Weird temp gauge behaviour
 

My temp gauge has started mis-behaving. If the car has been stood off for awhile when I turn the engine on the temp gauge goes into the red (off the scale) after about 5mins the temp gauge drops all the way into the blue (off the scale) then it gradually climbs to the centre were it remains.

Any ideas on whats going on?

As the gauge is controlled by the computer it could be anything. Try changing the sensor as that should be cheap and easy. After that it gets expensive real fast.

I don't think its the sensor as also the MPG gauge doesn't work properly either.
It only gets as low as 20MOG when flooring it and sits of the scale when normal driving... Not what it used to do.

I'm having some similar issues. In the cold weather issues thread I had determined my t-stat was stuck open. I spent $300 and had it replaced. Problem solved...Till it went from 54 degrees outside yesterday to 4 degrees this morning (to hell w/ Chicago). All of a sudden my gauge is burried in the red again...Any ideas?

-tstat replaced.

-a week later temp gauge burried in the red.

have your coolant flushed and thermostat replaced will solve the problem. while you are at it, get your water pump checked for malfunction or leak. if it is the case then replace your water pump. if all checked and replaced as mentioned above doesn't solve the problem. have your mechanic look at crankcase vavle or oil separator, if it is bad replace it before it damage your engine. if all checked and replaced and the problem still there, SELL THE CAR IMMIDIATELY.

I agree, mutiple simultaneous sensor failure is not likely. It sounds like this could be a harness problem but then again, you should still probably investigate each system individually just to be safe. If you can plug a scanner into the OBDII harness and see what the computer really sees as a coolant temp, that will tell you if the gauge is malfunctioning ot if your engine is really having issues.

I am concerned about the oil seperator now that I had the tstat done. Any cost estimates on that, done by self of shope?

The separator problem will not immediately damage your engine. It makes you run low on oil, mostly, and use too much fuel. But if you keep your oil level up and do not run the engine ragged, it will not destroy you. Do a search on separator.

Besides, that doesn't have anything to do with the temp gauge.

Curtis, it costs around 400 - 600 bucks at an indie shop. Its definitely important to do. Especially since its brutally cold out now. You can do it yourself too if you want, but its not the simplest job. Its not extraordinarily difficult either, as lojack will soon tell you. I'd get it checked out and dealt with. If not, one morning your valve cover gasket might blow when you start her up. Then after driving for about 10 mins your car will be a smoke machine at exponential levels. Mine Blew when it was 8 degrees out on my way to work. It was not fun.. that was only the begining, a few weeks later, I had to replace my oil seperator (directly related to valve to cover gasket blowing) Then a few weeks after that I had to replace the entire valve cover. it was cracked..

Thanks for the info guys. I'm a capable mechanic if I know what it is I need to work on. So I'll try to do this one myself if I can find a garage to do it in. I'm only in Chicago for a year and stuck in an apartment w/ no place for my tools or my X5.

Any ideas as to why a week after t-stat replacement my temp gauge pegged red again? The coolant was also drained and flushed. Bad t-stat maybe?

I'd first bleed the system to ensure there's no air pocket and ensure waterpump flowing properly. With the engine off and cool, remove the plastic bleed screw and fill the expansion tank untill coolant comes out of both holes. Then re-instal the bleeder screw and start the engine. There should be a stream of coolant flowing across the opening where the fill cap goes. If no stream there, have someone hit the gas to about 1500-2000 rpms and look for it. The stream should be pretty strong then. If no problems were found/corrected with that, then may be time to take it in for electrical diagnosis at the dealer as the temp readings and fuel mileage are determined by the DME and signale sent to the cluster via can bus. Not many simple circuits on the beemers, but the coolant temp sensor is a regular NTC sensor.

Even in the morning when the car is cold I can turn the ignition on and the temp gauge goods off the scale past red. So its not engine related.
Its more instrument cluster related, due to both MPG gauge and temp gauge acting up at the same time.

When you first turn on the key the cluster does a self check... all the needles do a full sweep then return to the normal positions. If that is all it is doing, then thats normal.

Does this happen only when you first turn on the key, or while driving?

mine never does a self check, and neither does my friends. The only time a self check happens is when power has been removed.
The temp gauge stays in the red for 5 mins, then drops to blue and steadily climbs to centre when warm.

what yr X5? engine? how low a temperature is your coolant good for? if you have alot of miles, has the water pump been also replaced (given you've replaced the t-stat)? - these are the 1st questions I would ask myself - everything else noted here already is good & maybe needs to be addressed also, but it helps to know the whole picture before giving advise