The common t-stat has a rubber gasket on the valve, the GM 131-158 has a metal to metal valve.

If it is not stuck open the temp sensor is suspect.

Folks. You might want to read the OP's other 2 threads they started and didn't come back to. Has a little more info like a recent engine change and electrical issues.
Me. I'll stay on the sidelines for this one.

Thanks for the heads up George.

Sassy, I've merged your 3 threads, for continuity. We still need a clear, concise history of the vehicle. You probably should just type it out, your voice-to-text is not serving you very well.

At this point, it's rather difficult to figure out what the problem is. The OP might want to clarify if the past problems are fixed or not, and what the current issue actually is.