How do I know that my thermostats is not working in a 91 chevy lumina with a 3.1 liter engine?

 

Lack of heat & overheating are two classic symptoms of a bad stat.

Should be changed every time you dump your coolant for service. It's cheap enough.

 

The best way I've found to test a thermostat is to put it in a pot of water and turn the stove on.
Using a temp gauge(the kind you would use for a turkey) check the temp when the stat opens.
If it's a 195 then it should open arond that temp and so on.
If it opens too early or too late (or not at all) it's definitely due for replacing.I use this method even on new ones because you never know.

 

Just in the last year, I have come into 5 bad thermostats. From now on, I will do a water bath test, like I did on suspect ones. There are some really bad auto parts, floating around under names we used to trust. Testing takes only a few minutes, compared to R & R after you already put it in.

Marturo