When a Valvetronic equipped engine is shut off the eccentric shaft will reset to a position that will allow greater valve opening than the minimum lift position to aid in starting. This is similar to how the idle air control motor is reset to a mid-travel position for the next start when the engine is shut off on GM fuel injected engines. This eccentric shaft position for starting is about 57 degrees on the in-line six cylinder engines.
One more area of interest to mention is compression testing a valvetronic equipped engine. Because the valve lift is controlled by the position of the eccentric shaft one would assume that BMW has a procedure to set the valve lift at a certain point to reach the expected compression pressure and this would be a correct assumption. There is a compression test procedure in the ISTA factory scan tool that has you adjust the eccentric shaft by turning the servo-motor with an Allen wrench until the eccentric shaft is at 176 degrees. This is done with the motor un-plugged. Once the eccentric shaft is set you continue with testing as you would any other engine. Normal compression values on the N52 engines are about 200-210 psi. Higher compression values can be seen with the eccentric shaft set at a lower lift position because the valve can close sooner and this will raise the effective compression stroke. I have measured over 220 psi at lower lift settings of around 100 degrees on the N52 engines.
The scope pattern seen in the accompanying illustration is the scan tool commanding a valvetronic limit learn procedure. The two lower waveforms are the voltage patterns from each of the two motor wires and the upper waveform is motor current captured from one of the wires.
|Waveform of a Valvetronic system learning the end stops.|
As the waveform shows, the bottom channel voltage is pulsed and the other channel above is held to ground to spin the motor to the end stop. When the current flatlines at the point marked 35 amps, the minimum end stop is learned. The polarity is then reversed, the middle waveform is pulsed to voltage while the bottom waveform is held at ground. The motor spins until the shaft reaches the maximum end stop and current flatlines again at 44 amps. These are known good values from a new servo-motor. The whole procedure lasts about 1.5 seconds.
The most common failures I have seen are bad eccentric shaft sensors. The servo-motors can go bad and will probably set over temperature codes for the motor drivers in the DME. Always check for service bulletins if a car sets Valvetronic system codes as there have been many programming updates to cure system codes. Get to know these systems because it seems they will be around for the foreseeable future on BMW engines.