What could be causing this Mercury Sable's heater to fail?

Dear Car Talk

Dear Car Talk | Feb 01, 2005

Dear Tom and Ray:

I've got a peculiar problem. My 1999 Mercury Sable's heater has been putting out less and less hot air for the past few years. This year, there was no heat. I flushed the cooling system; still nothing. Then I flushed the heater core, and now I have heat, but only at highway speeds. At idle, you can feel it cool right back down to nothing. Any thoughts? I'm thinking it's the water pump. Can you help out? My wife and kids refuse to ride with me anymore ... on second thought, maybe that's OK! -- Mike

RAY: My brother got his wife and kids to stop riding with him, too. But he didn't have to give up his heat to do it. Giving up his reflexes and judgment seemed to do the trick.

TOM: I think it's your heater core, Mike. Flushing won't help once it's really plugged up. I'm going to guess that the water pump is OK. It's turned by a belt that runs off the engine, so, at idle speed, it's supposed to pump more slowly than at highway speed. At normal idle speed, it just can't overcome the gunk that's clogging up your heater core.

RAY: The good news is that the heater core is easy to get to on this car. The bad news is that it's only easy to get to once you remove the entire dashboard.

TOM: So, you're looking at $600, Mike. A big chunk of that is labor. You have to make a decision here: Do you want to be frozen out by your car, or frozen out by your family? Good luck deciding, brother.

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