Coolant cap O-ring replacement

MidtownMike · 12-06-2023, 08:53 AM

In the two years that I have owned the car (2018 F30 320i B48), it has been gradually losing coolant.
After having the notorious cylinder head to expansion tank coolant line replaced (under CPO warranty), I investigated a bit more and I noticed some powdery residue on the inner ridge of the coolant filler on the expansion tank (above the bottom O-ring of the filler cap). Decided to replace the filler cap (1.4 bar) and when I received the new one, I was happy to see that it had nice, round, chunky, fat O-rings, where the ones on the old filler cap had been flattened to squares, taking the shape of the ridges in the cap and had lost significantly in outer diameter.
Coolant caps on these cars are notorious for slowly leaking because of the design.

I topped up coolant to the Max line with a stone cold engine, replaced the cap, and drove a few 100 kms. Screwed off the cap again and noticed that the new O-rings had been flattened already, and even had some little dents in them.

Even though the cap is not a super expensive item, it is not my intention to replace it too often just because of the O-rings, and I would prefer to have a little supply of spare rings.

Does anybody know if these can be bought separately and are proprietary sizes, or whether it is just a standard O ring. If so, what material and diameter/girth? I wish I had measured the new ones before installing the cap! I saw that many O rings are made of Nitril-butadiene, but that material is only rated up to 110 degrees C, whereas the coolant temp usually hovers around that level (108-112), so would a silicone rubber ring make more sense?

Thoughts are appreciated!

pshovest · 12-06-2023, 02:16 PM

O-rings aren't the only part of the radiator cap that can cause leaks. The pressure vacuum seals can also leak and they aren't replaceable.
Flat spots on o-rings are usually not a problem.
The o-rings on my 25 yo M3's radiator cap are original, have flat spots and don't leak.
In 42 years of owning (14) Bimmer's I've only had (1) leaking radiator cap that caused coolant consumption and needed replacement.

Check compatibility of silicone with ethylene glycol before you make a change.

quakerroatmeal · 12-07-2023, 09:49 PM

Are you sure it's not other areas causing the leak? Such as the water pump or oil filter housing?

12-06-2023, 08:53 AM	#1
MidtownMike Lieutenant 357 Rep 416 Posts Drives: BMW F30 LCI 320i M-Sport Shado Join Date: Nov 2021 Location: Netherlands iTrader: (0)	Coolant cap O-ring replacement In the two years that I have owned the car (2018 F30 320i B48), it has been gradually losing coolant. After having the notorious cylinder head to expansion tank coolant line replaced (under CPO warranty), I investigated a bit more and I noticed some powdery residue on the inner ridge of the coolant filler on the expansion tank (above the bottom O-ring of the filler cap). Decided to replace the filler cap (1.4 bar) and when I received the new one, I was happy to see that it had nice, round, chunky, fat O-rings, where the ones on the old filler cap had been flattened to squares, taking the shape of the ridges in the cap and had lost significantly in outer diameter. Coolant caps on these cars are notorious for slowly leaking because of the design. I topped up coolant to the Max line with a stone cold engine, replaced the cap, and drove a few 100 kms. Screwed off the cap again and noticed that the new O-rings had been flattened already, and even had some little dents in them. Even though the cap is not a super expensive item, it is not my intention to replace it too often just because of the O-rings, and I would prefer to have a little supply of spare rings. Does anybody know if these can be bought separately and are proprietary sizes, or whether it is just a standard O ring. If so, what material and diameter/girth? I wish I had measured the new ones before installing the cap! I saw that many O rings are made of Nitril-butadiene, but that material is only rated up to 110 degrees C, whereas the coolant temp usually hovers around that level (108-112), so would a silicone rubber ring make more sense? Thoughts are appreciated!
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12-06-2023, 02:16 PM	#2
pshovest BMW CCA 69606 89 Rep 228 Posts Drives: '15 435i,'10 535i,'08 X5,'98M3 Join Date: Aug 2014 Location: DE iTrader: (0)	O-rings aren't the only part of the radiator cap that can cause leaks. The pressure vacuum seals can also leak and they aren't replaceable. Flat spots on o-rings are usually not a problem. The o-rings on my 25 yo M3's radiator cap are original, have flat spots and don't leak. In 42 years of owning (14) Bimmer's I've only had (1) leaking radiator cap that caused coolant consumption and needed replacement. Check compatibility of silicone with ethylene glycol before you make a change.
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12-07-2023, 09:49 PM	#3
quakerroatmeal Second Lieutenant 119 Rep 283 Posts Drives: 2018 430i Grand Coupe Join Date: May 2021 Location: DFW iTrader: (1)	Are you sure it's not other areas causing the leak? Such as the water pump or oil filter housing?
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