Coolant Temp Warning - pegged to red. Holy crap.

[Meestahbig]

it was a long ride home in traffic for like 2 hours... in the tunnel it was slow, crawling, like 5 MPH. car has 145k on it, I replace the tstat two years ago b/c it was stuck open. when i pulled out of the tunnel, I parked it and checked to see if the main fan directly in front of the radiator was blowing - and it was. hmmm.

[Overboost]

Originally Posted by Meestahbig

it was a long ride home in traffic for like 2 hours... in the tunnel it was slow, crawling, like 5 MPH. car has 145k on it, I replace the tstat two years ago b/c it was stuck open. when i pulled out of the tunnel, I parked it and checked to see if the main fan directly in front of the radiator was blowing - and it was. hmmm.

On a really hot day here in the south I can hear my viscous fan clutch locked up and turning the engine fan but only for a minute or two. That fan moves a lot of air and given the temps you saw, your engine fan should have been noticeable by ear. If you don't remember hearing the "roar" from the mechanical fan when she was hot I think a simple test would be to roll up a newspaper and try to stop the mechanical fan. It should be locked in at temps above your center point on your gauge (115C) and you should not be able to stop the fan.

From the Bimmerforums:

1. Start the car (cold) with the hood open and note if the fan is turning, increase the engine RPM and note if the fan turns faster and the noise increases, if it does, first good indication, if it does not increase speed/noise, clutch is bad and needs to be replaced. (Remember, this must be tested after the car has been off for and extended period, over night etc.)
2. Leave engine running and note if the fan starts to slow down after 2-5 minutes, speed/noise should diminish and even raising the RPM, the fan should not make as much noise as when first starting, if it does slow, this is the second good indication. If speed/noise does not decrease, clutch may be “frozen” and should be replaced.
3. Leave the engine idle and watch the temperature indicator. When normal operating temperature has been reached, some increase in fan speed/noise should be noted, in particular when the RPM is increased. If temperature is fairly stable and the fan noise/speed increases or cycles, third good indication. If temperature indication continues to increase, with no increase in fan noise/speed, clutch is defective and should be replaced.
4. After the engine is at normal operating temperature or above, is the only time that the “rolled up newspaper” test that many people talk about should be performed! Take some newspaper and roll it up into a long narrow tube. Be carefull, keep hands and fingers away from the fan while performing this test! With the engine at full operating temperature and idling, take the rolled up paper and insert it on the back side of the fan and try to reach the hub of the fan avoiding the blades until close to the hub. Push the rolled paper at the fan increasing the friction to the hub area of the fan. If the fan can not be stopped easily this is the fourth good indication, if it can be stopped the clutch is defective and should be replaced. Again, this test can only be performed when the engine is at or above full operating temperature.
Testing can be performed in any order but just make sure the conditions during testing are those that are specified for that specific test.
Do not continue to operate the engine if the temperature continues to rise and certainly stop if the temperature approaches “redline”.