Heater issues

2000 GMC pickup with 160 k. Didn’t seem to get hot enough. Changed the thermostat and fixed that problem. Temperature is just below 210 degrees. Heater never did work well ( had it bout 6 months). After I changed the thermostat heater was cold. Ran it for a while and heater eventually warmed a bit but it doesn’t get hot. And sometimes it goes cold. It goes back and forth…cold to warm…never hot. Is my heater core stopped up or do I have a control problem?

That’s an indication that there’s air trapped in the cooloing system.

Try this. With the engine up to operating temperature, and with the engine idling, loosen the upper radiator hose clamp. Take a small flat bladed screwdriver and slip it in between the upper radiator hose and the radiator hose neck. Allow the engine to idle until all the air is purged out of the cooling system or until there’s constant heat coming out the vent system. Remove the screwdriver and retighten the hose clamp.


Thank you Tester, I was wondering if air might be an issue but never experienced it. I will certainly try it!!