Car sleuths: Can you solve this mystery?

On Monday I let my car ('02 BMW) run for 10 minutes to heat and defrost, it was 11 degrees outside. I drove for 15 mins car then noticed the gauge needle was at the hottest possible position.

I got home and hooked up the code reader. Nothing came up, despite the car throwing an oxy sensor malfunction a few months ago.

The next day I repeated the idle and drive, nothing happened. The gauge needle was where it always is (12 o’clock).

Possibly important details:

  • Temp went down to -5 night before, day temps around 10.

-02 sensor code went on a few months ago, but no code coming up now.

-Car runs amazingly well, gauge is working normally.

-Oil and antifreeze are topped up.

The coolant thermostat was stuck closed, then it unstuck and worked fine?

I’d say stuck thermostat as well. Frankly, I’m surprised if it is the original as they only last 10 years or so in my experience.

Yep, I think the thermostat, too. And, weighing the risks/rewards, I wouldn’t drive that car until I replaced it. The only car I ever overheated turned into an awfully expensive repair.

Another vote for thermostat. Replace it immediately.

Excellent, thanks guys!

And at least some BMW’s have complex thermostats. One of mine had a double valve unit, it controlled both the hot water leaving the engine and the cool water returning, so as not to stress the block casting from too cool water when driving at 100+ mph in freezing weather. Because it’s a BMW so why not drive like a bat out of hell in the winter.

Anyway, if air got trapped between the two halves of the thermostat bad things happened. My car had a rather complex bleeding procedure a full page long which included bleeding the heater pumps. Yes, it had variable speed pumps feeding water to the heating coils (plural). Because it’s a BMW so why not have 2 heat exchangers with water pumps.