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How to repair BMW Valvetronic systems

Friday, September 1, 2017 - 07:00
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The Valvetronic system also incorporates phasing into the valve opening event to improve charge motion in the cylinder and allow for better air\fuel mixing. Phasing can differ slightly among different generations of Valvetronic. At minimum lift, one valve can open while the other stays closed. As lift increases the valves may open together then one valve can lead, or open further than the other and then the second valve will catch up to the first near the end of the opening event.

The Valvetronic eccentric shaft has minimum and maximum end stops built into the cylinder head to limit rotation. On the 6 cylinder N52, the minimum stop is screwed into the cylinder head and the maximum stop is a cast portion of the head. The Digital Motor Electronics, or DME, computer will learn the end stops when an implausible value is determined during the start procedure. A scan tool may also be used to run the limit learn procedure. This should be done anytime the valvetrain is disturbed or serviced.

SERVICE NOTE:  Keep in mind the servo-motor must be removed to replace a valve cover gasket on these engines. The motor is under a preload with the engine off and the eccentric shaft should be placed in the minimum lift position prior to removing the servo-motor. The scan tool can command this position on 1st generation systems. On 2nd generation systems there is a 4mm Allen socket built into the end of the motor armature and is accessible at the rear of the servo-motor. The motor should be unplugged and the armature turned clockwise to the minimum lift position prior to removing. If the motor retention bolts are removed without performing this step the motor could be shot out of the cylinder head cover and the eccentric shaft gear teeth may be damaged causing a very expensive mistake.

The DME must know the position of the eccentric shaft at all times. This is accomplished with the use of an eccentric shaft position sensor. V engines will have two sensors, one for each bank, while only one is needed on the in-line six engine. This sensor is a complex magneto-resistive device with two sensors monitoring the position of a magnetic wheel mounted on the end of the eccentric shaft. One sensor is called the measuring sensor and the other the evaluation sensor. The measuring sensor is checked more frequently and the evaluation sensor is used for plausibility. This sensor transmits data and is not a linear device like a throttle position sensor, so scope testing the sensor will not reveal the eccentric shaft position to a technician although it is an interesting device to scope. The scan tool can be used to monitor eccentric shaft position which is usually displayed in degrees of rotation from zero degrees, (min stop) to about 225 degrees, (max stop).

SERVICE NOTE:  This sensor protrudes through the valve cover and is easily damaged during valve cover service. They are expensive so be very careful during service. Many of the N52 six cylinder engines have problems with oil intrusion into the sensor so a good idea during any service requiring valve cover removal is to check for oil in the sensor wiring plug connection. If oil is found then sensor replacement is recommended.

Eccentric shaft mounted on N52 head with valve cover removed.
Eccentric shaft sensor magnetic wheel seen with sensor removed.

The third generation valvetronic system saw major changes in design. Gone is the large DC servo-motor, replaced by a much smaller AC current brushless motor that is more efficient and responsive. Also gone is the eccentric shaft position sensor. The brushless servo-motor has an integral position sensor. The third generation design is more compact and the servo-motor sits in a well in the cylinder head and is exposed to engine oil. The rapid response of the brushless motor allows BMW to claim cylinder selective lift adjustment to improve engine smoothness and idle quality along with idle speed control.

Additional service notes

As already mentioned, the concept of throttling the engine at the intake port allows for high manifold pressure, or low vacuum, at idle or low load conditions which reduces pumping losses. While the engine could run with atmospheric pressure in the intake at idle, there is a requirement to have a small vacuum present to allow for crankcase ventilation and charcoal canister purging. The DME targets 50 millibar of vacuum at idle which is equal to about 1.5 in/hg of manifold vacuum. This vacuum is maintained by adjusting the throttle angle to about 3.5 to 4 percent at idle. While the throttle appears mostly closed, the airflow through the engine at idle is a function of valve lift, not throttle opening.

There is a differential pressure sensor on the intake manifold that reads actual vacuum, not Manifold Absolute Pressure like most manifold pressure sensors. The scan tool will display 0 to 1 millibar at key on engine off, not 980 to 1000 millibar which would be atmospheric pressure. Normal idle should produce 40-50 millibar readings.

If the engine is a turbo, the pressure sensor will read like a MAP sensor and show almost 1000 millibar at key on engine off so pay attention to what the manifold pressure reading is with the engine off to determine which type of sensor is used on the engine you’re working on. This is important because the pressure sensor value will let you know if the engine is running in Valvetronic control or throttle control. If an engine fault occurs such as a VANOS problem, the engine will default to throttle load control and the manifold sensor will read approximately 600 to 690 millibar of vacuum, the eccentric shaft will be close to maximum lift.  While idling, if the eccentric shaft position reads less than 30 degrees and manifold pressure is 50 millibar then the engine is running in Valvetronic control. It is crucial to understand the difference when diagnosing running problems on this system.

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