ok, well, i have NOT replaced my freeze plugs yet - timing is not good right now, so we DID have to try a quick fix - which DID work, and worked weel, actually - not a typical stop leak - but the coolant seal tabs from GM. These are so good, that everyone around here recommends them 0- including ford and dodge dealers, and foreign shops, as well.

Well, on to the issue - I have the correct 190 thermo installed now, and with the leak 'stopped' for the time being. However, the engine will only hold the correct temp on the gauge when the engine is running about 2500 rpm or higher - and lower, and the temp drops - at idle, it JUST above the cold line.

I know that I have to clean out the heater core, but this is WITHOUT heat applied.

What could cause this? I'd consider a pressure test, but afraid it would cause more trouble with my freeze plugs right now. The rad cap LOOKS ok - but could a poorly sealing cap cause this issue?

 

Sounds like your temp sensor may be bad. Are the upper and lower radiator hoses hot when its at temp?