Car Talk: Sneaky leak may be the cause of unwanted cold air

Dear Car Talk:

I live in Fairbanks, Alaska. I drive a 2011 Chevrolet Silverado, 5.3 liter. I noticed the other day that the inside of the cab was not heating up, despite driving for a while. I checked, and my antifreeze level was low, so I filled the reservoir to the “full” line.

I drove the truck this morning, and again, it was blowing cold air, even after 20 minutes of driving. I checked the reservoir, and it was down maybe an inch.

Then I noticed that while I was driving at speed, the heat would be sufficient, but when I stopped the truck to drop my daughter off at work, the air coming out of the vents got colder.

By the time I drove off again to go back home, the air was cold and wouldn’t warm up again until I was about a mile down the road.

Do you think I have a bad water pump? What could cause my truck to go from hot to warm to cold after a brief stop, and then warm up again down the road? — Joyce

Have you considered relocating to Honolulu, Joyce? I feel certain this problem will bother you a lot less there.

Here’s what’s going on, Joyce: When you’re low on coolant — or right on the borderline between OK and low — the speed of the engine can determine how much heat you get in the cabin.

How? The water pump (which circulates the hot coolant) is driven by a belt from the engine. So the faster the engine turns, the more pressure the water pump produces.

If you’re low on coolant, and the engine is idling, the pump may not produce enough pressure to push the coolant all the way to the heater core in the cabin. And if you don’t get coolant to the cabin, you get no heat.

When you’re driving along at a higher speed, the water pump is turning faster, and unless you’re really low on coolant, that masks the problem, and the heat comes back.

So your problem, Joyce, is that you have a coolant leak. Unless there’s a leak, you should never be low on coolant. What could be leaking? It certainly could be a water pump that’s going bad. Your truck is the right age for that. But it could be a lot of things, from a leaking hose to a bad head gasket.

Have your mechanic pressure test your cooling system and see if he can figure out where the leak is. If it’s a water pump, it’ll probably set you back about $400. If it’s a head gasket, look at the apartment rental listings in the Honolulu Star-Advertiser.

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