Mysterious coolant leak found... heater valve

[wayneawh]

In the past couple months, I was looking for a tiny coolant leak that caused me to add ~300-500ml coolant every month.
I was super certain that the m62tu is not burning coolant and radiator, water pump and pipes are good. (Well I overhauled the whole cooling system one year ago)
Finally I managed to the leak 2 weeks ago... and it turns out to be from the heater valve, auxiliary water to be exact.
Coolant was seeping out between the electric pump and heater valve housing.

There is an o-ring used to seal the impellers to the heater valve housing and that o-ring wall thickness is just way too small.
It could be due to old age as I suspected at that time but the o-ring was actually still very flexible and soft.
I soon ordered a heater valve repair kit (ebay has lots of it) and the supplied o-ring looked about the same.
Well, I did not put too much thought in it and put everything back together.

Unfortunately, coolant level warning came up 2 days after install and I checked the repair again.
And not surprised, coolant is leaking from the same spot again.
Well, this time I went to my work place and looked for a viton o-ring with slightly thicker wall thickness.
Took apart everything and installed the new thicker o-ring.
1 week after and no leak. My theory is that the housing or impeller got warped a bit due to old age/heat cycles..

If anyone is looking for mysterious coolant leak, remember to check this spot and use a slightly thicker o-ring if doing a repair.
IMG_5199.jpgIMG_5198.jpg

[Chedley]

Thanks...Good to know. Will replace that O-ring promptly.

Talking about O-rings, I found quiet a few O-rings in the cooling system of my 22 years old car that should or must be replaced by thicker O-rings :
Most tubes and hoses have hidden O-rings at their extremities. Replace them with a similar size but thicker O-ring, that could fix or prevent mysterious and potentially damaging coolant leaks.
These hoses and tubes : #1,2,5,6,and 7 in this diagram.
https://www.realoem.com/bmw/enUS/sho...diagId=11_2267
Also, there is a big O-ring on the alternator, and 2 O-rings on the tubes connecting the water pump to the back of the engine valley pan.

[MotorMouth93]

In the vast majority of cases you just need to replace the O ring with one of the same size. There's a reason BMW chose the O-ring sizes they did and unless you've got warped plastic parts like OP there's no good reason to change that.

Also, the alternator gasket is not a typical O-ring, it does not have a round profile.