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Steve S

Engine thinks it's overheating

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My wife was using the car today and on her way home and about 8 miles in to the journey she got a warning message saying  engine temperature high drive slowly to allow it to cool etc or engine overheated stop immediately. As the journey is about 12 miles and she was just over 1/2 way home I would have expected the temperature to be not even up to normal. When I have checked the temperature (using the hidden menu display thanks to this great forum) I have found it takes about 6 to 8 miles to get up to about 80 deg C and does eventually rise to about 92 or 93 deg C. I suggested she just gave it 10 minutes and try again. She did this and got about 1/2 a mile further and same problem. I thought about talking her through accessing the hidden menu, but it's too complicated for me to explain to her over the phone. God only knows why the BMW iDrive software programmers weren't asked to or didn't get round to putting the temps on the iDrive.

 

When I got to the car about 30 mins later I called up the hidden menu and selected the coolant temperature. Initially it gave a figure of -10C, which I thought was a bit strange as normally it shows the temperature even before starting the engine. I started the engine and it immediately went to 79 C and seemed steady for a while while it was idling. I pulled away and everything seemed fine until a short way down the road the temperature on the display started to rise and fall quite erratically  At some point it obviously exceeded a preset warning level because the display of the temperature was replaced by an amber graphic of a thermometer and I got the warning engine temperature high drive slowly to allow it to cool. I was already driving slowly so I carried on. Then I got a reg graphic of thermometer and with this a complete loss of power and the engine cut itself off or maybe I stalled it. I restarted and at various points over the 3 or so miles home I saw temperature of 80C steady or fluctuating temperatures of  up to 138C as well as the amber and red graphic of the thermometer and complete loss of power.

 

As it is impossible for the engine and the coolant temperature to heat up and cool down at the rates I saw it must be an instrument problem. It could be the sensor itself failing or a wiring connection fault anywhere between the sensor and the engine management system. Does anyone have any suggestions as to the most likely cause?

 

Maybe very few people have experienced this type of problem as I would hope that failure of a system that BMW rely on to allow the engine management system to decide to completely cut all power from the engine to protect it from over-temperature would be extremely rare. If my wife or I had been pulling across a line of traffic just as the engine cut all power it could have resulted in a serious accident.

 

Steve

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definitely a sensor problem! Never heard of that problem on here though.

 

Sounds like you would have stalled it, nothing will cut the engine off unless it's a proper mechanical fault

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Baus,

Thanks for the confirmation. I guess I'll just go and a have a look what I can find in the wiring and connectors first and if there's nothing obvious I guess its a new sensor required.

 

I thought I might have stalled the engine rather than it stop itself, but subsequently when the red thermometer graphic came on there was virtually no power, but id did keep idling.

 

Regards,

 

Steve

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Your coolant temp sensor sounds shot!

The car should not go into limp mode until the coolant reaches 120C.

When I had my M5 you would replace the thermostat and coolant temp sensor as a matter of course.

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I've just had a check of the wiring/connector and there seems nothing obviously wrong. I can't check the wiring condition as it's all enclosed in plastic conduit. While I had the connector off I checked via the hidden menu what the temperature was displaying and with the engine off it was still showing -10C and when I started the engine 79C, so these seem to be the temperatures that the system deduces from an open circuit. As these are the same as with the sensor connected I wondered if the sensor was open circuit, but when I put the multi-meter across it it's not open circuit, resistance is 36 Ohms. I haven't got a clue what it should be. Does anyone know the correct resistance and is -10C and 79C normal for open circuit?

 

Regards,

 

Steve

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are you sure you've got the right sensor? I've never taken mine out, but I'd have thought you can't remove it because all the coolant would come out!

 

 

edit: and if it is the correct sensor, check it again in the morning, and you should see a difference in the resistance when the coolant is colder...

Edited by Baus

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Baus,

It had crossed my mind that I might be looking at the wrong sensor when I got no change in the temp reading when I disconnected it and yet it wasn't open circuit. I'm pretty sure I got the right sensor though, it looks like the picture below with an oval connector and a 12 point star head all in plastic. I didn't remove it because as you say the coolant would come out, so I just put the meter across it in situ. What confuses me is that disconnecting it make no difference to the reading and I wonder whether it's a problem with the monitoring system rather than the sensor. 

 

1609320583864040_1.jpg

 

Regards,

 

Steve

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well it does have a range of other sensors, it has an oil temp sensor, it might even have two coolant sensors (inlet and outlet etc.) Could have been taking a reading from somewhere else. You should have driven with it disconnected, and checked to see if the temp went up or not.

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Checked the resistance cold and it was 33.2 Ohms. Again hidden menu gave -10C before starting and 79C as soon as I started it. Then after a minute it went to 20C which actually seemed to be an accurate figure. I disconnected it and it went to 79C. When I reconnected it it started to go eratic again and for a while seemed to give gradually varying temperatures in the range 95 to 105C. The engine was still pretty cold as it had only been idling for a few minutes.

Checked the resistance at this point and it was 118ohms. I can't rule out the connector, but I sprayed some WD40 and it made no difference. If 118 Ohms is the correct resistance for 100C then sensor must be buggered as it definitely wasn't that hot. New sensor I think is required.

 

Steve

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Seems to be all OK now with a new sensor fitted this afternoon. It's located at the front of the cylinder head. Removal and refitting takes just a couple of minutes, but finding a suitable socket is the tricky bit. It really requires a 17mm double hexagon extra deep socket, but a 19mm hexagon extra deep socket did the trick. The sensor doesn't seem to be very tight as it seals on a couple of O rings and not on the shoulder. Its a job anyone could do, but I got my friendly Independent BMW specialist to fit it while I was there just in case there were still issues with the monitoring system and he would have hooked it up to the diagnostic equipment. He initial said it was unlikely he could get one from stock because they rarely fail and it might have to be ordered from Germany. Luckily the main dealer had one, so maybe they do fail from time to time. The Indy got it delivered, which saved me a 24 mile round trip.

 

On the hidden menu coolant temp it was 68C steady as soon as it was plugged in and on the drive home (I took the scenic 5 mile route to give it a bit of a test) it went to 90C, so all is good with the sensor and the thermostats.

 

I did have to drive it with the sensor disconnected this morning because it went into the engine overheated range as soon as I pulled out of the drive. Obviously with the sensor disconnected I was driving with no over-temperature protection, which raises the question why do BMW fit such a system, which when disconnected or the sensor is open circuit or any of the wiring open circuit still gives a reasonable temperature signal. When open circuit with the ignition on it gives -10C and with the engine running it gives 79C, so I wonder why they didn't programme the computer to flag up a sensor fault if it saw a step change of 89C or thereabouts when the engine is started. All the info is there they just choose not to use it. As you may guess I have a very dim view of the BMW software programming department.

 

It's a real pain having to call up the hidden menu to see the temperature, but I will certainly be doing it more often. This issue has probably been occurring intermittently for a while and if there was a gauge or a readout I might have spotted it

 

Regards,

 

Steve

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