So I had an epiphany regarding the DME's VANOS low pressure errors last night.
The DME has no idea what VANOS pressure really is. None. Not Possible. Having taken apart more than a few now, I don't know why I never realized this sooner. There is no pressure sensor on the S85 VANOS high pressure oil system. If there was, the ISTA/D diagnostic routine for measuring pressure wouldn't have to connect a pressure transducer on the "T" fitting to carry out the diagnosis, it would just poll the DME for the pressure reading.
So, what causes this code then?
It seems to me that the only way the DME would know if pressure was low is if the VANOS actuators were adjusting timing at a slower rate than expected. This would lead the DME to interpret that as low control oil pressure, this triggering the code.
What does this mean?
I had this on my car 3 years ago, only on very, very cold Maine winter mornings. When my injector failed and I opened my bottom end, I replaced the VANOS pump, the high pressure line, the rod bearings, and switched oils. I also replaced 3 of 4 VANOS solenoids before I started the engine back up. This doesn't help in diagnosing at all, but I never saw the error again. I never received it during any summer months before either.
I know the VANOS pump wasn't the fault, I replaced it with a brand new one to upgrade the gears and my old one is running merrily around in another engine for 25k miles now.
It's possible the VANOS HP line was at fault, or I believe more likely is the VANOS solenoids.
It's amazing we (collectively) have never realized/discussed this before. It really has to make you wonder how many BMW techs over the years could have been lead down the path of pump/line replacement when really the solenoids could have been at fault. Often it accompanies two VANOS bank actuator errors and I know they love to replace those.
Just something I figured I should add to this thread. Still trying to work out the mechanics of the error in my head.
A very good addition indeed! The more info we gather, the better off we all are. I haven't seen the error in a while now, not since I've been garaging in a temp controlled environment and keeping the inside temp at around 60F. As long as I don't leave the car for two weeks (happens.. on long trips away), then I don't get the error.
There is merit to what you're saying. A lot, actually. If the oil seeps to the bottom of the block on a long timeline, the VANOS solenoids can be starved of lubrication pressure initially, at the very first crank. Just enough for the DME to try to slam everything to the "Cold Start" position and not get what it asks for immediately causing a CEL.
Either way, I tend to arrange my inspections in the warmer half of the year...