Had the exact same situation on my 2013 X5 and yes, it WAS the expansion tank! Leaked like that for weeks as I was searching. It doesn't fail in the same way as in an E38, E39 or even E53 where it is catastrophic within a few days.

AHA! Yeah this leak is definitely playing hide and go seek. Brutal weather coming. Taking it to my mechanic after this weather. It shows up sporadically. Governed probably by the way I drive. I wish it failed like my e39 does. Until you guys shared this, that what was changing my paradigm. Funny how cold weather totally fudges with our rides! %$#*&!

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Cold hurts my joints why not the car's also

So since I haven't done this expansion tank before, from you that have, is there anything else I should replace "while I'm in there" type of preventative maintenance thing...?

TIA!https://uploads.tapatalk-cdn.com/202...4968012a55.jpg

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I'm also seeing a small coolant leak forming under the front driver side of the vehicle. I am going to use radiator dye to find the source.

I had expansion tank replaced on my 09 E70 3.0 about 2 months ago. It wasn’t a super slow leak, but it did leak only while driving though. I also had vent pipe replaced because I broke it while replacing the tank. That hose was really brittle though and I’m happy I had it replaced. When I took the old tank out I couldn’t see any cracks, so still not sure where it was leaking from, but the new tank did solve the problem.

Thanks for this. No way it was the vent hose though right?

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If your doing an expansion tank? Some come with a new cap included. Some don’t. But make sure you replace both the cap and tank.
As above, not a difficult job. Worst part for me was getting that hose clip off the bottom of the tank. Once off, it was a quick job. Not worth the 12oo the dealership wanted.

You just reminded me of something I thought was strange when you mentioned the cap. The cap loosens itself... I put a little bit of antifree in [its a miniscule amount] and occasionally when I return to refill, the cap is loose! I make sure to tighten it "clicks" and it loosens by itself! Is this indicative of a specific problem OR do these caps go "bad?" Hell, maybe I should start with a new oem cap...

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Well, I am not an expert on this particular problem per se but, with that new information- if if were me I would certainly try a new cap. Especially since you are not overheating or losing large amounts at once and not preparing for a long trip that could render this catastrophic.