tpaulus

My 2006 550i E60 N62 keeps setting OBD II code P0128 which is a low coolant temperature code, that is the coolant temperature is persistently below the thermostat operating temperature. I watch the coolant temperature with a OBD II scan tool and it does remain low usually but not all the time. From cold start, I expect it to rise steadily to about 195F and then perhaps back off a little as the thermostat opens but recover eventually. What I see is that it rises steadily till about 140F and then it stays between 140F and 180F most of the time. I've seen it at almost 200F but not for long. The dealer has replaced the thermostat twice but I continue to get the code and see low temperatures. Could I have a problem with the coolant temperature sensor, sending low or otherwise inaccurate temperature reading to the ECU? I see what appear to be several temperature sensors in the engine compartment, one on the thermostat itself, one on the lower radiator hose and one on the engine coolant outlet. Can anyone provide guidance here?

tpaulus

I've been watching the coolant temperature more closely, recording it and graphing it using an OBD II tool (Torque PRO for Android) and I'm finding that most times, it behaves normally. Steady rise from cold start to ~200F.

However, there are times that the temperature will drop ~40F in an instant and remain about 40F lower than expected for some period of time, after which it will rise ~40F in an instant and return to normal readings. I observed this directly only once. However, I believe my initial observations described in my original post were made during one of these periods where the reading is ~40F lower than the actual coolant temperature.

The day I observed the instant drop/rise had several trips, each trip is detailed where a trip is from engine start to engine shutdown.

home -> work in the morning, normal readings observed.
work -> carwash -> barber, normal readings observed until full throttle acceleration leaving car wash coincided with sudden ~40F drop in coolant temperature reading
barber -> home, lower than normal readings observed
home -> post office, lower than normal readings observed
post office -> vet, lower than normal readings observed
vet -> home, lower than normal readings observed until full throttle acceleration from intersection between vet and home caused sudden ~40F rise in coolant temperature reading
home -> restaurant, normal readings observed
restaurant -> daughter's house, normal readings observed
daughter's house -> bar, normal readings observed
bar -> home, normal readings observed

One peculiar fact is that the ~40F drop coincided with a full throttle acceleration 0-60 mph leaving the car wash. The temperature remained low during my next several trips that day, through engine stops and starts and returned to normal later in the day during another full throttle acceleration 0-50 mph. I wonder if something is getting wet in the car wash causing the reading to go bad until it dries out sufficiently. The full throttle acceleration doesn't cause it to change in general, I can't just floor the pedal and observe a temperature change every time, however that is when I noticed it. I do a fair number of full throttle accelerations, so far two have coincided with the temperature offset.

I'll watch it a little longer to see if I can draw any conclusions about water affecting things. Our weather forecast is calling for rain today and if not I can run through the car wash on the way home and see what happens.