Start the engine and let the idle settle, pull the engine oil dipstick and listen for the idle to drop then recover, and a slurping noise from the dipstick tube. If you get that then its likely that the OSV requires replacing. Its not an expensive job, but it is a bit of a pain to do as its located between the firewall and the intake manifold.
Have a read of this and its spells out what you need to do, Rear Manifold Cover aka. OSV | BMW 740iL | Magnum
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