Leak under dash

Discussion in 'Classic Mustang Specific Tech' started by gismo7354, Oct 14, 2008.


  1. gismo7354

    gismo7354 New Member

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    I'm having a heck of a time getting my car (66 coupe) to stop leaking water into the floor boards when it rains. I've had the windshield resealed twice by two different glass companies in the past month to no avail. I also have the plastic cowl cover on it to keep water out of the cowl. But it is still leaking into the floorboards. I've tried looking up under the dash to find the leaks but I can't see anything cause of the factory air and heater box.

    I'm gonna eventually put it in the shop and pull as much of the dash, glove compartment and such off as I can then try to find the leak. I just want some ideas before I get in there of what to look for or what all it could be. It usually runs/drips down the sides of the kick panels. Stopping this leak has been my doom.
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  2. BornInAFord

    BornInAFord Member

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    if you pour water down the cowl vents, can you confirm the leak? This will narrow down where it is coming from. There are three areas for leaks: the cowls (rust through very common), the upper corners of the windshield (leaks down the sides behind the gasket and behind the dash from a pinchweld) and the heater core (you'd smell antifreeze and it would come from the base of the heater box). If it is the cowl, the cover over the cowl vent grill at the base of the windshield will minimize but not stop the leaking as the water also comes through the holes for the windshield wiper stubs.
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  3. Rusty66coupe

    Rusty66coupe New Member

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    My money is on the fact you probably have a cowl vent leak....very common. From what I understand it is also time consuming (i.e. expensive if paying a shop) to fix.

    There is an article on Mustang Monthly taking you through step by step on how to fix the problem but it requires welding skills, removing the wind shield, etc.

    Mine needs to be done but for now I just bought the cover that can be put on the cowl vents. My car is not likely to ever see the rain so I just use it when it is time to give the car a bath and then I am careful about not blasting water directly into certain places such as the base of the wipers, etc...
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  4. BarnStang

    BarnStang Founding Member

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    Based on the description, I bet on the cowl vent hat above the heater box too. This job is much easier now (shop wise) since you can buy the whole cowl top and bottom already welded together. If you find a good shop, they should be able to get it out pretty quick. I cut the cowl off the 68 Shelby in less than 3 hours. So, I can't imagine start to finish taking more than two full days or less. And you would need to have it painted of course. I put the left and right hat patches in my 66 project car before the whole cowl was available. An effective repair, but time consuming.
    You need to get the heater box down and out of the car to see what the damage is. You won't be able to see much with it in there. Oh, the fenders will need to come off too, if you replace the cowl. You can save some cost by stripping as much as you can yourself-fenders, heater, front glass, etc. Take some pictures and post when you get the heater box out.
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  5. Rusty66coupe

    Rusty66coupe New Member

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    "since you can buy the whole cowl top and bottom already welded together"

    I wasn't aware of that....where can you pick one of those up?

    Would make life much easier. I am considering doing floor pans soon and if I could find a way to make it more cost effective I might consider going ahead and getting the cowl done now instead of later.
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  6. jamison42166

    jamison42166 Member

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    do those one piece prewelded cowls have their insides painted and or any rust preventatives on the inside of them? id be concerned about having a new cowl having the same problems the old ones are notorious for.

    Just my $.02
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  7. BarnStang

    BarnStang Founding Member

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  8. Blu-Steel

    Blu-Steel Member

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    There is one other possibility; I know because it drove me crazy too and I didn't discover it until I stripped the car down. The firewall could have a hole rusted through near the top corners. I did all the things you did and still had a leak but when I stripped the car during my restoration I found the hole. Mine was on the fender sides at the top and the sound proofing hid it. The sound proofing had dried up trapped water and rusted so water flowed past the fenders openings and down the firewall on both sides. That's why the cowl cover did not stop it for me.
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  9. Platonic Solid

    Platonic Solid Founding Member

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    Agreed, outside of other structural rust issues, doing the cowl should be priority 1. No point in doing anything to the car until the cowl is fixed.

    When you say resealed, does this mean they pulled the windshield out? If yes, what was the condition of the window channel sheet metal?
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  10. wild70stang

    wild70stang New Member

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    On my coupe the lower corners of the rear window (on top between the stripping and glass) leaked. When I sent her off to be painted the shop resealed the window and 1 year later it started leaking again :mad:.
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