Identification of VANOS Reference position is performed identically by both Siemens and Bosch DME. Immediately after switching on the engine, in first 20 .. 30 turning cycles of camshafts (3 .. 4 seconds):
a) VANOS solenoids are turned off;
b) VANOS mechanisms are in a “relaxed” position, it means – camshafts are in positions, in which they are adjusted;
c) DME measures position of the camshafts, using the signal of the VANOS sensor.
The average value of all measurements (if they have been correct) is recorded in the DME memory for the current driving session. When starting a new driving session, VANOS positions are remeasured.
In the image: VANOS Reference positions are marked
DME allows the following offset of the Reference position:
a) +/-10 degrees for Siemens DME, the error message is recorded immediately if the Reference position doesn’t fit in the corridor;
b) +/-11 degrees for Bosch DME, but the error message is not recorded, if the value doesn’t fit in the corridor till/above 3 .. 4 degrees.
From my experience: Siemens DME processes the signal of the sensors more acurately, the Reference position is less dependent (or even is not dependent at all) from “sensitivity” of the sensor and its changes (more about this read here). By contrast, the Reference position, detected by Bosch DME, is affected by “sensitivity” of the sensor, when it is reduced (when the sensor ages), Reference position tends to float “to the left” (INPA visualization).
Several nuances of the Reference position topic:
a) as, during the time of indication of Reference position, the VANOS valve is closed, the Reference position is NOT AFFECTED by oil pressure (of course, if the valve is not leaking)!
b) turn on INPA, open VANOS menu, turn on the engine. In first seconds you will see, how the identified Reference position changes. If it changes for more than +/- 0.2 .. 0.3 degrees, obviously – either the VANOS mechanism is worn out (it has an angular free movement), or (less likely) the valve is slightly leaking or the sensor has increased jittering.
c) start the engine several times. If the stored Reference position changes significantly (above +/- 0.2 .. 0.3 degrees) – look for a cause of the problem. Cause of the unstable Reference position could be (sometimes) a leaking valve, or (less likely) wear of the VANOS mechanism itself or problems of the sensor.
Elements of both VANOS hubs (inlet and outlet) are identical. It allows for detecting the damaged element easily. For example, if the Reference position is incorrect:
a) if there is an error message regarding the signal of the sensor – switch both sensors (inlet and outlet) by places. Observe, if the error message also “follows” the sensor;
b) if there are no error messages regarding the sensor (its signal), but exactly regarding Reference position – switch the inlet and outlet VANOS valves by places. If the error message still is here – obviously, the leaking of the valve is not one to blame. Then you have to check the performance of the VANOS hub itself (possibly, it can stuck) and the camshaft positions for the turned-off engine.