Up until recently the engine was running too hot and overheating at idle, so I replaced the radiator with a Cold Case 2 row and added dual 11” Spal fans. Now the engine temperature will barely reach 160 after driving for 45 minutes in 70 degree outdoor temperature. I replaced the thermostat about a year ago with a duralast gold 180 degree. I haven’t tested it yet but I doubt it would be stuck in the open position since the car was overheating right before I changed out the radiator and fans. Any thoughts on what could be causing this?

It's a 1990 IROC with a 5.7, no modifications.

 

Thermostat stuck open

See my signature

 

New thermostats stuck open…..

ABSOLUTELY possible!!

 

I figure during my life time it has been a toss up on the way they have failed. Some stuck open and stuck closed.

 

Clear as day that is one of those garbage fail safe designs. It is a dumb design that causes a problem with something that was never an issue in the standard configuration. That thermostat was literally designed to catch in the open position even with a mild overtemp running condition. The tabs designed to latch on to the thermostat disc are readily visible in the above picture. You can remove those tabs and that thermostat will likely be problem free, I have done it several times to those POS's.

 

I pulled the thermostat out and its working properly. I put it back in and let it run for a while. The hose from the thermostat housing got pretty hot after about 12 minutes. I'm guessing that the temperature gauge is not showing the correct temp. What's a quick and easy way to check the engine temperature and get a semi accurate reading?

 

for $8 on amazon you can get a infrared temp gun to put on the thermostat housing.

 

did you put it in a pan of boiling water and watch it open at the temp and close when you removed it from the hot water?

--point being, it probably isnt working correclty, id put a new one in

 

Yes, I tested it in boiling water and its opening an closing properly.