1992 chevy G20 van

I bought a used van to run around town with. It sat for a year, I change the thermostat, hoses, and flushed the radiator. When i drive the temp gage spikes until I put the heat on. Then it returns to around 120. I need your help because my wife is standing on the porch with her hands folded looking at me

Well, the first thing I would do is give the wife a boyish smile and say “what’s life without a little adventure?”

Then I would check the following: a) make sure that all of the air has been bled out of the cooling system; b) check that the cooling fan is actually running when it is supposed to; c) check the fins of the radiator for crud and blockage d) make double sure that the thermostat went in the right way (backwards can sometimes be easy and is bad). Its probably also worth getting an actual read on the coolant temp to check up on the sender.

Is it keeping its coolant? Or losing it?

Is 120 Celsius or Fahrenheit? There is definitely something fishy going on if it’s dropping over 100 degrees (F) by just turning the heater on.

It is keeping coolant when before I installed the new thermostat I backflushed the radiator because the van had been sitting for a year. I will check to see if it was installed backwards