No heat when stopped???

[sydwys]

Hello everyone,

I have a 1992 SC400. Towards the end of last winter, I have noticed "no heat" standing still with the heat dial cranked all the way to "hot". Heat would return as soon as I got rolling, only to go away and blow cold air when I stopped again. Since I have not used the heater again during the summer, It completely escaped my mind. Well! The winter is upon us here in the West Coast of the US, and the other day, when I turned on the heater for the first time, I noticed my problem is still there. I wonder if any one of you know if this is related to the same heater control unit being mentioned on other posts. I would very much appreciate your thoughts or experiences on this nagging problem of mine. Thanks.

~Noah~

noahi@tutsys.com

[AWJ]

So there are guys talking about a vsv (vacuum switch valve) in the heater system. Try topping off the engine coolant reservoir first. If you know something about heating systems there is a tweak for the vsv you might be able to try. Or possibly, a new vsv is needed.

[DJF]

Here's a few things to try.

1. Does your temperature gauge reading change when you are stopped? One possibility is that the belt that turns the water pump is slipping and you have very little coolant circulation at low RPM.

2. You may have a vacuum leak. Vacuum at low RPM is much less than at high RPM. There's a vacuum controlled baffle in most climate control systems that regulates the whether the air blowing into the car passes thru the heat exchanger. With a vacuum leak, that damper may close at low RPM so that the air bypasses the heat exchanger. The valve itself may be defective.

3. There's a valve that stops coolant flow to the heat exchanger when the A/C requires maximum cooling. Again, a vacuum leak may be causing that valve to close at low RPM.

[sydwys]

Thought I would update this post. The problem turned out to be a low coolant level in the radiator. Because of its location in relation to the radiator, the heater core would get no warm coolant thereby blowing in cold air. At higher RPMs, coolant would flow in providing hot air. Topping off the coolant fixed my problem.