When sitting at a couple of lights the temp will quickly go from normal to pegged in seconds! Then drop almost as quickly once moving. High speed fan working. All fluids good.

Dunno. Wisdom accepted.

 

Have you heard the fan turn on?!
Pre 2005 mini's have the rad fan and the ps fan on the same fuse...the ps fan gets jammed...blows the fuse...then the radiator fan can no longer turn on...the wiring was changed/some cars retrofitted starting in 2005.

 

Just saw you said high speed fan is working...
Any odd noises? Rattling chains? In the near end case of the dreaded pto gear failure, rpms can help the pump pump enough coolant to be ok driving...but not enough idleing...just a guess...coolant full, no bubbles right?!

 

PTO gears, you should be able to hear the rattle from the cabin, even before you get to the point of overheating - a screwdriver or a stethoscope can help you figure that noise out.

If you're sitting at a light and then the temps drop once you're moving, you're probably getting enough airflow over the radiator to drop the temps to below what is needed to even kick on the low speed fan. If you can get your hands on a scanguage, that's a great way to tell since you'll see a voltage drop and the low speed fan kick on at around 221F.

Like Zippy said, coolant full, no bubbles. I'd lean towards a dead resistor on the rad fan. Especially if you can drive more than a mile or so without the car overheating. Remember that gauge on the dash is a dummy gauge, when it hangs out in the middle, it's a general operating temperature, anywhere from 170-230. (High speed fan kicks on at 231, normally)

 

Dead resistor was re-routed via a wirewound resistor, .33ohm 100watt. Low speed fan working.

 

If there's power to the new resistor and the low speed fan is coming on, you said your coolant levels were good, doubt there's air pockets in your coolant system. Have you noticed a dip in levels?

Try cranking up the heat at idle and see if the car is still overheating?
Very possibly could be a bad thermostat... I'd get a stethoscope and see if you're getting any odd noises, specifically from the water pump. How many miles on the Mini?

 

So it is overheating/nearly overheating with the fan running? Are you hearing it switch from low to high...or seeing it turn...or just know it works from testing it?
the low speed should be keeping the temp in check....then the hi kicks in in hot/prolonged low/no airflow situations....with a big cast iron block the mini can hold lots of heat...any track day person has seen this first hand...
I'm kinda wondering about the thermostat ..simple, cheap...
But also basics like has the rad ever had the fluid changed...is it crusty...or maybe failing water pump even...heck for it to need movement and not have the hi speed fan be able to keep the motor cool...that is some serious heat....

 

High speed works bc I can hear it. Low speed works just by turning on the AC to test.

Rad fluid has been changed a while ago, looks clean.

 

If everything else seems to be alright, next culprit is the thermostat.

 

Just started about a week ago... could hear the high speed radiator fan come on when idling... then just today... high speed fan kicked on while sitting in line at bank, and temp gauge started to spike. Repair site is 2 hrs away... am hoping that as long as I can keep moving.. I can make it there with no prob.

How hard is it to replace thermostat?

 

Has never had coolant flushed... resevoir shows between Max and Min on coolant bucket...

 

I am actually going to do it this weekend. It is definitely the thermostat, leaking and or sticking as it is 'loosing' coolant. Not leaving a puddle and not in the oil and I can smell it after it is running around the engine.

 

Ah Ha! Unfortunately, it is a bad radiator. I hit a piece of truck tire on the highway a while ago and it bent it and smashed all plastic housing around it. It is a seeming very small leak.

Unfortunately I also figured out the power steering fan is not working, also there is an oil leak coming from the head/block interface, also the bushing on the tensioner strut is bad, and I am sure I am forgetting something else. Sheesh.