Serious issue with e46 m3 coolant temp gauge?

[jacobm3]

So I was driving my 2002 e46 m3 home tonight and something strange happened. When I got home, I noticed the coolant gauge was in the red zone. I immediately shut the car off and checked both fans and they were intact and spinning. I also checked the bottom of the car and the reservoir and I hadn't lost any coolant.

I got in the car and restarted it and turned on the heater all the way. The coolant temp rapidly started dropping, the gauge dropped way past operating temperature (to the tip of the blue) and the auxiliary fan started spinning extremely fast. (Doesn't make sense, since the engine (according to the coolant temp gauge) was running cold so I can't understand why the fan would start spinning faster when I turned the heater on.

When I turned the fan off the coolant temp rapidly started rising again.

The second I turn the heater back on the temp drops past operating temperature until it's cold but the second I turn the heater off it off it climbs to the red.

If the thermostat was stuck closed and not allowing water in, turning on the heater wouldn't make the needle drop below operating temp. The car would still overheat regardless if the heater was running. Correct?

I scanned the car with my peak tool and there was no new codes.

What could this be? Did my car actually over heat or is the gauge malfunctioning? I checked my oil and there was no coolant in it. I will do a compression test ASAP. I'm really worried I damaged the head gasket but I don't know if the car actually over heated. Please help me guys.

[MauiM3Mania]

Did you discover the issue?

[bmwdirtracer]

My hunch is that there is air in the system. This would explain the dramatic temperature swings, since the sensor cannot read the temperature of air/steam very well. Switching on the heater / auxiliary coolant pump might cause the air bubble to move into direct contact with the sensor, whereas when this electric pump is off, the air bubble might just be at the (mechanical) water pump impeller. Or, perhaps, the mechanical water pump is bad, as stout as the S54's unit is.

When the engine is completely cooled, how low is the coolant level?

[WadiM3]

+1 bleed the system!

[N0Roof330Cic]

Jacobm3, what was the culprit, please? Did you get this resolved? I am having a similar issue.

[Shift4]

I had the exact same issue with my 2004 M3. The plastic water pump impeller had separated in two pieces. The part with the blades was no longer spinning and not circulating coolant. I believe these cars have an auxiliary pump for the heater system. With the engine off the aux pump can circulate coolant to keep hot coolant in the heater core and keep you warm longer. In this case the coolant in the radiator cooled down significantly and only circulated with the heater on. So the culprit is the water pump with these symptoms. At least in my case it was.