Where is this interior leak coming from?

When it rains, passenger side gets wet down in the corner angle where your right foot would be. Looks like it’s coming from around the heater core on the right side. Any ideas where it’s coming in from on the outside without me tearing everything apart first?

Thanks!

Test it. Pour water over the Windshield, if it stays dry pour water in the cowl. This way you can find where the leak sits.

Assuming it’s water and not coolant, my money is on from the seam in the cowl and possibly blocked drain hole in the bottom corner of the cowl.
As said above. When it’s dry, start selectively pouring water to help pin point the leak. Take the valance off the cowl so you can see in there. There’s a transverse seam that I believe tends to be a common leak spot. Or rust through hidden somewhere in the cowl or bottom of windshield frame (under the gasket).
Your truck looks real clean though, so not sure if it has any of the damage I mentioned above. But you didn’t mention how it’s been stored over the years.

The cowl drains get clogged eventually and water stacks up and comes in the kick panel area. Under the truck behind the front fenders is where they drain. You may have to remove a plastic trim piece and pull some foam out of the way. I've had to clean, flush, clean, flush, etc until I got all the debris out.

Thanks for the recs! The truck sat outside, but not under trees, but near them, for 7 years. Since I’ve had it, it’s been in the dry until now. It’s a pretty clean truck with minimal rust (rockers), but I’m believing y’all that it’s a clogged drain and coming in the kick panel area. The vent in the kick panel was caulked around the perimeter before I pulled it out and restored all my trim pieces. I’ve wonder if it was coming from that vent ever since, but the insulation around the heater core under the dash is very wet…. I’ll be checking this tomorrow hopefully. Been sick, so haven’t addressed it yet.

Good to know about the cowl drain holes. I have similar leaks on both sides, despite a new windshield gasket.

There is seals on the hood hinges or should be

My truck gets water in through the doors as well with a good rain. The seals are poopy.

Doors will be a whole project themselves someday haha.

On my 82, seam sealer under the cowling was not applied very well at the factory, so it eventually cracked and leaked. In may case, water ran down the drivers side and rusted out part of the fuse panel on its way down to rusting out part of the side panel below the fresh air vent. It's worth a look.

MikeB said:

On my 82, seam sealer under the cowling was not applied very well at the factory, so it eventually cracked and leaked. In may case, water ran down the drivers side and rusted out part of the fuse panel on its way down to rusting out part of the side panel below the fresh air vent. It's worth a look.

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This is kinda what happened... the vent is sealed around the kick panel from the factory, and when I removed the passenger side kick panel when I was restoring all the plastics, I noticed silicone caulk around it, but never replaced it. The truck always sat in the dry then, so I never noticed a leak when I was done restoring the pieces. Well, since it sits outside now, I did.
I found the leak to be from the vent from the water that runs down the cowl and down the side and drips and hits the vent window on the kick panel and drains in. I added a ton of Lexel to both the kick panel and the sheet metal on the cab and put them back together. Putting them back together is a PITA, so I hope I got a good seal. Gave er a quick wash after with no leaks, so now that it's all dry, I need to dump a ton of water down the cowl and see what happens.

Screen shot from the video I took

Good work so far. My only comment is I feel fortunate (knock on wood) that both squares are leak free and appear to have been since new.
I was sketched out a little about replacing the windshield on the 86 w no sealant between the gasket and cab, as there was some there originally. But decided that no way I could get the windshield in without making a total fcking mess out of it if sealant was involved. So I didn’t use any and she no leak. (Can’t imagine how it could as the windshield fit in so ****** tight I thought I’d tear the seal).
Same with back window. No sealant under the gasket, no leaky.

I have a theory that sealant may trap more moisture than without. As both presumably original gaskets had sealant under them and both windows had rust issues along the bottom.
I’m fortunate that our trucks likely won’t see long term exposure to rain as I’m able to garage them and that’s where they live 95% of the time. But just a theory.
The otherwise rust free 86 had the rust issues. The 77 that sat with a truck camper on it (kept water off the cab) for decades has no rust in those areas either. Original gaskets as well from 77.

C10SLEY said:

This is kinda what happened... the vent is sealed around the kick panel from the factory, and when I removed the passenger side kick panel when I was restoring all the plastics, I noticed silicone caulk around it, but never replaced it. The truck always sat in the dry then, so I never noticed a leak when I was done restoring the pieces. Well, since it sits outside now, I did.
I found the leak to be from the vent from the water that runs down the cowl and down the side and drips and hits the vent window on the kick panel and drains in. I added a ton of Lexel to both the kick panel and the sheet metal on the cab and put them back together. Putting them back together is a PITA, so I hope I got a good seal. Gave er a quick wash after with no leaks, so now that it's all dry, I need to dump a ton of water down the cowl and see what happens.

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AuroraGirl said:

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Woahhhh… where did you find this?

C10SLEY said:

Woahhhh… where did you find this?

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looks like non AC models only?