[ronburgandye92]

I have a 2011 335i N55 e92, my car the other day threw a light and said it needed coolant which is normal after a while. I bought some coolant threw it in and in a matter of 2-3 days the light popped up again saying it needed coolant so I put more in again. After I began to think I had a small leak so I brought it to my friends body shop to lift up the car. When lifting the car A lot of coolant seemed to be leaking from the bottom of my car near the front passenger tire which the coolant tank is on the car. so I had it brought to BMW and they cannot seem to find the leak at all. Checked the car with a pressure test, drove the car and let it sit and also checked the hoses and water pump. BMW told me there is no absolute leak in the car what so ever. IDK how this can be because I saw the coolant leaking out with my own eyes but now its gone.

Has this happened to anyone before?

[JETmn]

The expansion tank is a common issue, as is most of the coolant system on these cars. My whole system has been replaced under CPO. It is probably a hairline crack that is opening up when it gets hot, but closes when it is cold. I would replace the expansion tank.

[blau iii]

I don't know if the n55 is different but in my n54, i had the same problem. There was a small line that ran under the snorkel by the grill that had a pinhole leak.

[olivojoe]

Originally Posted by JETmn

The expansion tank is a common issue, as is most of the coolant system on these cars. My whole system has been replaced under CPO. It is probably a hairline crack that is opening up when it gets hot, but closes when it is cold. I would replace the expansion tank.

OP, the exact thing happened to me a few months ago, and above explanation is spot on.

The dealer did a pressure test to 110% operating temperature and couldn't find the leak. After topping off again and going back to the dealership, they took off the expansion tank and visibly identified a hairline crack that was opening up based on combined pressure and temperature, not just pressure. Apparently this is relatively common with these polymer expansion tanks. Also, the dealership replaced the aged rubber hose that attaches to the tank even though it wasn't damaged, but the SA said that once they are removed, they usually fail shortly thereafter so it is worth just replacing the hose with the tank all at once.

Good luck. Also if you are not still under warranty, it is not a very difficult DIY from what I hear.

[ronburgandye92]

Originally Posted by olivojoe

OP, the exact thing happened to me a few months ago, and above explanation is spot on.

The dealer did a pressure test to 110% operating temperature and couldn't find the leak. After topping off again and going back to the dealership, they took off the expansion tank and visibly identified a hairline crack that was opening up based on combined pressure and temperature, not just pressure. Apparently this is relatively common with these polymer expansion tanks. Also, the dealership replaced the aged rubber hose that attaches to the tank even though it wasn't damaged, but the SA said that once they are removed, they usually fail shortly thereafter so it is worth just replacing the hose with the tank all at once.

Good luck. Also if you are not still under warranty, it is not a very difficult DIY from what I hear.

I keep hearing that I will check next time I drive the car to see. The dealer is giving me back the car today because they could not find the problem. I will def check if it is a crack in the expansion tank because that is exactly where its leaking from where the tank is. According to the dealer they check the expansion tank and found nothing so I guess ill just have to see for myself.
Thanks !