Q. I have a question about my mother’s “91 Grand Am. She has discovered water (and a lot of it) on her driver’s side front floorboard and passenger’s side back floorboard. There is no evidence of leaking around the windshield. The cloth seats and dashboard do not get wet. The car does sit outdoors and we have had a lot of rain lately, but we can’t figure out why the water shows up only in those two areas. Any idea?

Bev Larson Needham,

Allamakee, Iowa

A. There are three possible causes of wetness beneath the instrument panel: coolant due to a leaking heater core; condensation from a plugged-up air conditioner evaporator drain; or rain that has entered through a windshield or cowl leak. Either the Grand Am has a second leak or the right-rear foot well wetness might be the result of a large quantity of fluid seeping rearward beneath the insulating pad.

A heater core is a cigar-box-size heat exchanger located deep within the instrument panel. A leaking core will usually soak the passenger side carpet with engine coolant but a centrally located core could make a mess of both sides. Another symptom of a leaking heater core is fogging of the windshield, especially when the defroster is started during a repeat trip. Pulling back the front carpet edge might reveal an identifiable pool of green, orange or red fluid (depending on the coolant type) and pinpoint the heater core as the cause of the wetness.

A plugged-up air conditioner evaporator drain also can wet the under-dash area, with the right side again being the most likely to get wet.

This will be a clear fluid, indistinguishable from rain water. Try checking for proper evaporator drain function by observing the under-vehicle (approximately below the glove box) after an air-conditioned trip. If dripping condensation isn’t seen, the drain might need to be cleared of debris.

A water (rain) leak also is possible, and will be the most difficult to diagnose and repair.

Your best bet here is to pull back the carpet on both sides and watch for incoming leakage – from an inverted body position, with a flashlight – as a helper floods the windshield and cowl area with water (starting low, working upward). A leaking windshield seal, body seam or rusted air-intake box would be my best guess here with each requiring skillful repair techniques.

Your first priority is to thoroughly dry the carpet and insulator beneath in order to avoid mildew and rust problems.

To do this right will require removing the front seats, seat-belt anchors, and door sill plates. This also will allow better access to view the source of the leakage.

Readers respond: Virginia Thorsvig suggests the use of Lysol to cure stinky air conditioner odors as it’s easier to find than Pure Air.

Kenneth Scholz points out using the air conditioner is only more efficient than rolling down a window when driving at moderate speeds or higher.

Brad Bergholdt teaches automotive technology at Evergreen Valley College in San Jose, Calif.

E-mail him at under-the-hoodjuno.com or write to him in care of Drive, Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. He cannot make personal replies.)

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AP-NY-06-11-04 0615EDT