I have a 1984 318i and I was stuck in traffic recently and noticed that the temperature was going up. I figured that if I ran the heater it might help cool down the engine. But when I turned it on, it blew cold air.
Once out of traffic and back up to speed, and the car cooled down almost immediately. And then later I tried the heater again and it worked.
A mechanic said that when the water pump starts to go, it will still work at speed, but won't pump at idle. I would think that a water pump would just go, all or nothing.
Also, I did notice that the coolant was down, and have added half a jug. I haven't tried to test this in anyway since then.
Is there some test I should do?