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Discussion Starter · #1 · (Edited)
Probably should make a post here about what I recently experienced on my 2001 E39 and my findings in diagnosing the problem.

It started with strangely high engine oil temps but coolant temp was normal and gauge was dead center. I've never seen her oil temps run that high before, especially with moderate city driving, but I thought nothing of it and just assumed it was the blazing Australian summer getting to her (This was the first time I am driving her through 40'C+ weather)

Then a few days later, I noticed the temp gauge went past the half way mark. That's when I knew something was seriously not right. Those who've lived with E39s will know the coolant temp gauge has a huge centre bias for about 20'C or so to keep the needle in the middle to appease paranoid housewives about their fluctuating engine temps.
Immediately, I unlocked my gauge cluster's diagnostic readout and got out the KTEMP reading to monitor the temps to the degree and I started noticing these traits (and highlighted the key points one should look out for in identifying this problem):

  • The engine oil and coolant temps are always within a degree or two of each other. This indicates that the coolant temp sensor has not lost it's mind and the engine really is running that hot (or somehow both temp sensors have died at the same time but that's very unlikely)
  • In city traffic or out on the highway, in the hot summer day or the cooler evening night, the temp remained stubbornly high. High speed, low speed, idle, prolonged engine braking, the KTEMP refused to budge more than a degree or two and, even when it did, it wasn't reflecting the current driving conditions.
  • Getting home, I found that the auxiliary electric fan doesn't activate despite the high temp readings. However, turning the AC on activates the electric fan, which rules out the fan being faulty. Even with the AC running with the aux fan blowing at the radiator, it doesn't make a dent in that 100'C+ reading.
After taking the time to read all the related forum posts here and research about the S62 cooling system, along with studying the design of the thermostat housing with all the pictures of DIY jobs (Thank you, all of you DIYers!), I learnt how the thermostat and it's cylindrical design works to isolate off the radiator and divert the engine's hot coolant straight back into the water pump, but not without going past the coolant temperature sensor first. Added to that, I learn that the auxiliary electric fan ran on a seperate sensor that's at the bottom of the radiator, I came to the conclusion the hot temps were due to the thermostat stuck shut and keeping the hot coolant from getting to the radiator and because the hot coolant never got to the radiator, the radiator's thermostat never activated the electrical fan.

And so I went ahead on this hunch; buying needed parts and rubber seals and took the time to pull apart the thermostat housing and what do I find:
937032

I was right on the nail. Old thermostat is on the left, new is on the right, after being left in a pot of boiling water.

Swapped it out, replaced all the seals and following timmay77's DIY guide (Bless ya!) to the letter on how to remove and stuff those 3 (well lubricated) shafts back into there and she's now running as cool as ever!

After all that, I have to give credit and utmost respect to whoever designed the S62 to be able to avoid catastrophically overheating immediately despite a stuck closed thermostat and have that coolant temperature sensor in the best location to identify it early on. She's such an engineering masterpiece!
 

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Weird, they almost always fail open. Think this is the second or third one I've heard sticking closed - I'm sure there have been more, just thinking of m5b threads over the years...
 

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Just curious, do you know if your temps were in the normal range before they spiked?

Weird, they almost always fail open. Think this is the second or third one I've heard sticking closed - I'm sure there have been more, just thinking of m5b threads over the years...
I've heard the same, but when mine went, it failed closed too. It's been a few years back, but if I remember correctly I first noticed the oil temps staying lower than normal (as if it was starting to fail open) on the longish drive out to a you-pull-it yard, and then begin to fluctuate oddly between low and normal on the way back. The next day, I only got a few blocks from home before it became obvious it was stuck closed and I shut the engine off to keep it from overheating. I did the pot of boiling water test too at the time with the same result.
 
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