Chevy & GMC Vans

Hey guys i just wanted to know something. Since the end of last winter i wasnt getting much heat. But winter was almost over and i just assumed it was the thermostat that was stuck open, so i was planning of just flushing the coolant and replacing the thermostat. Well i havent had a chance to do that this summer because i was busy working on other issues on my van. Now its starting to get cold so i need to do it. But i have a feeling that its not just the thermostat now. I think it might be the heater core as well. I reason why i dont think its the thermostat is because i just noticed today the the temperature gauge was climbing up to just under the yellow, and i still had absolutely no heat at all.(before the temperature gauge was low) Im a little confused at this point and wanted to know what you guys think. Is it the heater core as well or even something else?

Thanks!

the heater core could be clogged or you could have blocked the airflow inside the airduct box. My 1990 sucked up a napkin from the passenger side floor when I was using the Max AC setting. It caused a bit of a blockage in the heater.

When the van is warmed up, go under the hood and feel the coolant lines going to the heater core. Make sure you have the temp setting turned all the way up as well as have the selector on defrost. If the lines are hot, coolant is flowing. If the lines are cold, there is a diverter valve that may be malfunctioning. It is run by vacuum I think. Not sure where it is on a 91 but the heater core coolant line(s) will connect to it.

You can always pull the heater core coolant lines off and see if you can flush garden hose water through it.

what temp does the engine run at? If it is below 180* then you should replace the Tstate with a 195*.