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Found this thread from another.

I just want to update with some facts about the E46 non-M thermostat. I have done a considerable about of research and testing using the OEM thermostat for my track car. Testing include controlling the OEM thermostat from a different source other than the ECU and data from numerous track events.

1. Yes it is true that the map cooling will received a signal from the ECU to open the thermostat. The heater element takes in 12V with a current of 0.8A. The thermostat opens fully in 70s and once the signal is cut, it will close completely in 90s. The ingenuity of this design is that ECU will send intermittent signal (1 for x seconds, 0 for y seconds) to the heater element such that it can control the right amount of coolant flow into the radiator. The controllable range of ECT is actually 80C to 103C.

2. Yes it is true that ECU monitors certain parameters before sending a signal to this heater element. Parameters include ECT, TPS, RPM and I believe possibly STFT.

3. Just to clarify. The thermostat itself should open at about 103C regardless whether the ECU sends a signal to the heater element or not.

4. Just to clarify. You can disconnect that heater element plug on the thermostat housing and the car will still crank up and run. Thermostat will open at 103C. I don't recommend driving the car as this since it is less optimal. As mentioned, I did it for data acquisition.

Hope this helps.
 

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Mr Paddle.Shift, thanks for the clarifications! I had a problem quite a while ago, randomly (either highway cruising, under load, stop n go traffic, nothing specific) the coolant temperature - verified through the OBC digital readout - was varying from 85 to 55 degrees, making the needle move from half mark to 1/4. The problem appeared completely random.

Disconnecting the t-stat plug made no difference at all, replacing the thermostat with another one, still nothing, replacing sensor on lower radiator hose, still nothing. Was really puzzled, until I changed the coolant temperature sensor below cylinder 6 which immediately solved my problem.

What is the contribution of that sensor to the overall cooling system? I was coming to the conclusion that it affects the t-stat operation, but since I unplugged it, the ECU had no "control" over it but was still getting the back and forth coolant temperatures. Is it safe to conclude that the sensor in question (below cyl 6) is only used to provide coolant temperature to the instrument cluster? I did not measure coolant temperature with a normal thermometer, so I can never know if the readings I was getting were real.
 
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