BMW E39 5 Series Oil Fill Cap

I replaced the oil fill cap on my E39 530i because it was very difficult to twist and remove. The rubber seal had hardened into plastic and covered in sludge. The seal is bad so I did not use silicone grease on the seal to ease in removal.

The two plastic prongs over the metal tabs are missing on the original cap. I don't know if it came from the factory without the two prongs and were added to new caps or if the previous owner broke them.

I have been working on resolving the intermittent P0171 and P0174 codes as described here and have been reading various BMW forums for solutions. I came across a few posts that say a bad oil filler cap seal will cause these codes. It seems like a stretch but I needed to replace the cap anyways.

Picture of the old oil filler cap versus new.

Oil filler cap Part # 11127509328 

• Compatible with E36, E46, E60, E53, E34, E66, E65, E70, and E85 BMW models.

BMW Oil Filler Cap Leak - E90

The seal in the BMW oil filler cap will begin to harden and deteriorate which allows oil to leak. Judging by the amount of posts I see in BMW forums, this seems to be a very common issue.

To stop the leaking:

1. Buy and replace the oil filler cap. The new cap comes with a seal. 
• Part # 11128507153 for E9X BMWs with the round oil filler cap. Earlier E90, E60, E85, and late model E46s use this square oil filler cap.
• Part # 11127509328 for earlier model BMWs with the square oil filler cap.  
2. Carefully remove the existing seal. Clean and reinstall by reversing the ring. I would only try this if the leak is very small and I don't want to spend $15.

My leaking oil filler cap:  

I changed my oil and the oil level read max in iDrive. A couple of weeks later, the oil level dropped and it didn't seem right that I would lose oil that quickly.

I noticed dried oil around the oil filler cap and oil leaking down onto the valve cover area. I pulled my oil cap and it looked like mustard. After doing some research and asking some friends, I came to the conclusion that my oil filler seal was bad and condensation was coming in and mixing with the oil and looking like mustard on the cap. I replaced the oil filler cap and oil consumption is now normal and no more weird mustard looking oil under the cap.

Below is what my old oil cap looks like and the seal that is indented. There is caked oil all around.