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Discussion Starter · #1 ·
For the m140i?

I cant find one that fits mine apart from NON-OEM ones, unless 'first line' is the oem brand?

I just need a bmw expansion tank coolant cap to replace mine to see if it stops the coolant loss.

If it doesent work god knows what the issue is i dread to think!!
 

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Don't smear it with grease or try and adapt the cap at all. Presume you remember all my issues with this? Fix it properly.

You have to go to BMW and get a proper cap. I couldn't believe it was just the cap that was the issue, I was starting to worry big time.

I went to a dealer and ordered one - think it was 18 quid odd. Problem solved instantly. Only thing I have ever done with a dealer.
 

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Discussion Starter · #4 ·
Don't smear it with grease or try and adapt the cap at all. Presume you remember all my issues with this? Fix it properly.

You have to go to BMW and get a proper cap. I couldn't believe it was just the cap that was the issue, I was starting to worry big time.

I went to a dealer and ordered one - think it was 18 quid odd. Problem solved instantly. Only thing I have ever done with a dealer.
Thanks bud ill pop into my local dealer and ask them 👍, fingers crossed its that cant see any obvious residue around the cap but theres no other symptoms.

Replaced a few coolant caps over the years on other cars.
 

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Don't smear it with grease or try and adapt the cap at all. Presume you remember all my issues with this? Fix it properly.

You have to go to BMW and get a proper cap. I couldn't believe it was just the cap that was the issue, I was starting to worry big time.

I went to a dealer and ordered one - think it was 18 quid odd. Problem solved instantly. Only thing I have ever done with a dealer.
There is absolutely nothing wrong with a smear, not a dollop, of silicon grease on the O rings.
I'll pretty much guarantee the new cap O rings aren't dry, but lubricated, you are just relubricating!
Plastic on plastic threads aren't the best idea, and a bit of lubrication just stops them binding and becoming stiff.
 

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There is absolutely nothing wrong with a smear, not a dollop, of silicon grease on the O rings.
I'll pretty much guarantee the new cap O rings aren't dry, but lubricated, you are just relubricating!
Plastic on plastic threads aren't the best idea, and a bit of lubrication just stops them binding and becoming stiff.
Agreed - I added silicone grease to my new original BMW cap and it's never leaked since then, with the bonus that it's very easy to remove the caps now.
 
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You could argue there is no point replacing the cap . . . you'll just get another one the same!!!
My car "uses" a small amount of coolant but not to such an extent that it remains a problem- The issue is that both coolant caps aren't conventional caps insofar as you tighten them down against a sealing washer - if they were, there wouldn't be any problem at all.
For some reason, BMW decided to reinvent the wheel and both caps "seal" (if only they did) by virtue of O-rings which are an interference fit in the neck of the reservoir bottles. These O-rings have a tendency to grip and ruck up when the cap is both tightened and/or removed. In turn, this will not only severely damage the O-ring and result in significant fluid "loss" (mainly evaporation) but can lead to the cap being very difficult to remove.
My car was new in August 2016 and about 3 months into ownership, curiosity got the better of me and I started fiddling around under the bonnet.
One of the first targets of my curiosity was my coolant reservoirs and I decided to check they were both to the correct level. In the absence of a manual, I attempted to remove the cap from the main reservoir but to my horror, it just got tighter and tighter and tighter and to such an extent that I began to doubt that it unscrewed in the normal manner. Anyway, I persisted and eventually the thing came off. There was loads of dried coolant deposits withing the cap, neck and O-ring. After studying the thing for a minute or two, it was patently obvious how it was designed to work and so I pulled the O-rings, inspected them for damage and cleaned up the cap as best I could and the neck of the reservoir bottle. Having removed all signs of dry residue and cleaned it up perfectly, I wiped a thin smear of silicone (in my case I used Dow-Corning M111 Molycote) onto the O-rings and reservoir neck. I then wiped this off with a soft cloth to leave nothing more than a shiny surface in each case as I was concerned not to get silicone into the coolant just in case. The caps turned almost without resistance, right up to the final click.
Periodically, I check for a build up of deposits, clean as necessary and reapply grease before wiping clean again . . . hardly ever get deposits these days but still lose a little coolant which is not surprising with such a crappy design.
ONE EASY MISTAKE to make is that peeps might think that silicon grease is just any old grease and they will reach for their tub of Castrol or Comma chassis or multipurpose grease . . .BIG MISTAKE!!!!!
Be very, very careful to use only silicone grease - if in doubt . . don't use it. It must be silicone-based. Normal greases (of all varieties) are based on mineral oils and they will cause the O-rings to swell. If this happens, they will ruck up even worse and assuming you can even get the cap off, they are bound to ruck up and damage themselves.
I suspect that anyone who has cleaned their cap and is still losing significant quantities of coolant either has a problem elsewhere (always possible) or more than likely, their O-rings were fubarred on first removal of the cap.
If a new cap is only £18 - probably easier to buy one than fanny around trying to source the correct sized O-rings but I would lube the thing on first use and thereafter, keep it lubed.
The only alternative is to pay the price and accept the cap is an arse to remove and that your car will "drink" coolant . . .
 
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