The plastic oil separator is mounted on the side of the engine, below the intake manifold. Several plastic hoses connect it to the intake manifold, valve cover and dipstick tube. Depending on the engine configuration, it may be possible to get to the oil separator by removing some surrounding components, but without removing the intake manifold. Or if you’d rather not torture your hands, remove the intake manifold as an assembly to gain access to the oil separator.
Engine heat exposure may have made the plastic hoses brittle, and you may find them difficult to disconnect and remove without damage. Your best bet for a long-lasting repair would be to replace all of the related plumbing while everything is apart. The job will also go faster because you won’t have to worry about breaking any of the plastic parts during disassembly. Now is also a good time to take a look at the dipstick tube and O-ring, as oil leaks in this area also are common. BMW offers an updated dipstick tube assembly for some models.
In addition to trapping oil in liquid and vapor form, the oil separator may also trap water vapors. Under normal conditions, water vapors are consumed by the engine, as the oil separator directs them, along with oil vapors, to the intake manifold. In extremely cold weather conditions, water vapors accumulating in the oil separator, the oil dipstick guide tube hose or the orifice in the dipstick tube may freeze. If the oil separator’s internal valve is stuck closed when this occurs, higher-than-normal crankcase pressures resulting in valve cover leakage and/or breakage may occur. If the oil separator’s internal valve is stuck open when this occurs, it could cause an oil hydro-lock condition and internal engine damage.
BMW has addressed this problem with an updated, insulated oil separator and connecting hoses. The oil separator looks like it’s been wrapped in an insulated blanket. An updated dipstick guide tube is also available to address this problem. If you have the engine apart to deal with a failed oil separator, make sure the updated parts are used, to prevent later cold weather problems.