I have a 91 Formula running a new L31 (with intake adapter base plate for the vortec heads) as a replacement for the original L98. I originally had a lot of idle issues with the car that seem to have been helped by a new throttle body.

Recently the car started running consistently cold at highway speed or really at any constant speed (below 140 according to my dash). The car would idle fast, seemingly because of the temperature being low. If allowed to sit at idle and warm up the idle would eventually settle into a nice steady range at about 800 rpm.

Assuming this was a thermostat issue I just had a new 180 degree thermostat installed. Now the car seems to run too hot suddenly (above 240 consistently) and started running too lean as a result, dying at a stop with rpm too low.

Does this sound like it could be a coolant temperature sensor issue or a problem with another sensor? Thank you for your help, this new engine has caused a ton of problems.

 

It sounds like you swapped a bad thermostat for another bad thermostat. Can double check things by using a scan tool. The ECM uses a different coolant temp sensor then the dash gauge. Can also use an infrared heat gun to check engine temperature. Even a hand across various points on the radiator can be helpful.

RBob.

 

This is pretty much it.

Based on what you describe, it sounds like you removed a thermostat that had failed stuck open and now have one that won't open. Even before you just toss another thermostat in, if you feel it is not opening at the right temperature range, you can verify this by putting it in hot water on the stove and get an approximate temperature of when it actually opens. If the thermostat is behaving normally, then you know you have another issue.