Stuck thermostat possibly. 2006 325xi.

[sshivaram]

These are the symptoms (on my 2006 325xi).
1. I opened the expansion tank cap and ran the bleeding procedure (Key in, start button pressed to ignition position(don't start car), set heat to MAX, fan speed low(to save battery), and press gas pedal to floor, in 10 seconds the coolant flow start, shows the thermostat opens up allowing coolant to circulate back into the expansion tank from the top of the radiator.
2. Start car, heat turned off(to close thermostat), and when coolant temp hits 90-93, no coolant is flowing thru the radiator, showing thermostat is stuck closed. Coolant temp reaches around 107Centigrade and still doesn't open thermostat. Coolant starts leaking from top of the expansion tank. Close cap, and turn on heat to max, and it opens up thermostat.

I keep losing coolant from expansion tank cap(too much pressure and it starts oozing out).
If thermostat is working, the coolant temp should stay at no more than 93-95C.

There is heat sometimes through the vents when thermostat opens up while driving(press gas pedal to floor for 10 seconds) and coolant temp drops from 106 to 79 rapidly when the coolant flows into the engine and back thru the radiator.

Has anyone seen these exact symptoms ? I will replace the thermostat. Since the bleeding procedure when engine is cold works correctly, the water pump is clean(it was replaced as a maintenance item a few months back).

[sshivaram]

Update on this:

I had a misunderstanding of how the electric pump works in the 2006 325xi, along with the thermostat. I replaced the thermostat, and the new one had the same behavior. I later came across a post here that explained how the DME controls the speed of the electric water pump to run the engine at different temperatures.

The coolant clearly leaks from the top of the overflow tank. I am currently running the car with half the capacity of coolant to prevent further leaks.

The leak happens when the coolant is filled to the specified level and completely bled per the procedure. Its very easy to replicate this problem: Run the car at aggressive speeds on the highway. At a certain point, the coolant temp drops rapidly from a high of around 105-110C to less than 85C. This rapid cooling causes the thermostat to shut off, and at this point, I think the electric pump is still running causing the coolant to get pumped into the overflow tank at full speed. This causes the leak from the top of the overflow tank(from the cap, there is nothing wrong with the cap).
I am currently running the car with near zero coolant in the overflow tank and the same temperature behavior exists today, but there is sufficient space in the overflow tank to hold the coolant when it gets pumped with the thermostat closed, till such time the electric pump is halted. There is a definite lag that I observe, which may be causing this leak.

Anyone knows if this theory makes sense ? The car has this issue only under aggressive driving.

here are some images from my Torque Pro coolant temperature graphing:

Aggressive driving..hitting highway speeds and revving up the engine in lower gears...vroom vroom:


Driving at constant highway speed and constant engine rpm:


Idling in neutral:



Driving from a cold start:


Driving on a hot day (around 93F) at constant highway speed: