'92 M50 with ~133k on it. While cruising, my temp gauge shows right around the 12 0'clock position. If cool out, slightly under, if hot out, slightly above. Pretty stable.

However, occasionally when I come to a stop after driving a bit, the temp gauge starts moving up QUICKLY. Very scary stuff. The strange thing is, if I put it in neutal and rev it a little, the temp goes back to normal, again quickly. This is a matter of maybe 5-10 seconds. Or, once I am in motion any my revs exceed 2k or so, everything returns to normal.

It has happened before, which scared the hell out of me. I checked the coolant, and its completely full. No signs of leaks anywhere, and frankly the engine is behaving normally. Tonight, I pulled over when it happened, and popped the hood to feel the radiator hose. It was not anywhere close to overheating, simple normal warmth. Again, no coolant, no smells. All seems normal.

This leads me to believe that I have a faulty sensor, so I'll probably wind up replacing the coolant temp sensor for the guage. If I'm reading realoem correctly, there's also one for the ecu right next to it? If so, I'll replace both, since they're fairly inexpensive.

I'm just concerned about other aspects of the cooling system. The only other thing that I can think of is the water pump not delivering enough flow at idle, but I would think the upper rad hose would feel very hot to the touch (the gauge is reaching the red zone) if the gauge was accurate.

Anyone have any insight on this? I may very well just change the thermostat and water pump for peace of mind, but it'd be nice to know I'm not totally here.