trying to digest that all... I struggle being able to accept this was just some random "big" that just appeared out of nowhere and then simply went away. I do agree it was odd that the coolant temp cooled down right away once I restarted the car, yes, in like less than 15 seconds, it went back to the noon position.

Why would it matter if the AC was on? I am curious about that question.

If the waterpump goes, does it get intermittent? If that stopped working and then restarted, with the fan blower on, would that not cool down the coolant temp right away. There is no engine/oil temp gauge. Same goes for the fan, if the fan stopped working, wouldn't the coolant temp go right up and then come down if the fan started blowing again?

thanks.

Engines hold a lot of heat. It would take 30 minutes for a stopped overheated engine to cool to normal running temperature.

AC can cause an engine to overheat if the condenser (like a radiator) is in front of the engine radiator. If it’s side mounted, then it won’t.

Water pumps fail at the bearing, causing leaks, or the plastic impeller can disintegrate causing loss of water flow and over heating. But it’s not intermittent. If you remove the overflow tank cap and water is flowing, then the pump is working.

Fans help move air at low speed driving, but don’t contribute much at freeway speeds.

The AC question was to eliminate the Aux fan that comes on with the AC as a potential failure point. You have two fans, one is mechanical, spins with the engine rotation. The aux fan (electric fan) comes on with the AC compressor,also comes on if the coolant temp gets too high. Not sure but likely your AC won’t come on if the electric fan is not functional.


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thanks. I checked the coolant level tonight (since last DIY I did on the car was replacing the coolant expansion tank). coolant level was perfect, not low and not leaking. hmm. well heading to the beach this weekend, 100 mile drive and back home. lets see how it goes.

Seems to me you got the temp sensor glitch. But the conversation will get you ready for the next time.
Happy motoring.


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Since it is intermittent at best at this point and there is no coolant leak, I would not expect it to be a water pump issue, or low coolant. Suggest a check if belt is in good shape. A bad thermostat usually sticks open which would not cause overheating. I test a mechanical fan after the engine has been at operating temp to see how easily it turns by hand. Should be some resistance, won't spin even a full turn and feels smooth. First let the engine run at operating temp for 10 minutes or so without the air on and again with it on. The higher the outside temp the better. If it overheats when not running carefully remove the cap and confirm temp. I think more troubleshooting is in order before buying parts and easy to this before your trip.

It's "laws of physics" impossible for the car to do what the instruments displayed that can. ONLY mean the instruments were wrong. I'll see if I can find the tsb from bmw


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https://www.m5board.com/vbulletin/e3...ture-gage.html

Here are the details about the problem. The M5 and X5 have very similar brains and the X5 has a nearly identical tech service bulletin.

FYI, I wasn't suggesting the cause, just simple troubleshooting to confirm or eliminate previous suggestions.

Head gasket/cracked head also possible.