right side. I experienced the same problem for this reason. Not sure what to suggest for the left side.
Scott is right about the right. Also, if the drain in the heater box is plugged, it will overflow into the passenger compartment. On my 82 I found it full of mud and pine needles. The driver's side leak could be one of two things that I can think of. One thing could be the connection for the wiring harness just below the wiper motor. Or, my guess is this: The wiper motor mounts to a plate welded to the fire wall. This can collect water. That is why the factory cut the corner off so that it will drain. But like the heater box, this can get plugged up. All you need are a couple of small leaves and some dirt in there, and this holds in the water, and the next thing you know, rot has eaten a hole through the firewall. My 82 had this also. Good Luck.
Brian
check out the new leak article. Be prepared for some fun...not...
http://www.gnttype.org/techarea/bodygauges/rustrepair.html
also coming soon is a floor pan rust repair page...
Thanks guys! I've looked/felt for the HVAC drain from the top but can't find it. Is it obvious from below? I know the crevice at the wiper motor you're talking about and try to keep it clear, will check again, good tip! Guess I'll just have to check -everything- that goes through the firewall on the driver's side. Sure hope there's no rust; usually lots of dry out time and full bore heater action between incidents.