Dear Tom and Ray:
Our mechanic states that the heater in our 2000 Ford Focus sedan is clogged. He says there is nothing he can do, since attempts to unclog it by shooting air through it would cause damage to the whole system if the inside is corroded. Is there a way to put an auxiliary heater vent in the inside of the car? Do you have any suggestions? — Phil
Tom: How 'bout a hibachi, Phil?
Ray: No, don't do that. I'd have to disagree with your mechanic. I think flushing it out is worth trying.
Tom: The easiest thing to do is simply reverse-flush it by hooking a hose up to the heater core and pushing water through in the opposite direction from how it usually runs.
Ray: You also can start by adding a chemical flush to the entire cooling system and, after letting the engine run and the coolant circulate for a while, you can drain that out and then reverse-flush the heater core with clear water.
Tom: Finally, you can blow compressed air through the heater core. You want to follow that up with a flush in case you loosen any crud that could flow into the rest of the cooling system (that's probably what your mechanic is worried about). But the truth is, as long as you flush the heater core well, you have very little to lose by trying.
Ray: In the worst-case scenario, you damage the heater core and cause it to leak. If that happens, you'll have to replace it. That's an expensive pain in the neck, because it's up under the dashboard. But that's probably what your mechanic is suggesting that you do now, anyway.
Tom: So why not try the easier, cheaper stuff first? To be honest, we find that flushing a plugged heater core works in only about 40 percent of cases. But that 40 percent of car owners leave the shop feeling happy and "flush" themselves, so we think it's worth a try. Good luck, Phil.
(Car Talk is a nationally syndicated column by automotive experts (and brothers) Tom and Ray Magliozzi. Write to them at the Car Talk website.)