[jrods2500]

Hi all. Need some help from you gearheads.

I've been stumped by a cooling problem in the last few weeks with my X3 F25 (140k miles). Engine overheats. I checked and all this is ok: Coolant, thermostat, oil/coolant temperature sensor, and the issue might be the water pump, but I’m not sure it is the root cause.

There are three codes, apart from the engine overheat, and I think they are not related to the overheating issue:
C90C87 Rain-light-solar-fogging sensor: overtemperature
DE8C5E: LIN Sine, communication failure
E7144D: Signal (0x226) received invalid: Intensity_Regen

I’ve been logging data with Bimmerlink and cannot find a correlation between data other than as follows (all temps in C):

1. In normal conditions the coolant target temperature varies between 90 (driving at slower speed) and 110 (faster) most of the time. Between the fan and the pump the coolant temp stays right on target (chart, red line). Oil temp follows coolant, no prob.

2. At random times (this is what stumps me, don’t know what triggers it), the ECM changes the target temperature down to 80 (which is known as High Operation and Current Supply to the Map Thermostat). It has happened driving on residential areas at around 30mph or highway at 75mph. See around second 550 in the chart.

3. The pump (green line) and the fan start working at almost 100% and the temperature starts to lower slowly but it does not reach the requested 80 when the pump stops working, and thus the coolant temperature starts going up and up and engine overheats (not shown in this chart). In the chart I parked (around 850 sec), turned engine off and restarted after a few seconds and everything went to normal, meaning target coolant temp was 90.

4. Is this normal operation and root cause is a failing pump that probably overheats and thermal protection stops it? Or the root cause is something else that makes the ECM request a target coolant temperature of 80 when it is unnecessary, or because of other condition?

I think the problem is not the pump as one time I was driving on the highway and target temperature was set to 110 and suddenly it changed to 80. I do not see a reason why it needs to change after being driving in the highway for 20 minutes at that speed.

HELP!!!

[Hellshalfacre]

"The pump (green line) and the fan start working at almost 100% and the temperature starts to lower slowly but it does not reach the requested 80 when the pump stops working, and thus the coolant temperature starts going up and up and engine overheats"

Everything you are describing are the symptoms of a failing water pump. I wouldn't be too concerned with the ecm target temp as that will change relative to a bunch of inputs (rate of temperature rise or fall, ambient air temp, intake air temp etc.) In other words I don't think that your ECM is faulty UNLESS you are able to verify by other means that the car isn't actually overheating (temp gun on the top and bottom radiator hoses.) Remember that overheating is generally not a result of the ECM not doing its job, it's the result of the engine exceeding parameters that the ecm monitors (signal to make the water pump run notwithstanding.)

If the radiator and water pump are original, 140k is a pretty good run. Have you tried to activate the pump for a coolant bleed procedure to observe and listen to its operation? While this test isn't entirely conclusive, it could tell you right away if there is an issue with the pump. Failing water pumps are the number one cause of overheating in these cars. Good luck to you.