Dear Car Talk:

I have a 1992 Ford Ranger with a four-cylinder, 2.3-liter engine. I bought it off my father as a spare vehicle. At some point, my father had a used motor installed. He thinks it came out of a Mercury, but isn't 100 percent sure. When I needed to replace the thermostat, the one that's listed for a '92 Ranger didn't match. After some searching with the auto-parts clerk, we found a 195-degree thermostat from a 1975 Pinto that matches. I installed it, and it fits.

However, the motor doesn't warm up properly: The needle on the temperature gauge climbs to the "C" mark, then goes back down. This continues no matter how long I warm it up or drive it. Also, the heater will not warm up as it did before. How can I find out the exact type of motor I have so I know I'm getting the right part? And do you have insight on why it's not warming up? -- Mike

Yes, that probably is the same engine Ford used in the '75 Pinto. Now you can see how much technological progress Ford made during that era.

I'm not sure how you can tell exactly what year it is, other than by having an experienced Ford mechanic take a look at it. Cars and trucks come with decals on the undersides of their hoods that identify the engine. But when you swap out the engine, the decal stays behind, so that's of no help to you.

But it shouldn't matter. A Ford 2.3-liter engine is the only engine that will fit in there without major modifications, so I'm sure that's what you have.

And for vehicles of this vintage, Mike, a thermostat is a thermostat. If it fits in there, and you didn't put it in backward, it should do the job.

So I'm guessing there's something else wrong. It could be that the thermostat you bought is faulty. Maybe it's been sitting in the box for 40 years. But even if it's been manufactured more recently, it could be stuck open and allowing the coolant to flow to the radiator when the engine is cold. That could explain both the needle in the gauge and the lack of heat in the car.

But you say that doesn't change no matter how long you run the car, which leads me to another suspicion: that your cooling system has air trapped in it. That provides a more likely explanation for why the temperature gauge doesn't work: The temperature sensor is supposed to be immersed in coolant; instead, yours is surrounded by a pocket of air. Air would explain the lack of heat, because air is blocking the coolant from getting to the heater core in the passenger compartment.

So start by trying another thermostat -- it'll cost you 20 bucks. And when that doesn't fix it, you have to bleed the air out of the system. Or, if that's not a skill you possess, take it to a shop and have them do it for you.