PSA - Expansion Tank Cap
 

I did an entire cooling system refresh last month. All OE/OEM bits.

I was getting the deflated upper hose after cool down. Often associated with improper bleed of system. But, my temps were good and heating was good. No air in system.

One cause could be a bad expansion tank cap. Swapped the old one back.

That was it. No more deflated hose.

The cap, mahle behr, was bad. The old cap is uro.

So, watch out for bad bits even when buying quality parts.

Let me guess, you are that BMW maniac at the front of my subdvision?

Hi neighbour, lol.

P.S.

As we discussed already in person, I have also replaced my cooling bits including Mahle / Behr exp. tank cap. I got mine from FCP Euro and I don't have that issue.
So, who knows...it could be several different reasons...

Yup.... I'm the guy on the corner who always gets harrased for working on my bmws!

Not a huge deal about the cap.

Sometimes it's just a bad batch of caps that get through.

Just an info note for the less experienced to take note to check the simple things that may not be obvious after buying new parts.

Agreed.
Yes, this is global economy and unfortunately the quality of parts, regardless of being genuine BMW, OE or OEM, is simply lesser.

Which MAHLE/ BEHR cap did you have this issue with? I have a few different versions that I purchased. There are 2 with a fully black cap and and orange underside where it shows the 200 one made in the united kingdom and one made in Austria and the "PREMIUM LINE" cap that has the colorful images on the top of the cap that also says Made in Austria. For some reason my Tapatalk app is having issues uploading images.

It was the premium line version.

The cap holding a bit of vacuum that collapses the hose has nothing to do with it being faulty. In fact the cap which doesn't hold a bit of vacuum is more likely to be faulty and not allow pressure to build which is the primary function of the cap. It would be nice if they designed the cap so that the vacuum valve would work with gravity and not need a spring but it's probably not considered a "positive" enough seal. The cap that doesn't collapse the hose probably(hopefully) just has a weaker spring on the vacuum valve.