Barnstormer

I replaced my thermostat (and did a flush at the same time). I think part of the problem was the o-ring in the housing with the thermostat was not installed correctly when I took it apart. I think although it was keeping the coolant in the engine, it was allowing coolant past the thermostat at all temps, so when highway driving (high engine speed, high airflow over the radiator, not much load), it was allowing enough coolant past the thermostat to let the engine run cool.
The thermostat should be against the block, the o-ring on top of it, then the coolant neck. In mine, the o-ring was below the t-stat in the housing. Did'nt leak out of the engine, but I think allowed coolant past the t-stat.
My car now warms up to 12 o'clock quickly and never varies from that temp, no matter the driving.
Brian

Allroady DungBeetle

That makes complete sense and it wouldn't surprise me if a mechanic put it in that way.

heretic

I reversed it Wpump>Tstat>O-ring>Flange & That worked. BUT my Haynes manual has O-ring reversed so it went Wpump>O-ring>Tstat>Flange and it leaked like crazy.

SilverA4

I have the same issue - if I let my car idle, it will warm up fine over time. If I start it and drive, it will never warm up.

Thanks!