Is my thermostat going bad?

[Iforgetalot]

99 540i 6speed.

Few days ago I noticed my coolant temp hovering lower than usual, in the low 90C. My car topically operates around the 104 -107C range, a bit higher if in traffic. The highest I’ve seen it is at 110. So when I saw the lower temperature readings on the coolant I was pleasantly surprised. This only lasted for two or three days and now it is back to normal temperature range. I monitor coolant temp on the cluster menu, #7.

Could this be a sign of my thermostat or temperature sensor going bad? I think maybe it was stuck open for a few days and that’s when it was showing the lower operating temperatures. My new cheap scanner says this.

C4B160FE-ADE5-4621-8B5E-2D48D2018031.jpg

[Dave Sal]

My thermostat went bad a couple of years after I bought the car new. The temp gauge would only get to around 11 o'clock and the heat from the vents was lukewarm. At the time I didn't know how to use test 7 to check the coolant temps so I have no idea just how low the temps were.

[edjack]

Bad 'stat is my guess.

[rbelton]

Yeah the thermostat is probably bad. How long does it take for the car to reach operating temperature (temp needle in the middle of gauge) from a dead-cold start in the morning?

If the warm-up time is unusually long like it was on our X5 in this video, its probably a stuck thermostat. Really common, even on our X5 where the t-stat was only a few years old.

Not a V8 E39, but the concept is the same, hope it helps:



link: https://youtu.be/JAR7UDSDUew