Cooling System Fan

[tomj76]

I have a strange issue on a 2003 Mitsubishi, 2.4 L v4 engine.

When first started, the cooling system fan is off. When the coolant temperature (via scan tool) gets to ~200 degF, the fans come on. The coolant temperature may drop as low as 196 degF, then stays there. The fans never turn off.

When I feel the lower (inlet) hose, it is cool, but the upper one (outlet) is hot (thermostat is closed!). When I put a thermocouple on the upper hose tube (on the engine head), it reads ~130 degF to 150 degF, not 198 degF like the scan tool reads. Is this a bad temperature sensor? The scan tool reading is OK when the engine is cool. The instrument panel temperature gauge reads mid-range in this situation.

When I disconnect the power connector from the cooling fan controller and I allow the coolant temperature (via scan tool) to increase to 225 degF, the thermocouple on the outlet tube reads 212 degF. This is confirmed with water dripped on the metal, since the water boils at that point. When I plug the power to the control module back in, the engine cools to 196 degF (scan tool) and the fan turns off. The fans turn on again at 201 degF. This will repeat three or four times, but each time the outlet temperature (read by the thermocouple) is a little lower than the previous cycle. Cycling seems to stop (fan stays on) when the lower inlet hose (to the engine) goes cold, which shows the thermostat is now closed.

On first take it looks like the engine coolant temperature sensor may be reading the wrong temperature, especially considering the drift in the shutdown temperature. The fact that the instrument panel temperature gauge seems to confirm the sensor reading implies that the sensor is not the problem.

Is the problem with the thermostat, possibly closing at too high a temperature? Why doesn't the fan shut off once the reading at the thermocouple is down to 150 degC?

I haven't probed the control wire into the control module just yet.

I'm getting ready to shotgun this with a new thermostat AND sensor (drain he coolant just once!), but I'd like to understand what the problem is.

[tomj76]

I haven't yet replaced the parts, but a few more observations.

If the cooling system control turns off the fans at a temperature that is lower than where the thermostat closes, then the fans will never turn off. This is because once the thermostat closes with the fans on the temperature at the sensor will stop decreasing and the shutoff temperature will never be reached.

So, there are three possible causes: (1) Sensor reads higher than the actual temperature [can be due to sensor or the circuit] (2) Thermostat closes at too high a temperature (3) The engine control computer is getting the correct temperature but does not command the fan to turn off until a lower temperature than the thermostat closes.

Known:
-The thermostat for this engine should open at 190 degF.
-The temperature reading from the sensor (which is located in close proximity to the thermostat) is greater than ~196 degF.
-The thermostat is closed at this temperature
-The outlet temperature is much lower than the scan tool reports
-The temperature gauge seems to roughly agree with the computer sensor.
-The sensor temperature is correct for room temperature.

Together, these seem to point to the thermostat closing at a higher temperature than it should. A conclusive check would be to monitor the temperature with the thermocouple next to the temperature sensor.