Overheatin' Mini Cooper

My 2002 Mini Cooper has been a faithful trooper since we bought it in 2002. It has carried me around at track days embarrassing faster cars at the track (pretty much anything else that shows up at a track day) thanks to its handling, picked up the groceries, and ferried the kids to countless activities in the last 12 years. It has just about 90K miles. Since this spring it has been giving the impression that is overheating after running for as little as 15 minutes. We’ve changed the thermostat, radiator and had the head off to replace the gasket and check it for warping. We’ve also bled the system thinking it could be air trapped somewhere. My mechanic says that it isn’t actually overheating - there is no fluid boil and it isn’t using any coolant up. There are no leaks. He thinks it could be a bad gauge. I called MiniMania and they agreed that it is not overheating really and suggested that it is an electrical issue. They doubted it is the gauge and suggested running the engine at idle with the radiator cap off so I could check the temperature of the coolant with a thermometer as the temperature gauge did its thing moving into the red. Any ideas for how to determine what is going on and fixes would be most appreciated!

Have someone take an infra-red thermal gun and point at the upper radiator hose with the engine running when it’s indicating an overheating condition. If temperature reading is 210-220 degrees the engine isn’t overheating.


hmm … besides the excellent idea above, if that didn’t identify the problem, I’d install a new coolant temp sensor sender. Those gadgets are usually just thermistors, where the resistance correlates to the actual temperature. I’d calibrate it first w/a pot of hot water on the stove and a mercury thermometer, see what the resistance vs temperature curve is. Then I’d measure the resistance with it in the car, and see what the coolant temp is actually getting to.

One more idea. Besides what you’ve already done, did you also replace the radiator cap? A failing radiator cap can cause this symptom.