Dear Tom and Ray:

I have a 1991 Toyota Corolla wagon with about 69,000 miles. I have had all the major scheduled maintenance done by my Toyota dealer and have had the oil changed every 3,000 miles. Until last fall, it would get 28-30 miles per gallon, mostly in city driving. Then the mileage dropped to the low 20s before the onset of cold weather, when it dropped even lower. I recently had a tuneup done by my trustworthy mechanic, but that has not improved the mileage. Could this engine simply be nearing the end of its useful life? -- Alan

RAY: I doubt it's nearing the end of its useful life, Alan. 69,000 miles is considered middle age for this particular Toyota engine. It's much more likely that all you need is a new thermostat.

TOM: After a while -- say, 69,000 miles or so -- it's not uncommon for the thermostat to break in the open position. That means the coolant is always circulating through the radiator and cooling the engine.

RAY: But the engine SHOULDN'T be cooled all the time. An engine that's always running too cool will run less efficiently, pollute more and -- get this, Alan -- get lousy gas mileage.

TOM: If the temperature inside the cylinders is too cold, two things happen. First, the computer thinks the engine is still warming up, so it keeps sending in more gas. Plus, some of the vaporized gasoline that the fuel injectors send in will condense on the cold cylinder walls. And that fuel doesn't get burned -- or at least doesn't get burned completely. That's why your mileage stinks.

RAY: So have your trustworthy mechanic slap a thermostat in this baby, and I bet it'll run like a frightened sardine. Whatever that is. Good luck, Alan.