528e Heater Help

[sailor1313]

My 1985 528e has a strange heat issue. The heat works fine below about 1700 rpms, above 2,000 the heat completely goes away. It is obvious that no coolant enters the heater core at the higher rpms. Slow down for an exit ramp or just run about 55 on the highway and there's plenty of warm air. But who drives 55?
Any ideas? It's getting cold here in New England

['89 635 csi]

Welcome! Let me ponder your question for a while.

['89 635 csi]

The only thing I can come up with is that maybe you have a very low temp thermostat in it and the faster you go the faster the water (i.e. water pump is moving faster) is circulating and it's not getting hot enough to warm the water efficiently. Or your heater core is on it's way out. These are the only things I can come up with.

[sailor1313]

Originally posted by '89 635 csi@Nov 16 2004, 11:47 PM
The only thing I can come up with is that maybe you have a very low temp thermostat in it and the faster you go the faster the water (i.e. water pump is moving faster) is circulating and it's not getting hot enough to warm the water efficiently. Or your heater core is on it's way out. These are the only things I can come up with.

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[sailor1313]

Thanks. The temp change is alomst instantaneous. The thermostat is new and the correct temp. I ruled out the heater core because it does work fine as long as there's hot water going into it. Is there a heat control valve that is vacuum operated?

['89 635 csi]

Yes. However I don"t think that the temp would change that quickly if it was the HCV. But I would check it out anyway.
Please keep me updated.