e39 M5 Thermostat Replacement

[e21Keith]

I've replaced the thermostat and in doing so have twice damaged the O rings that seal the three pipes at the back of the thermostat housing resulting in a coolant leak.

I've viewed all the videos on YouTube, read all the posts on all the forums but still don't understand what the trick is to make sure the seals don't get damaged. I cant see how you can line up the housing and get the tubes to go in straight.

The thermostat sits on top of the water pump, so maybe if I remove the waterpump and fit the thermostat to the pump before installing that will get everything to line up and it won't damage the seals?

Anyone done this job and can lend some advice?

[Hereward]

Have you lubed up the O-rings with a bit of silicone grease before assembly?

Edit: Have you watched this from about 25:35 onwards? Any help?
https://www.youtube.com/watch?v=KWr-YtDDM3c

Edited by Hereward on Saturday 14th May 10:58

[Depthhoar]

I too went through all this frustration when replacing the 'O' rings on my M5's thermostat housing. Like you, I'd read all the forums and tales of woe people had attempting this job. I concluded that luck seemed to play a part in success!

After a few abortive attempts, I eventually succeeded using this approach:-

1. Carefully clean the oxidation off the recesses where the 3 short alloy tubes eventually fit into the thermostat housing and the engine. Don't go mad with this. Avoid removing too much where the 'O' rings are actually seated - those areas didn't seem to have much corrosion anyway. I reckon quite a few 'O' ring failures occur due to them hanging up on the corrosion when the tubes are pushed home. I found out that most BMW dealers use a brand new thermostat housing when doing this job thus avoiding some of the corrosion-related difficulties.

2. Use some lube on the 'O' rings and orifices to ease insertion (ooo er!). Silicon grease, Vaseline or even KY jelly: you don't need a huge amount.

3. Insert the 3 short alloy tubes into the engine side orifices, not into the thermostat housing. Practice this without the 'O' rings to get the hang of it. They're a snug fit even without the 'O' rings. Once confident, do it with the 'O' rings in place on the tubes. (Best to have the 'O' rings on both ends of the tubes at this stage now). Push the tubes in carefully, squarely and with firm hand pressure. You'll feel them seat.

4. With all 3 tubes now in the engine side of the cooling system, tilt them up a few degrees but not much. The 'O' rings will squirm a little but they'll be fine. This helps later when mating the tubes to the thermostat housing.

5. This next bit is key, I think. Drop the thermostat capsule into the circular aperture of the water pump housing. (Best to rotate the thermostat in the w/p housing to get unrestricted water flow across the part of the thermostat that contains the wax.)

6. Having cleaned the corrosion off the 3 thermostat housing orifices, now lube them up.

7. Next stage involves lowering the thermostat housing down onto the thermostat capsule whilst at the same time lining up and lightly engaging the 3 tubes sticking out of the engine side. (With the tubes at a slight angle this should be fairly easy to achieve).

Keep the t'stat housing square to the 3 tubes and gently lower the t'stat housing down onto the t'stat capsule. At the same time use your fingers to lift the t'stat capsule up into the t'stat housing and snug it home so that it's a pretty much flush fit in the housing.

Now with firm pressure, and keeping everything square, push the t'stat housing towards the engine and rotate downwards onto the water pump housing.
Everything should now be lined up including the 3 bolt holes that hold the t'stat to the w/p.

8. Fill with coolant and test for leaks.

My early attempts failed I think because I put the t'stat capsule into the t'stat housing first and in so doing couldn't get the combined assembly to clear the lip of the water pump housing without massive pressure...and shearing off a couple of 'O' rings. Dropping the t'stat capsule into the water pump housing first was key to success, I think.

Looking back, I reckon I could do this complete job in about 2 hours now having taken more than double that on the first attempt.

Hope this helps! (I did a forum posting on this job on BMW5 a few years ago but the forum has been down for a couple of months but if it does come back up search for 'Cool Runnings..thermostat' and you will find it, with pictures!)

Edited by Depthhoar on Saturday 14th May 12:47

Edited by Depthhoar on Saturday 14th May 12:48

[e21Keith]

Thanks for the advice, really helpful.

I considered removing the water pump but after the dealer supplied two damaged water pump gaskets I gave up on that idea anyway.

I ordered another set of O rings from the dealer and stripped it all down again. Much easier to see what you're doing with the plenum off and having been there a week earlier I took it off again.

The housing was really difficult to get off because I'd trapped one of the O rings and damaged it. This time everything just went together like it should have the first time around. I left a coolant system pressure tester in place over night and no leaks, so I finished assembling everything this afternoon and ran it up to temperature. All good.

[Depthhoar]

Well done.

I remember clearly the feeling of relief when the car started up and was leak-free after my second attempt at the 'O' rings!

Apart from the frustration of the leaking 'O'rings, the most difficult part of the job was separating the thermostat housing from the engine when attempting to disassemble everything at the start of the job. It felt welded on. It eventually succumbed but only after a few taps from a plastic hammer and some desperate grunting and wrestling!