Surprise! AC Problems are More Prevalent in Hot Weather

Dear Car Talk:

We live in New Mexico and have a 1995 Jeep Grand Cherokee Orvis Edition. The Jeep has 135,000 miles and is in very good condition.

Last summer, my wife and I went to run errands around town. I went inside a store, and my wife stayed in the Jeep with the air conditioner running. It took me about 45 minutes to come back.

When I walked outside, I found my wife standing outside of the Jeep waiting for me. There was steam coming from the engine when I opened the hood, the water in the coolant reservoir was boiling, and it took about three hours to cool off.

It got so hot that the coolant cap of the reservoir came off. I took the Jeep to a mechanic and he installed a new thermostat, but it overheated again. Any idea what is causing this? -- Ernest

Probably summer in New Mexico, Ernest.

Actually, since you've already replaced the thermostat, the next thing I would check would be your cooling fans. You have two fans in this Jeep. Which is more fans than I have for this newspaper column, Ernest.

One of your fans is an electric fan that comes on whenever you use the AC. Running the AC makes the engine work harder and causes it to heat up more. That's compounded by hot weather, which, I've found, is when most people use their air conditioners. Pretty observant, huh?

Anyway, whenever the AC is on, this electric fan is supposed to go on with it, to provide extra cooling and help keep the engine from overheating.

You can check that yourself, Ernest. Have your wife sit in the Jeep while it's running, and with the hood up, have her turn on the AC. You should see -- and hear -- one of the cooling fans in front of the radiator start up. If not, that could be the problem.

The other fan also could be faulty. That's your regular cooling fan, and it's controlled by a thermostatic clutch. That's a fancy way of saying that it goes faster as the engine gets hotter.

It's possible that one, or both, of your fans is no longer working. Or working well enough. That won't be a problem when you're moving fast and blowing a lot of air through the grill. Or in the winter, when the ambient air is cool and you're not using the AC. But it'll be a big problem in hot weather when you're using the AC and sitting still in a parking space.

Start there, Ernest. And if it's not a fan problem, write back, and we'll run up your bill with some more expensive suggestions. Good luck.

Todays Car-o-Scope

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