Does this sound like a thermostat issue? - update, nope.

[g300d]

RB, yeah that would be nice to know, please do!

[srmmmm]

My original, and three subsequent replacements done by the dealer, have all been Behr. I would agree with the "soft failure" comment as the last two replacements were based off of stored codes and not a visible temperature gauge indication.

2002 X5 3.0 312,300 miles
2014 428i 22,800 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles

1970 Firebird - Under restoration

[Ricky Bobby]

^OK good to know - mine was replaced but I don't think the dealer so probably a Wahler, its been fine, but if I do the water pump preventatively when I do the rad hoses and Exp tank due to age, I'll get a new Tstat in there as well -

[Zulu95]

Fan clutch?

[g300d]

Fan clutch is new.

I was thinking that might be it so I checked how the fan is running when the temp goes up and when it goes down.

Air flow was the same, difference was when the temp is going down the air being pulled by the fan is hot.

Which leads me to theorize its a thermostat problem.

[Ricky Bobby]

Well as promised I got my idler and tensioner pulleys replaced with new INA units and new Continental belts - with 30k miles but 5 years on the ones I replaced previously both pulley bearings were making noise when spun which was my cold squeal at startup - all quiet now

I did get to peek at my OE (BMW stamped) Thermostat and it is in fact made by Wahler - would be interesting to see if a currently produced BMW thermostat is a Wahler or Behr unit, perhaps mine is older and the newer dealer units have been superseded by Behr.

[g300d]

So I guess mine is not a thermostat issue. Replaced it with a new Mahle-Behr one and I still dont have my old 92-93c average temps.

'Stat was a BMW-marked Wahler. No infamous leak on the heating element.

[spadge]

Maybe there is some air left in the cooling system?

Just another guess: do you have automatic transmission? If yes, there is another thermostat #4 on this pic: RealOEM.com - Online BMW Parts Catalog
Don't know if this one might cause your temperature issues...

[X53Jay4.8is]

Originally Posted by spadge

Maybe there is some air left in the cooling system?

Just another guess: do you have automatic transmission? If yes, there is another thermostat #4 on this pic: RealOEM.com - Online BMW Parts Catalog
Don't know if this one might cause your temperature issues...

Yes that thermostat should be changed especially if the vehicle is over 100K miles. Good piece of mind maintenance

[g300d]

Gawd these cooling systems are complicated. Ive been spending a couple of days reding up on them.

That thermostat only affects flow to the transmision oil cooler.

When engine is cold:
- The engine thermostat shuts cold water flow into the engine so it can warm up faster.

- The trans thermostat allows water from the top radiator hose through the expansion tank and to the trans oil cooler, so the gradually warming water heats the atf along with it.

When the engine is warm:
-Engine thermostat opens to allow cooled water from radiator into the engine.

- The trans thermostat shuts flow of water from the expansion tank to the oil cooler and simultaneously opens another coolant path from the radiator, so now it cools the trans fluid.

But yeah, anything slightly out of wack could explain slightly elevated but not overheating temps I guess.