Q: I have a 1998 Chevy S-10 and I am getting the smell of antifreeze coming from the air vents. It seems to be stronger when the heat is on. There is no antifreeze leaking anywhere in the engine or under the dash. The heater core hoses are tight, and they are both hot after warming up the engine. I just recently bought the truck and a short time after that the A/C quit working. I don't know if that would cause the smell to occur. I would hate to replace the heater core if I am not sure that that is the problem. Any advice you can give would be greatly appreciated.

A: Your thoughts about the odor coming from the heater core are well founded as this is the only area within the truck cab where antifreeze exists. It's possible also, but less likely, that leaking coolant odors are coming in with the ventilation air from an under-hood leak.

Typically, when a heater core begins to leak, one may notice fogging of the windshield when using the defroster; damp-slimy carpet beneath the right/center of the instrument panel; and a citrus-like odor, similar to what you've described. There would also be a loss of engine coolant, but the rate may be less than you'd expect -

it only takes a pint or so of leakage to create the above symptoms - and this might not be immediately evident looking at the coolant overflow bottle level. A leak from under the hood should be easy to spot on the ground.

Your heater core is a cigar box-sized mini-radiator that transfers engine heat to the passenger compartment via circulating engine coolant. This part resides deep within the heater/ventilation system, and most aren't very pleasant to renew, as quite a bit of instrument panel disassembly is often required. Most heater cores are made of thin-wall aluminum, with a large network of finned passages that provide maximum surface area for heat transfer. Over time, accelerated by deferred cooling system maintenance, internal corrosion will find a weak spot, and pressurized coolant will begin to seep into the surrounding housing, and eventually to the carpet beneath.

I have a hunch your heater core may have a tiny leak, and most of the liquid may be vaporizing, causing the odor, or hasn't yet reached a seam of the heater housing. To be sure, you may need to drill a small hole in the lowest part of the plastic housing. Slide a piece of hose or tubing on the drill bit so it can't bore more than one-quarter inch deep - you don't want to cause new problems. Then, after shutting off the fully warmed up engine and waiting perhaps 15 minutes, check for signs of coolant leakage at the test location. Even one drop of coolant means trouble.

You could buy some time by adding a single dose of Bar's Leaks or AlumAseal, which should close up a small leak at least for a while. Your S-10 heater core is about average in replacement difficulty, taking 3 to 6 hours depending on skill level, tools available, and fortitude. It's within bounds for a practiced home mechanic armed with good service information.

I can't fathom how the air conditioning working or not would be a factor regarding this odor. Refrigerant is odorless, should leakage occur. It's likely a coincidence the A/C has recently quit.

Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at under-the-hood@earthlink.net; he cannot make personal replies.