Mysterious Leak Pass Footwell

I have a Mysterious Leak in the Passenger  Footwell a puddle forms in the floormat. Source unknown?

I don't see water or moisture above it at the windshield?

Heater core
To stop the leak by pass the heater core
There are 2 hoses that run along the passenger side to the heater core
Run the hose coming from the left front corner of the manifold (most likey the heater control valve it'll cool the engine same but won't have heater) to the water pump and done.
The heater core should be on the passanger firewall and not behind the dash like other cars. 
Hope I helped

I can guarantee you its the heater box. Those are notorious for leaking, both of my caprices leaked, along with thousands of other folks. The firewall gets all rusty and water seeps through, and water also seeps through the heaterbox itself. Those are common rust and rot spots. I just repaired mine last year, you cant get the firewall seal so you have to use the weatherstrip foam tape that you can get at lowes or your local auto parts store, and for the heater box, take it apart and silicon seal all the seams up, and it will never leak again. I posted this awhile ago: https://www.gmbbodyforum.com/t46-leaking-heater-box-disassembly#124

Yepp mine leaked too right at the firewall to heater box foam seal.

On another Canadian classic car fourm that I frequent, someone found a GM Oshawa released TSB for the 1977-78 sedans and coups built at that plant that explained how not enough sealant was used and a new larger thicker gasket was issued to all dealers of the affected VINs

Can I replace the box from in the car? If so I'll buy a new seal and heater core.




Slightly off topic but why waste space anyone have a pic of the ashtray assembly with the light?

Too many parts laying around can't find what I need.

Myloth wrote:Can I replace the box from in the car? If so I'll buy a new seal and heater core.




Slightly off topic but  why waste space anyone have a pic of the ashtray assembly with the light?

Too many parts laying around can't find what I need.

I would have to say these cars are probably the easiest heater core to change I've ever done. You simply get at it from the engine side. There's no need to go inside the car and take out half the dash or the whole dash like some cars. You'll just need to separate the top of the air box and it's accessible right there. It's held down in there in a metal frame with a couple of bolts and it pulls right out. While you have it all apart, you can re-seal the box too. If you want to re-seal the entire air box, you'll have to pull the passenger wheel well and fender.

Do you smell or feel anti-freeze? You shouldn't, that's a new heater core.
It's probably the leaky windshield.  leaks on both sides, but it's worse on the right side.
did it ever since I had it replaced.  SOB's at American Glass wouldn't warrantee it.

It's the glass then I checked last night(plain water) it's a bad seal . I can try and cob it though for now car is

running good for the moment the new rearend seems solid. I thought the core was ok heat seems good except it

took forever to warmup I just changed the 160 thermo to 195 for the winter now it warms up quick. Yeah I'll just

reseal in the spring properly if needed.

Am I missing something here or no seems odd there isn't some kind of cover something?

nope. that's all that's there.

Seems weird though the 60s to early 70s Caprices\ Impalas have covers and so do the 90s B bodies

but none for the 77-91. That grill\vent does that just let water go down past the firewall to the ground?

Did you change that seal for the Heater Box or no?

IIRC, the whole heater box wasn't removed, only enough fasteners were removed to allow enough space to switch out the cores. It was sealed back up with automotive seam sealer.

I'm not 100% positive, but I think that vent/grill area has to do with the air intake for the blower.

This conversation reminds me of something. Idk if the info was passed on to you or not, but there's a broken orange (i think) vacuum line along side or below the passenger side hood hinge. Put direct vacuum to it and the HVAC controls will work again. It's one of those plastic lines that crumble over time. It broke when I owned it, but the control was set to defrost which I was fine with so I wasn't concerned about fixing it. It was one of those "I'll get to it when I do this other thing" things.

Yeah thats everything, theres nothing missing that i see in the pics. My 79 is like that and the 84 i had was like that.
The neat thing about the heater controls is if you lose vacuum for the little plastic lines, the default position is heat/def so youll always have heat, same with the heater control valve, default position is open. Someone at GM designed it like that and I give them an A+ for it!

I still can't see where the water is coming from but it doesn't seem to be the windshield at least on top section.

I can feel moisture underneath the dash on a piece of the vent though. It leaked when snow\ice was on melting

where the wipers are. Not a lot though there was more when it rained.

I wonder if a rust hole developed under the front of the vinyl top and that's how the water is getting in?  Running down the "A" pillar and under the dash?
Years ago I had a 4dr Caprice that did just that.

I'm just about to go out there and take off the dash I might pull the headliner and check (need new one anyway)

just to see if that's true thanks for the steer.



ARRGH everything including the pillar is bone dry no rust, no moisture I even checked the entire strip of

insulation under the windshield can't find a drop frustrating.

I caught the drip today in the act! It's coming from the insulation below the heater box which means it's leaking from the heater box. I suspect the ice is getting into it then melting into the car as the car was warm inside from the sun (it's 33f here).


Anyone have condensation trunk issues? I'm getting massive condensation on the bottom lid of the trunk was there an insulation liner(none on mine)?

I don't think any of them came with trunk lid insulation.

I used to keep a bucket of desiccant in the trunk to absorb any moisture.
Here's the stuff i use - http://www.wayfair.com/DampRid-Refillable-Moisture-Absorber-Dehumidifier-FG01K-10.5oz-L846-K~YHB1006.html?refid=GX50899355580-YHB1006&device=c&ptid=75695838300&gclid=COTE2tD1xcICFWho7AodIDEAGQ

True Value Hardware sells it.

Thanks I'm going to need something like that way too much humidity.