Hey guys, ended up in some stop and go traffic on the highway last night, mostly standing still and first gear. I looked down and the temp gauge was pegged and the light was on. I got off on the next exit, and pulled into a neighborhood. As I did this, the temp went back to normal. I let it sit there idling while I popped the hood, and it started to creep back up. I got back in, jacked up the heat, and took off to go home in some direction where the car could keep moving fairly constantly, and the temp remained normal all the way home.

Today, I went through the normal battery of tests:

- Coolant is fine(changed a couple months ago) and half way up the reservoir.

- Fluid in the power steering pump reservoir

- Fan fuses are fine

- Power steering pump fan working fine

- Radiator fan spinning low and high when playing with heat and AC.

I took the car around the block to let it heat up and then let it idle in the driveway for 10 minutes, everything was fine. Any thoughts? Thanks as always.

Bad temperature sensor?
Stuck thermostat?

Any noise coming from the supercharger? The water pump attaches to the pto gears and might indicate the seal has gone and the gears are wearing due to lack of oil (google cooper pto supercharger)

How does the underneath of the oil cap look? Any foamy white goodness indicating a head gasket problem?

water should be flowing back to the reservoir if the water pump is working, im not sure 100% how to check this on mini coopers though

Sorry, I should have mentioned that the supercharger and water pump were changed a few months ago and the engine is purring like a kitten.

There's 2 fan temp sensors. Low speed and High speed. High speed kicks on when you run AC.

Low speed is definitely temp driven. I don't have my R50 any longer, but I recall that the same symptom. Car would peg the temp gauge. If I turned on the AC and kept moving, the car would cool down.

Look for the giant thread here about low speed fan sensor problems and there are some diag tests that can be done.

You probably have a busted low fan speed resistor. Was the AC on when you were stuck in traffic?

If you know how to access the ECU through the guage info display, you can get the actual coolant temp readout in celsius. Never rely on the coolant temp needle. More like a dummy light. It's really easy to access it.

Thanks guys, I'll check out the low-speed fan threads. Now that I think about it, I did smell kind of a burning electronics smell the other day while coming home from work. Maybe that was the resistor going poof.