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96 Subaru Legacy Overheating When Heat On

Basically this car runs fine for 20 miles or 20 minutes (depending on number of red lights) with the heat on. Runs fine when it is sitting idle. Runs fine when the heat is off, as long as I'm not forced to stop at every red light half a block apart for 10 miles. When it overheats, however, it pushes the coolant into the reservoir. I've read that this can be a head gasket, but that doesn't make sense to me because I can drive for hours without a problem as long as the heat is off (which sucks to do in the winter). It would seem that a head gasket would be a problem regardless of the heat. Furthermore, as I understand blown head gaskets, it wouldn't even start in the winter time...

Here are the repairs so far:
Visit 1: Flush radiator. Mechanic couldn't get it to overheat, said it was probably an air pocket. Overheat with overflow into reservoir 10 minutes later.
Visit 2: Replace radiator cap. Overheated with overflow 5 minutes later.
Visit 3: Replace thermostat. Mechanic said that part of the radiator and the lower hose was cool, so it had to be the thermostat. 20 minutes later, overheated-overflowing reservoir.
Visit 4: Replace radiator. New mechanic said the radiator was cool, sure sign that it was clogged. 30 minutes later... well... you know the story.

I'm not really in a position to dump a bunch of money into narrowing down the problem by fixing everything but the problem itself until someone finally stumbles onto it. And it seems like everyone is ignoring the fact that it runs fine when the heat is off. Anyone have this problem before?

Comments

  • ateixeiraateixeira Posts: 72,587
    Before I read your repairs I though to myself - T-stat, then water pump.

    You did the T-stat already, so it's probably the water pump.
  • xwesxxwesx Fairbanks, AlaskaPosts: 8,679
    Yeah, I'd look to the water pump as well. I really don't think the use of the heater has anything at all to do with the problem. The only thing that happens differently when you use the heat is air being blown over the heater core; it doesn't change the flow of the coolant through the system.

    The water pump went out on my '96 Outback at -25F, 0200 on a Sunday morning. That wasn't so fun. It started overheating ~20 minutes after startup, radiator was cold. I replaced the water pump the next morning, and it was good to go.

    If it is an air pocket due to combustion gases being forced into the coolant, you should be able to see signs of it in the overflow bottle in the form of black crud sticking to the sides of the bottle.
    2010 Subaru Forester, 2011 Ford Fiesta, 1969 Chevrolet C20 Pickup, 1969 Ford Econoline 100, 1976 Ford F250 Pickup, 1974 Ford Pinto Wagon
  • ateixeiraateixeira Posts: 72,587
    My '93 Miata had the same problem in 2007 or so.

    The heater helps a bit merely by getting rid of some of the excess heat, but that's marginal and risky. I'd fix it ASAP.
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