1995 BMW 530i
Motronic Diagnostic Codes 1222 & 1221

M60 V-8 engine abruptly idled roughly.  Check engine light illuminated.  Soon heard a loud noise similar to a slipping belt.  At city street speeds, engine seemed smooth and sound disappeared.  Motronic reported 1222, Oxygen sensor lean/rich control stop, and 1221, Oxygen sensor #1, i.e., passenger side.  The significant DME code is 1222, oxygen sensor lean/rich control stop.

To avoid destroying additional and expensive parts, stop using the vehicle until repaired. 

With symptoms, check fuel pressure, at idle.  If poor idle and too high fuel pressure, check vacuum.  If vacuum too low, remove oil dip stick and listen for sucking sound.  If sucking sound heard, you have confirmed an air leak from the crankcase to the intake manifold.  The likely cause is the vacuum plate on the rear of intake manifold.  The vacuum plate is part of the positive crankcase ventilation system.

Don't wait for the car to exhibit heavy black smoke on acceleration, another symptom of vacuum plate failure.  If you wait, you will need to clean the intake manifold of liquid oil.  The heavy black smoke can coat your oxygen sensors and can negatively affect your catalytic converters.

If you don't need to clean the intake manifold or if the mess is small and toward the rear, you can avoid removing the intake manifold.  Similar to the water pump, there is a pipe that runs under the intake manifold from the front of the engine to the lower driver's side of the vacuum plate.  Use a 90 degree pick to lift the retaining clip on the rear pipe end and slide the pipe slightly forward to disconnect the pipe from the vacuum plate.  Remove the passenger side overflow hose from the cooling system reservoir and unbolt reservoir from the firewall before folding reservoir to driver's side.  Disconnect the two vacuum hoses on the rear of the intake manifold.  Remove the engine insulation between the firewall and the engine rear.  Remove the internal Torx headed bolts attaching the vacuum plate to the intake manifold being extremely certain to fully seat the Torx wrench squarely in the heads.  The vacuum plate will now easily separate from the intake manifold and lift out.   Clean any mess.  Replace the gasket.  Replace the vacuum plate and put everything back as it was.

Before taking my vacuum plate apart, I shook it and there was a loose item inside.  The new vacuum plate had no such sound.  The old vacuum plate was internally clean at ~85,000 miles.  Poor oil changing intervals or poor quality oil can gum up this part causing it to fail.  On the back of the vacuum plate is a large diaphragm with a spring and large fiber washer protecting the diaphragm from the edge of the spring.  My fiber washer broke in two with one half of the fiber washer loose in the chamber.

I am not totally certain how the vacuum plate is designed to work so consider this paragraph as speculation.  At idle, I believe the diaphragm seals the intake manifold from the crankcase.  When the intake vacuum decreases with a more open throttle, I believe the diaphragm opens the port to the crankcase permitting positive crankcase pressure to be transferred to the intake manifold.  Since the vacuum in the intake manifold is low when the throttle opens, the oil separator can do its job.  If the vacuum plate is defective and remains open at idle, the intake manifold vacuum is almost totally lost and because of the otherwise high intake manifold vacuum, oil is sucked through the oil separator into the intake manifold.  Fuel pressure is regulated high because of the low intake manifold pressure (to keep the relative pressure constant) causing a rich mixture plus oil sucked from the crankcase.  This is a major adverse malfunction.

If you dislodge the pipe running from the front of the engine under the intake manifold to the vacuum plate on the rear of the intake manifold, you can reseat it without removing the intake manifold.  The small stub pipe to which it attaches in the front is visible in front and under the intake manifold.  When you have the long pipe reattached, you will also see it under the front of the intake manifold.  To avoid dislodging the pipe, separate the pipe from the vacuum plate by moving the pipe forward before moving the vacuum plate rearward to achieve clearance needed to lift the vacuum plate from the engine compartment.