Off topic water cooling question ???

Hello, my other car is a 1994 BMW 320 i and I have noticed, in this really cold weather, that the heater blows hot when the revs are up such as when the vehicle is driving. As soon as the car slows and idles at a set of lights for example, the heater starts to blow cool air. Once revving again, the heater goes full heat again. Is this a low coolant problem, water pump, or leak in the system ?? Thanks for any advice.

Jerry S

1981 930
1994 BMW 320 I

My (former) 93 325is did the same thing. Also, in the summer in stop and go traffic, the engine would start running excessively hot until you pushed the rpms up to around 2-3k, then the temp would start coming back down. I think part of it is the nature of the water cooling system in the 325, but mine got better (a little but not perfect) when my dealer/mechanic put a new water pump in under warranty. I've also heard that the overflow cap could be part of the problem on those cars as it also serves as another valve. I wonder if that would have completely fixed mine. Also, low coolant is a complete possibility in those cars as flushing the entire system is a real bear. If you've recently flushed/refilled, it could be air pockets.

As much as I loved that Bimmer, I don't miss water cooling.