1985 745t

Brickers:

Over the past few months I've been having an internmittent problem with my turbo and car's driveability. What happens is that when the turbo enters the yellow or spools up and starts boosting, the engine seems to retard. The more boost I give it the worse the engine seems to perform, until it actually seems to kind of stumble under full boost (floored, pinned at top of yellow on boost gauge). When the car is experiencing this problem the best performance is usually found by keeping the turbo needle right at the top of the turbo gauges black area.

The problem is generally in place for an entire trip, meaning if it happens it will happen non-stop from the time I turn the car on until I turn it off for a little bit, with no let-up whether I drive easy for a bit or not. Also, when this happens I usually end up seeing a wet spot around the oil filler cap on top of the crankcase cover. Whether that wet spot is symptomatic or just coincidental I have no idea.

At first I figured the problem was just excess crankcase pressure, and replaced the oil separator, cleaned all the tubes, and replaced the plastic split. This seemed to help for a little bit but didn't solve the problem. I'm now thinking this is a faulty knock sensor issue, and that the knock sensor is incorrectly retarding the timing. However, would that cause excess crankase pressure?

Is it possible the oil return line from the turbo is getting intermittently blocked somehow, and that that is causing the engine to stumble under boost and leading to excess crankase pressure? What would cause it to happen infrequently AND to last for the duration of a trip?

Would a faulty CBV working intermittently cause the car to stumble under boost and increase crankcase pressure? Maybe the CBV is somehow getting stuck open or closed?

I'm at wits end here, and am eager for any advice folks can offer. I'm planning on replacing the knock sensor regardless because I think its dying at 220k miles and is worth swapping out, but would like to finally fix this problem once and for all.

Thanks,
rt