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Cool Runnings: E39 M5 thermostat 'O' rings learning curve

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'Cool Runnings' was a crap movie. But it's a snappy thread title to describe the operation of my slow-to-warm-up E39 M5.


The problem is the well known progressive failure of the thermostat that results in it opening sooner and remaining open wider than it should. Thermostat replacement has been well documented. There's several long DIY threads on M5Board and many, many tales of woe from a lot of people describing their botched attempts, mangled 'O' rings and coolant leaks. Reading the threads, luck seemed to play a part in whether the outcome was successful since everyone seemed to follow more or less the same processes and procedures, yet many still failed. What's going on? This couldn't have happened in the BMW factory in Germany back in the day; had to be right first time, every time. So there has to be a way of installing the thermostat without chewing up innumerable 'O' rings. Surely?


Was feeling confident that I would find a way. Actually, really confident since I would be removing the air intake plenum so would have excellent access to remove the thing and re-install it. Simples then! What could possibly go wrong? Started well.....



(Above) Here it is. Hours of fun ahead. Early disassembly is fast. Air intake tubes from the MAFs to the plenum removed + the plenum cover. After that drain coolant out of the expansion bottle and the top of the bottom hose. A siphon makes this much easier. Definitely no need to drain all the coolant. I think I got out about 4 litres (of a total of 10 litres) which was enough to carry out the 'stat removal/replacement. (Tightening torque values for the plenum nuts, in two stages, 5NM, starting with the centre four nuts followed by the nuts around the perimeter at 5NM. Then 10NM, again starting with the centre four before moving onto the outer nuts.)



(Above) Plenum cover off reveals the air intake 'trumpets'. Loads of screws/bolts to remove. Trumpets come off first. (No torques values given for the trumpet securing nuts in TIS. I'd recommend 'hand tight' on a spanner or ratchet. Remember the trumpets are just plastic so don't go mad. The nuts are 'nyloc' with a flange and BMW TIS recommends using new ones when reassembling). I also replaced the 4 plenum 'O' rings and the big plenum rubber gasket (visible above) to ensure against un-metered air entering the intake.




(Above) Trumpets off. Stuff some kitchen towel down the intakes to keep dirt and debris out. Loads of plenum bolts to remove now. (Torque values: two stage tightening again - 5NM starting with the inner rows then moving to the outer. Repeat using 10NM.) Detach all oil separator rubber hoses from the front and underneath the plenum.



(Above) Got my crayons out for this one. The plan was to replace a load of stuff under the plenum that typically give trouble down the road. Preventative, or planned, maintenance. Usual suspects highlighted above. Basically anything rubber that carries vacuum or tank vapours was changed.  The Throttle Position Sensors (TPS) were also changed and they were a bit of a mare to extract, as was the Fuel Tank Breather Valve.


Back to the thermostat. You can just see two of the (three) shiny alloy tubes that connect the thermostat to the top part of the engine - bottom of above photo.




(Above) Thermostat removed. With the plenum out of the way access is great for removal. Remove the VANOS oil junction box that is bolted on to the thermostat (two bolts). 3 bolts hold down the thermostat housing to the top of the coolant pump. (Tightening torques for these 3 bolts: 10NM). Be patient removing the 'stat. It'll resist if it's the original and been in there for the best part of 20 yrs. Try to rock it up and down a bit then side to side and pulling. Final removal requires you to pull out and up. A sort of rotation, not a straight horizontal pull.  Swearing is optional. I did. A lot. If you haven't replaced those small diameter oil separator pipes (arrowed above) then now is the best time to do that job since they're an arse to do with the thermostat and plenum in situ.




(Above) Thermostat housing (inverted here) and alloy connecting tubes removed. Pull all the alloy tubes out of their recesses - wrap in cloth or tape if using a tool to extract them. They're made to tight tolerances so be careful extracting them. You'll want to replace all the 'O' rings. Buy a spare set since they're cheap but you shouldn't need the extra ones if you do this right. Factory original thermostat in view above. When I eventually put my new stat back in the housing I orientated it in the same position. This orientation seems to give optimal coolant flow across the brass capsule containing the wax.




(Above) Gently clean up the recesses for the tubes in the housing and engine side. The tubes are a very snug fit even without 'O' rings so you need to be careful to remove only oxidation, not metal. Lube them up. I used Liqui Moly automotive silicone grease. Some use Vaseline. I've read KY jelly works, too.


Well, I screwed up my first attempt and I didn't just nip the edge of the 'O' rings either.....




(Above) 'Fools rush in....'?


This is what I did wrong:


1. Put the alloy tubes (with 'O' rings either end) into the thermostat housing first.

2. Then tried to achieve the impossible by rotating the thermostat housing (with projecting alloy tubes) down and up to meet the other 3 engine-side recesses, and at the same time tried to get the projecting thermostat to clear the lip of it's own circular aperture at the top of the water pump. Reminds me of trying to assemble something like this:-



There's very little, if any, dimensional tolerance to allow a clean engagement of the 3 tubes into the engine and seat the thermostat down into it's aperture all at the same time.


3. I persisted with this approach. (Taking inspiration from Homer Simpson perhaps?) Eventually, after much wrangling, the 'O' rings 'popped' into place (Health Warning: 'popping' is not a good noise when locating these 'O' rings! Read on.) and the thermostat seated down onto the pump.


Buttoned everything up, refilled with coolant, fired up the engine and very quickly had coolant dribbling out from the 'O'rings. Doh!


I had extra 'O'rings but I wasn't going to attempt another go at putting the thermostat in by taking the plenum off again. Resigned myself to doing it with everything in place.


A better, more logical and successful approach:


1. Put 'O' rings onto all 3 alloy tubes. Carefully push the lubed up alloy tubes into the engine-side recesses, not the thermostat housing. Get the tubes absolutely square on to the recesses and press them gently but firmly home. If you don't do this absolutely squarely, with even pressure, you'll nip and edge of an 'O' ring. Practice first it without an 'O' ring. Once you get the hang of this the alloy tubes will slide home with their snug fitting 'O' rings. They don't 'pop' into place. Popping probably means you've nipped an 'O' ring. 


2. Make sure the alloy tubes are pushed home right to the end of their recesses. Now that they're squarely and fully seated you can push each one so it's pointing up slightly just a few degrees. It's not much but it does help. Don't worry, the 'O' rings will still be fully seated. Make sure the alloy tubes are perfectly spaced and aligned ready to eventually engage with the stat housing.


3. Don't put the thermostat into the thermostat housing instead drop it into the coolant pump aperture like this:



(Above) 'Stat sat on top of the coolant pump aperture. Make sure the projecting brass wax capsule of the 'stat is down into, and hard up against edge of, the circular aperture (of the coolant pump) nearest to you. Arrow shows one (others obscured) of the 3 alloy tubes ready in place in their engine-side recesses. 


4. Make sure the VANOS oil line is zip tied to one side. Ditto the large coolant hoses: you need some room to manoeuvre. You only need to undo 2 banjo bolts to swing the VANOS oil line stuff out of the way but new copper washers will be needed for reassembly - 4 washers in total, 2 different sizes. Realoem has all the part numbers.


5. With the 'stat sitting in the coolant pump aperture (as per photo), very carefully move the thermostat housing down onto the 'stat. At the same time lightly engage the 3 recesses of the stat housing with the 3 engine side alloy tubes. Keep everything square and aligned.


6. With your fingers underneath the stat housing,  lift the stat up into the thermostat housing. Seat it fully up into the housing with your fingers - should be an almost flush fit. As you're doing this, continue to lower the 'stat housing down. Maintain firm control, and remaining square,  push the stat housing horizontally onto the 3 engine-side alloy pipes. If everything was lined up everything will slide into place and be a nice snug fit, plus the 3 bolt holes on the pump housing (shown in the photo above) will align perfectly with ones in the 'stat housing.


Once I'd learnt from my initial error the above method was a cinch. They must have done it this way in the factory. 

Buttoned everything up, filled the system, fired up the engine with cabin temp and blower fan to max. Air auto bleeds out of the cooling system. Ran the engine for 20 minutes, hot enough for the pusher fan etc to kick in. No leaks or coolant loss!


Other thermostat  related stuff:



(Above) Replaced the inner seal inside the stat housing. Bought a cheap bearing puller.




(Above) Needs to be pulled out in two stages, rotating through 90 degrees for the second 'pull'. 


The new seal needs pressing in. Carefully.



(Above) Cheap seal press kit worked well.


(Above) Might be cheaper to just buy the thermostat housing new from BMW since it comes equipped with a pressed in seal!  (The new tools will get used for other stuff no doubt.)




Did a little side by side test on the old and new stats:






(Above) New stat vs old 'stat at 85 degrees C.  


With the new stat the car now warms up faster and runs warmer. Temperature gauge sits at '11.45' instead of just '10.00' (IYSWIM)


Quick photo tour of the other under plenum work carried out: Throttle Position Sensors (TPS)



(Above) Back of the engine hard up against the wiring conduit next to the firewall. The two screw heads hold the sensor in position - Pozidrive screw heads, not hex head unfortunately.








(Above) Did use hex head bolts when fitting the new TPS. So much easier to fit (or remove for that matter!)



(Above) Bit of butchery required to ease extraction of the old TPS on the nearside bank of the engine.


(Below) Vacuum hose for the fuel pressure regulator nearing the end of its useful life. It sits in the 'V' between the two banks of cylinders and gets comprehensively cooked over the years:-




(Above) It's a common source of vacuum leaks on our engines.


Round up photo of replaced parts:-



(Above) The largest hose (bottom hose for the radiator) in the photo is actually the new one since I'm letting my garage guys change this as it's best replaced with good access to the underneath of the car.


Had been putting off replacing all the above as it's often bloody awkward work best suited to people with childlike hands but with 10" long fingers. Mine aren't like that. At all. The thermostat - once I'd worked out the best method - was not that difficult and I reckon could be done quite easily in 2 hours with the plenum remaining on. Getting the stat housing off the car was probably the hardest bit.





Edited by DepthHoar

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Hi DepthHoar,


As per above, that's an excellent writeup and very very useful for M5 owners indeed! I'm sure knowledge of this thread and your information will filter through to plenty of owners over time and, assuming you are on M5board as well, it'll help those guys too as well as the rest of the examples over the rest of the world!


I'll be sure to reference this post if/when I hear of anyone having issues or needing to know all about the M5 thermostat issue! Thank you for this thread/knowledge :)


Cheers, Dennis!

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Thanks for the comments, guys.


For those of you who've read the above thermostat DIY  and are put off by the perceived complexity, don't be. You're basically standing in front of the engine compartment (not lying on your back under the car. Bonus!) undoing nuts/bolts/hoses. Can be achieved without taking the plenum off as well. There's some wrestling involved, for sure, getting the 'stat out but it's not a particularly technical job at all. Have confidence. Different matter for the TPS and Fuel Tank Breather Valve (and its associated pipe work) - they were a bitch of a job mainly because of difficult access and me having banana-like fingers.


Once you get hands-on and into the 'nuts & bolts' of thermostat replacement, the suggested sequence of operations in the DIY write up will make a lot more sense than if you're just a casual reader of the thread who's never done the job.


Hope the write up assists those attempting to find the 'Holy Grail'  of trouble-free M5 thermostat replacement! Certainly worked for me....after the initial trial & error.

Edited by DepthHoar

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Good write up for those who might be a little hesitant to do this themselves. I echo your comments though, looks tricky but isn't really. I have had the thermo housing off twice now (once to replace the stat and once to replace the oil separator hoses) and I never had any issues with the o rings at all. Just use vaseline to ease their passage back in, easy peasy if you take your time and be careful.


Very handy DIY on the hoses, good work!

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I just wanted to update this thread with a little thank you note to DepthHoar. Using his send "better, more logical" method I did the job super quick and without any woes with the o-rings.


Just a tip to others - I didn't find it necessary to remove the plenum, which saves a fair bit of work. It took a little persuading of the thermostat, but as the two halves are sprung there was enough give that I could flex it a little to pop in.


Thanks again DepthHoar! :D

Edited by chrisw_uk

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Yes, plenum was in place for my second, final and successful attempt. Failed miserably on my first attempt with the plenum off - see the post for details . Took the plenum off to give access to all the other bits I was going to replace at the same time as the 'stat. Taking the plenum off does give better access to the stat but removing the plenum is not absolutely essential.


Chrisw_uk: Glad you found the post useful and that it worked out for you!


Did you replace the round metal/plastic seal in the thermostat housing?

Edited by DepthHoar

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