1989 suburban thermostat issue
 

I have been through three thermostats. Put new thermostat in, burb heats up, gets up to red line, thermostat finally opens, engine cools to normal. Finally after about three weeks of doing this the thermostat fails open and needs to be changed. Why is this happening?

Air in cooling system...
TStat butterfly valve hanging up on gasket or housing...

Before I'd buy another Stat, I'd get a pot of water, a A/C thermometer, and check the temps it opened when you heated the water.

Operating temp should be 185 deg, is that what it's at when it does open ?
Are you using a genuine GM Stat ?

frontwards backwards upside down??

Thermostat issue
 

Thanks guys! I think it was an air issue. Refilled and let run awhile with radiator cap off. Seemsvto be doing ok now.

Thanks again.