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Have done the same and while doing so replaced all the TPS and CPS sensors too. Took the plenum off and cleaned the throttle bodies too. Once the plenum is off the thermostat change is quite an easy task. No need to remove and vanos lines. As for the 3 tubes with orings to get them back it id say there is some force required. The seal ring didnt look that good after 160k miles so changed it. That was more difficult as changing the TPS sensors but no leaks after some miles.

If i would need to do it again every time plenum off so you could check the vacuum lines too.
 

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I just did the thermostat. I second removing the plenum. I also removed the vanos lines. So far no leaks.
The job is not overly difficult.
 

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I would say that you can get away with not having to remove the plenum, as I did not want to and did the job just fine. Also, freeze the O rings overnight if you can and pull them out only once you're ready to put them in. Honestly, the hardest part for me was removing one of the side pipes that was seized onto the thermostat but it was just an inconvenience if anything.



Also, I picked up a hand pump from Home Depot to siphon fluid out of the reservoir and that worked well for me.
 

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...
What did work
- Using silicone lubricant (just a little bit) on the o-ring before putting it on
- Installing the tubes into the engine first, then pushing the thermostat onto it
- I took a small hand file and manually added lead-in chamfers on the thermostat housing, smoothed it out with 200-grit sandpaper, then cleaned metal shavings out thoroughly with coolant
- Having a friend with a flashlight inspecting all around each of the tubes to check for o-rings squeezing out

Contrary to what people in this thread have said, any time I heard a "pop" it meant I had torn an o-ring (verified after coolant leaked and thermostat was pulled out). The time it finally worked I heard no pop, but rather the thermostat housing slid into place nicely.
Well, I tried spraying the o-rings with Gunk Heavy Duty Silicone Spray and that made the o-rings expand to be really loose on the outer 2 pipes. There are a lot of other ingredients besides silicone in that spray, to include petroleum distillates and ethanol. I have a feeling something other than the silicone in that spray is what caused the swelling of the o-rings.

I heard a pop when putting the housing straight on the pipes that were fitted into the ports on the engine.
Pulled it off and both outer o-rings were in 2 pieces each. What happened was the slack hung down on the pipes and the opening of the thermostat housing cut the o-rings.

My second attempt was with new o-rings with no lubricant applied, but I put Permatex dielectric grease on the lead-in chamfers of the thermostat housing openings. Pushing the housing straight in worked wonderfully. All I did was rotate the thermostat in the housing so that the bail was parallel to the front axle, as others have suggested.

I did have to take a piece of wood and hammer to the front of the thermostat housing toward the rear of the engine to seat the tubes into the engine and housing openings enough to be able to get the bolts from the thermostat housing to go into the top of the water pump.

Once all of the cooling system hoses and new dual temp sensor were installed, I borrowed a Snap-On Cooling System Pressure Tester and pumped it to 12 psi before even adding coolant ( I had only removed enough to do the job, pumping it out of the pressure bottle). I cranked up the volume on my hearing aids (lol) to the max and could not hear any air escaping. Added coolant to the correct level and retested. Dryness and joy.
 

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Here are my observations:
1) Stant T-stat doesn't work without using some kind of a shim or extra o-ring to create a sealing surface. You can try without, but the system won't hold pressure if you do a pressure test.
2) Behr and Stant rubber gaskets are not interchangeable.
3) Pressure tester is a must have. You can rent one and if you have Advance Auto Parts, rent theirs. The adapter for BMW is orange.
4) I froze the pipes and used KY Jelly as a lubricant. I did the job 3 times and haven't lost a seal once (had an issue with t-stat itself) despite pretty rough handling of pipes and housing.
5) If I were to do it again, I would get an rubber o-rings ahead of time and use Stant t-stat. I would also get a new housing with the metal ring pressed in.

Your experience may be different of course.
Is that actually available somewhere?
 

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How much coolant must be drained to remove the t-stat? Radiator only or is it necessary to open one of the block drains? Thread search function is not working.
 

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How much coolant must be drained to remove the t-stat? Radiator only or is it necessary to open one of the block drains? Thread search function is not working.
C’mon vet, use the trusty google site search method! ;) The answers vary, but these 2 short threads will help you decide what’s best for you.

https://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/149327-draining-coolant-thermostat-change.html

https://www.m5board.com/vbulletin/e39-m5-e52-z8-discussion/150558-advice-draining-coolant-stat-change.html
 

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Also the next job to do for my GreenBeast ... I already bought the t-stat and the o-rings (1 pkg of the smaller one = 10 rings and 4 rings of the bigger one for the inner pipe).

Thanks to all your descriptions and tips but I have one question for the thermostat pros:

my plan is
- to dismount the airbox (many screws but simply job)
- dismount the visco blower (I already have the special tools)
- to lift up the front of the car and
- to drain the cooling system -

but how many liters (or gallons) will I have to let out to get the thermostat dry?
 

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about 1 pint of coolant will come out of the thermostat housing itself. And about 1 pint of coolant from the coolant lines.
Thanks,

so if I drain the cooler (blue screw) about 1 pint and the coolant lines the thermostat housing is dry? (I dont want the coolant flow down over parts of the engine when I open the thermostat housing)!
 

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Thanks everyone especially to jfoj. Decided to remove the plenum after having leak on my first try with broken o-ring (see picture). So much easy to push the 3 tubes to the engine first and then push the thermostat housing to the tubes and using liberal amount of water based KY jelly. I used a silicone lubricant on my first try but it is not as slippery as the KY. I also replaced my radiator due to crack (see picture). (y)(y)
 

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