Engine temp oddity

[Marshy]

UK 540i auto, build 02/93

Over the last few days, my temperature gauge hasn't got up to the 12 o'clock mark where it normally lives (yes, I'm aware that 12 o'clock covers a multitude of sins on these things...). If the gauge is 0% cold to 100% shes-gonna-blow, I'd say it's mostly sat on about 40%.

Now, previous experience would suggest that this is potentially the thermostat stuck open, causing excessive cooling at speed. However, it's still sitting at 40% when sat in traffic. It's been nearly 30C here of late, so the aircon's been on full-pelt too.

So far from the temperature reading varying with speed (as it does with my other car which I *know* has a duff stat), it's as though the needle's been moved on its spindle...

All the fluids are topped up, still runs clockwork (165k miles, uses no water, no oil... that's jinxed it, of course

Any ideas what this might be before I ask someone expensive to look at it?

Thanks,

--Chris

[infinity5]

You could just change the thermostat and see. I did my 530 and it's not really bad at all. Draining the coolant is annoying, but if you only drain the radiator its not so bad. You could switch to the metal thermostat housing while you're at it.

It'd save you a ton of money and might solve the problem.

[joshua43214]

The aux fan will cool the engine down to whatever temp the thermostat is opening at. So a t-stat that is opening early will run cooler in traffic if the aux fan is running, which it is with the ac on.

T-stats are cheap and easy to replace, it will need to be replaced sometime anyway, so no harm in doing it now and it turing out to be something else. If you don't drain the coolant, it will probably take less time to replace than pinpoint testing with a multimeter.

[gale]

Most likely the thermostat. They're more likely to fail in the open position than in the closed position. If you leave the cap on the expansion tank & are quick about swapping t-stats, you'll lose a minimum amount of coolant & can merely top it off afterwards & rinse off the spillage. The m60 motor is self-bleeding.

[gmannino]

Could be your tempature sending unit is going bad, its easy to replace and I'm sure its pretty cheap too.

[BillionPa]

40% on the gauge is actually not that bad, if that was happening to me, i would leave it alone during the summer and fix it when it starts to get cold.

[Marshy]

OK, so I'll get the stat changed - thanks for all your input folks: useful and reassuring at the same time.

BillionPa: I realise 40% rather than due north on the gauge isn't necessarily a problem per se; it's just that it's been pointing straight up when warmed up under all conditions for the last 2.5 years and 50k miles. Hence the change has made me suspicious, especially given that its behaviour doesn't match other cars I've had with a duff stat.

[KenB]

That's where my guage sat when my temp sender went bad. The danger is that it doesn't move and will not tell you when it is running hot. It was a pretty cheap fix IIRC, replaced t-stat same time to be sure.