What would cause water inside passenger floorboard 2004 Pontiac Grand Prix GTP?
Replaced heater core, still fog inside front window and a lot of water build-up on passenger
side floor board, now front & back. Never seems to dry out.
There is a drain outlet from the vent screen under the hood, just forward of the windshield, and also a drain outlet from the interior AC air box. One, or both, is probably clogged. The under-hood vent takes in air for the AC and heat air box. You may have to remove the screen to see the duct leading to the airbox. Gently run a small amount of water into that duct and look to see if it's draining under the car. It should come right out, instantly. I'm betting that's the one that's clogged. To clear it will require either getting under the car and finding the drain so you can run a wire up into it and clear it out (very easy to do), or removing the airbox inside the car and clearing the drain tube from inside (not an easy job)
Is it coolant or water? Knowing that will help isolate the problem.
Good. That rules out the cooling/heating system. If it has a sunroof that could be the source. They are known for leaking. Open it and check that the channel around the glass is clean and c.ear of debris. There are a couple of drains. Pour a little water into them to make sure they too are clear of debris. Another source as Norm mentioned is at the cowl vents at the base of the windshield underneath the backside of the hood. With my car I have noticed it only leaks when parked facing uphill. That could be the case with yours too. Try to avoid that in rainy weather. Also take steps to dry out the interior of your car so it doesn't start to smell musty. Taking out the floor mats until its dry helps. Of course if the weather it not good the drying process may need to wait till spring, but do make sure it gets dried out at some point.
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