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My daughter has a 1999 Grand Prix that leaks rain onto the passenger floor whenever there is a heavy rain. Mechanic and body shop can’t find where it’s coming from, even when they ran a hose on it. Any suggestions about what to check next?

Spot welded seam in the wheel well or firewall opening up?

Is the windshield wiper motor inside or outside the firewall? If inside, check the seal where the shaft goes through the firewall.

Got a sunroof? Where does the drain hose come down from the sunroof cavity? Is that hose pulled up into the body?

Is the passenger side door filling up with water when it rains? What about the rocker panel? Pull up the door sill and pull back the carpet to see if it is coming from the door or the rocker panel.

Does she have to be driving it for the leak to occur or will it leak with the vehicle parked??


Sunroof? Is the car parked where leaves and such can get into the grilled area in front of the windscreen? Does it appear when the A/C (or defrost) has been on? Is she using the recycle or fresh air on the heater/AC?

The only question I know the answer to is there is no sunroof. It seems to be coming in under the dash.

It leaks in both instances, but only if it rains really hard.

If a mechanic and a body shop can’t find a leak with a garden hose, how can we who can’t run a hose on it or see the car diagnose it? You need a new mechanic or body shop.

Thanks for the helpful comment. I was looking for ideas on what it could possibly be to have checked or if someone else has ever had the same problem. This is an online site, so obviously you can’t see the car to diagnose the problem. Maybe you should stay offline and go hang out in a garage.

Has the car ever been in an accident?
Have you owned it since it was new, or did you buy it used?

I’m betting that there is some hidden crash damage that you don’t know about that is letting the water in from the engine bay (probably around the right front wheel), or around the lower floorpan/firewall area.

You will need to have a shop put the car on a lift, and look for a seam that wasn’t sealed, or a crack in those areas.


A leak that happens only when it rains really hard eliminates a few of the possibilities. A leaking windshield or body seam would not care how hard it is raining.

I would focus on things that are supposed to drain, like the door, the air inlet at the base of the windshield, and probably the rocker panel. Leaves, dirt, critter nests, or pet hair can partially block drains so that in a heavy rain, the water flow exceeds the ability of the drains to clear the water, so the cavity fills up and water goes somewhere that it shouldn’t.