My Z4 has a map controlled thermostat. When you stomp on the gas the coolant temperature drops extremely quickly and when the pedal is released jumps back almost as quickly. We are talking 4/5 secs to shave almost 20 degrees celsius.

At cruising the thermostat is barely open to maintain operating temperature, particularly in a diesel that generates little heat and with the massive radiator that the X5 has. If it opens all the way, for example because the spring can't hold the increased coolant flow the temperature will tank. That's why I took that swag about the thermostat opening up...

I suspect you are correct. I will enable the hidden menu water temp read out (if I can find the info and understand it!) just in case my scan gauge is reading the wrong temp for some reason but I don't think it is.

All gasoline BMWs after 1999 or so have electronically controlled thermostat. But what you wrote doesnt add up. When you LEFT OFF the gas the temp is raising 20C :yikes: From where the heat will come if the engine is not loaded? And even more so not loaded diesel in a cold weather.

I was simply using the example of my Z4 which has a 2.5l gasoline engine and an electronic controlled thermostat as an observation of how quickly the temperature can change in the engine when the thermostat is open very quickly (and conversely shuts quickly when the ECU stops manipulating it)... I really didn't want to sidetrack the discussion, just drawing a parallel to what appears to be a odd behavior that London Lad reported.

If it's not the thermostat, then what's not working right?

To warm engine or water 20C you need to take energy from somewhere. In the case of idling diesel this energy is just not there. A 6 cylinder engine full of 3 gallons coolant has a huge thermal inertia and this kind of fast warm-ups and cool downs are not plausible. My opinion is that "London Lad" is getting a false reading due to the tool that he is using or due to electronic problem in his car.

OK well I have checked the reading against the dash hidden obd and another obd hand-held and the results are the same. Its as if the increased RPM forces the stat open and allows a large inrush of cold water. Running at 70 mph at light load, drop 2 gears and floor it, temp drops 15C in 5 to 8 seconds. Lift off and drop back to normal 'drive auto' and the lost temp rebuilds in about 20 seconds. All other temp readings during warm up etc look normal. No OBD fault codes are being logged.

:popcorn::popcorn:

I got my thermostat replaced. On its first or second day, the temp stays around 89-90 degree most of time. Then the following days, it varies from 65-80 on highway, 84-94 on local. Outside temp is about -10 degree these days.

Is this normal? Do you guys have a steady 88 degree most of time?
I purchased thermostat from ebay with under $40. My local dealer asks more than $150.
Where do you guys buy it from?
I doubt the one I bought does not have a good quality...

Thanks.

$150? Wow.
My local dealer quoted $90. Assuming OEM (is OEM even a good thing anymore?).

ZetaTre/lcapital,
Is the temperature on yours still happily 89-91 since t-stat replacement?

Mi wife drives the X5 so I don't monitor it on a daily basis, but whenever I drive it, the temperature is constant in the operating range. I'll ask her if she lets me drive it for a couple of days and I do some logging, including see if it holds temp when you floor it.

HOWEVER my dad is back at having longer warmups after about 70K km (45K miles). He had the dealer in Italy warranty the old one and now has to replace it again. We live up in the Alps and he said that no matter what was the outside temperature the car would get and stay in the operating range, but not recently and again while cruising at 70 mph on a flat he still sees the temp dropping below 80/75.

We'll see when he replaces it again if things get restored, but if that's the case, these thermostats are pretty much cacca

And by the way people, the MSRP of the thermostat is $79.05 so don't get stiffed by your dealer... Granted they shouldn't be jacking up price 2 fold, but for the love of God, think with your head not your wallet... Seriously...