Due to differing levels of internal friction from one engine to another, each individual engine has a slightly different air requirement at idling speed. Thus the valve travel of the variable valve gear at idling speed is dependent on the engine's internal frictional losses.
The minimum valve travel is limited to a specific practicable distance. This is necessary in order that manufacturing differences in components do not result in severely uneven distribution of intake air to the individual cylinders. If the amount of air required to maintain idling speed is less than that achievable with the minimum permissible valve travel, the throttle valve closes by an amount sufficient to reduce the idling speed to the required level.
If the DME then nevertheless detects uneven running at minimum valve travel, the minimum permissible valve travel is increased. When valve travel is greater, the differences between individual valves are proportionally lower.
Example1: Minimum valve travel: 0.3mm, Difference in valve dimension: 0.03mm = 10%
Example2: Minimum valve travel: 0.6mm, Difference in valve dimension: 0.03mm = 5%
There is also a maximum limit to adjustment of the minimum valve travel as above a certain valve opening, the desired consumption-reducing effects of the Valvetronic system are lost.
Under certain conditions, wear can occur on individual intermediate levers in the variable valve gear. As the wear only occurs at specific points on the intermediate lever contact surface, it cannot be detected under all engine operating conditions. For that reason, there is a special system test for diagnosing intermediate lever wear which spans the entire range of adjustment.
In order to be able to start the system test, the following conditions must be met: Engine at normal operating temperature, air conditioning off, engine at idling speed, vehicle road speed = 0, accelerator pedal not depressed, engine not choked (Valvetronic not open).
During the system test, the engine is changed from unchoked to choked mode at idling speed. If there is intermediate lever wear, misfiring will occur during the change-over. The misfires are added up for each cylinder. If specific limits are exceeded, worn intermediate levers are diagnosed.