Foggy glass, soggy floor: heater core

Q: I've noticed my windshield has steamy areas and fog, and the defroster doesn't do much to make them go away. What causes this?

A: It sounds like your heater core has begun to leak. Other symptoms are a sweet syrupy odor and wet, slimy front carpet.

This cigar-box-size part deep within the instrument panel serves as a small radiator. Engine coolant circulates through its many passages, which transfers heat to the passenger compartment.

Heater cores are vulnerable to leaks because of their thin-wall construction and soldered/brazed connections.

Electrolysis is a major cause. Periodic cooling-system service is the best way to prevent electrolysis, and should include a pH and voltage test of the coolant. Among other causes: excessive engine movement caused by a faulty engine mount can tug on the core's delicate hose fittings.

An inspection of the bottom of the right-side instrument panel will usually confirm the leak. A clinging drop or two of engine coolant can be distinguished from plain water by its sliminess, color and odor. Plain water in this area could be caused by an obstructed air conditioner evaporator drain or windshield/cowl leak.

Renewing a leaking heater core promptly is a good idea, as a substantial and difficult-to-clean mess soon develops beneath the carpet.

Labor cost can vary widely, depending on the vehicle, from two to 10 hours, and the part costs $75-$150. The easy ones drop down; the tough jobs require dash disassembly.

If the cost is daunting, a radiator stop-leak additive might provide temporary relief.

E-mail: under-the-hood@juno.com. Sorry, no personal replies.