89 Truck Heater Question

Is the heater valve opening (easy for the control cable to slip off the valve lever). Is the heater valve clogged? Is the heater core plugged? If so, try backflushing it with a garden hose. Is there air trapped in the cooling system? If so, try and burp the air out of the system, it'll rise to the highest point in the system, which is often the heater circuit.

The valve is opening. I will try backflushing it.

Thanks.

I had this problem, couldn't even get the windshield to defrost... changed the thermostat and it works now.

is the temp gauge in the truck going up at all?

Brush wrote:

I changed the thermostat and flushed the system and it didn't help. I then reached up under the dash and played with the cable attached to the hot/cold lever and was able to open the flap to let the hot air in. Finally it works again.