V12 high oil useage update (archive)

About 2/3 months ago Mikael posted regarding high oil useage and if this was normal. The responses varied greatly. I have some new information. I was using about 1 quart every 400 miles. There were absolutely no indications there was a problem. I found a symptom by wanting to add oil while engine running and noticed a hissing noise emitted from the oil fill cap upon removal. Using my hand next determined this was pressure and not vacuum. Replacing those PCV valves has eliminated the high crank pressure and tonight after checking the oil since I last added oil about 1200 miles ago and the oil level has not changed. A 1200 mile run may be premature but I'm claiming success especially at $7/qt AMSOIL synthetic.

There were no engine oil leaks anywhere except the filler cap baseplate. I'd have a teaspoon or so in the little catch weekly. I almost hate to guess what and where, the previous owner and I have been filling up in this car with all that oil. Actually I think the engine had to be burning it and the catalytics completing the burn. Strangely there was never any smell or smoke from the exhaust system. Those cats must be very good at what they do.

A little theory on the PCV system as I think it works. The actual valve on the valve cover is a two stage valve. If you blow through it in the proper direction, very tasty BTW, a low air flow will go through easily (1st stage), whereas a high flow air pulse (blow real hard) will cause the valve to shift to a low flow condition (2nd stage). The high vacuum of the intake should cause the valve to goto stage 2 as a normal condition and allowing a restricted flow from the crank to the intake. If it didn't I could see a situation where the intakes would draw too much oil vapor and again high oil useage. There is an additional input to this valve coming from a check valve located in the boot between the MAF sensor and DK throttle motor. And in all normal circumstances this check valve is closed and not allowing any flow toward the PCV valve. It's my bet that BMW built in a safety feature so if for some reason the PCV valve malfunctioned and remained in stage one, then the check valve would open and allow fresh air into the intake via the malfunctioning PCV valve. If this safety circuit were non existent you would be losing alot of oil going into the intake and your MAF reading would be off since now the crank is providing some amount of air instead of being drawn in by the normal pathways. With a bad valve your MAF reading is still good as the check valve is down wind.

If this interp is wrong please correct me. I don't have pressure readings as I thought throwing $80 away on new valves just to see what happens was easier than a spaghetti setup of fitting, hoses and guages to nail down exactly the finite details. My hunch worked. No more positive crank pressure and no oil useage, so far.

Guy Simpson

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