Heater goes cold at idle

beezee

beezee

True Classic
The heater in my X works fine when I am cruising at speed, but when I come to a stop and the engine is at idle I get cold air. Is this normal?

Brian
 
Could be an air bubble in the system that collects at the heater circuit when the water pump pressure drops at idle. Have you tried bleeding the system with the heater circuit open?
 
How low is your idle speed? The water pump may be turning so slowly that you have limited flow.

You may have a out of tolerance water pump installation as was demonstrated by DR Jeff. This is where there is too great a gap between the pump housing and the impeller which causes a low flow issue, most notable at low rpm.
 
I’ve bled the system a few times so I don’t think that air in the system is the problem. I replaced the water pump when I rebuilt the engine several years ago and I suspect that impeller clearance could be the issue, but I wanted to ask the forum if the symptoms I am seeing are typical (I guess not). The car does not overheat in hot weather and while stationary the radiator fans will come on and drop the engine temperature when the temperature gauge goes over 190. This leads me to believe that the water pump is doing its job, but perhaps it’s not quite generating enough flow to push the water through the heater core at idle.
 
Actually, I'm going to take the position that this is normal for Xs and to a lesser degree 124s. At idle the water pump is just not pushing enough water too keep the heater core hot. Try an experiment. Next time you get in this mode, step on the gas an goose the idle to about 1500 rpm and see if that makes a difference.
 
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