540i running hot at highway speeds in freezing temps

[joshs540i]

Ive noticed over the last couple of days that when im cruising around 80+ mph the temp gauge will be slightly to the right almost to the first right line. the obc reads from 116c to 120c but when i back down my speed the gauge goes back to center. this only happens when the outside temp is below 40F.

The low coolant warning warning also comes on but the coolant level seems fine

the only thing i could think of being at fault is the thermostat, just a guess.

[MPOWER228]

I'd guess it's your water pump, being that it only will heat up when there's alot of load on the engine. but it still could be your thermostat or radialor is a bit clogged.. FIRST, try bleeding your system. It's very possible that there could be a bubble in the system causing a low flow at higher rpms. and that would explain your "low coolant" light.. That's what mine did Give it a try..

[T1TT910]

Dude!!! mine is doing the same thing. I checked all rad & fluid... all fine.

I am not getting the low coolant light though. (@ 28F)

[02Pilot]

Overheating at speed suggests a coolant flow problem. Water pump is the obvious first guess, thermostat or air in the system possible. Another oldie but goodie is the lower rad hose getting soft and collapsing under suction at high RPM, restricting flow. I doubt the last one is the case, but I've seen it happen (not yet on E39s, however).

Regardless, unless it has very low mileage for a 98, if your car has not had a compete cooling system overhaul by now you are well overdue.

[thrilos5]

sounds like a leaking head gasket or worst case a cracked cylinder head, do you had any previous overheating problems??
compresion from the motor would force itself through a leaking head gasket and won't let the coolant circulate throughout the motor causing a rise in coolant temps as the rpm rise (highway cruising, etc...) in the beggining it might not seem as a big problem and you might have to top off the coolant once in a while, but as time goes by and you don't take care of the problem is going to get worse,

on the bright side it could be sticking thermostat,
do a leakdown test and a coolant system pressurised test to see where you at

[T1TT910]

I noticed it this last night when I was stopped at a light. it was slightly higher than normal. Then again in 4th @ 3500rpm

[ohlins8990]

I don't have a BMW, but if I had to guess, I would guess the thermostats getting stuck towards the closed position.

[T1TT910]

I don't have a BMW, but if I had to guess, I would guess the thermostats getting stuck towards the closed position.

Anyone know the failure rate of the 540i thermostat??

[joshs540i]

Thanks for all the suggestions.

I bled the system this morning and got it to take about another quart. but that didnt seem to change anything.

I payed alittle more attention when stuck in thanksgiving traffic and noticed that the temp would fluctuate between 96c and 116c while traveling anywhere from 2 to 15 mph. It seems to have little to do with load on the engine.

are these sympoms consistent with a stuck thermostat or a bad water pump?

Thanks again.

mileage is 141k and the radiator was made in 4/03 so im assuming the cooling system was overhauled around then.

[BMW 528]

Had sam problem with 2001 Jetta 1.8T.

Water pump!

[ArmySweitzer]

my guess says stuck thermostat, you can take it out and should be able to test it in some really hot water. Seems like your in need for an entire cooling system overhaul, thats assuming your engine doesn't need serious work.

[joshs540i]

well i replaced the thermostat and bled the system this morning. and on my 30 min drive it seemed to have done the trick.

the temp was pretty steady at 110C after warm up went as high as 113 and as low as 105, i think that is all in the range of normal, I know the thermostat is 105C

I took a bunch of pictures and im going to write a DIY tonight because i couldnt find one. although theres a bunch for the 528