The BNW B58 has gained a reputation for drinking coolant.
There are a number of causes, not least of which is a failure of the plastic body of the oil filter housing but by far the most common cause of coolant loss is due to simple evaporation through the reservoir cap.
In a conventional design, the cap is securely tightened down against a rubber washer but the BMW relies on the use of O-rings which creat an interference fit in the neck of the expansion vessel.
With time, these O-rings dry out and fail to seal. The resultant build up of crusty deposits serve to make matters worse. The cure isn’t to replace the cap and achieve more of the same. Simply remove the O-rings, clean all the deposits off the cap and inside the filler neck of the expansion tank with a soft cloth dampened with a little kettle descaler or similar. Finally, lubricate the O-rings with a little silicone electrical grease ( I used Dow Corning DC4 electrical compound grease or Dow Corning Molycote 111 ) and also the inside neck of the tank. Wipe off all the excess to produce a sheen.
I retreat mine annually and it’s a ten-minute job.
No coolant loss and the car is now 6 yrs old and still using the original 2 caps!!
