Variable Valve Lift Monitoring (Including Switching Valves)

11. Variable Valve Lift Monitoring (including switching valves)

11.1 Variable Valve Lift System

11.1.1 General Description
In order to optimize engine torque, performance and emissions, the vehicles of this OBD group are equipped with a two step variable valve lift system on the intake side. (VVL)

The electronically control of the VVL position depends on engine speed, temperature, battery voltage and camshaft position.

The mechanical switch of the valve lift takes place via regulation of the oil pressure by a three-way valve affecting the switchable bucket tappet.

The monitoring of a correct switching takes place on two ways:
- diagnosis by behavior of the lambda controller
- diagnosis by camshaft oscillations

11.2 Switch VVL diagnosis by behavior of the lambda controller

11.2.1 General Description
This diagnosis detects, if one cylinder bank sticks at low or high valve lift (e.g. due to a mechanically defective solenoid valve).

During low valve lift the diagnosis value A is calculated (difference of the lambda controller output values of both cylinder banks at low valve lift).

After switching to high valve lift the diagnosis value B is calculated (difference of the lambda controller output values of both cylinder banks at high valve lift).

If the difference between the diagnosis values A and B exceeds a certain threshold a failure is detected.

11.2.3 Flowchart






11.3 Switch VVL diagnosis by camshaft oscillations

11.3.1 General Description
This diagnosis detects, if two cylinder banks stick at low or high valve lift (e.g. due to a mechanically defective solenoid valve on both banks).

The valve actuation, i.e. opening and closing, produces oscillating friction torque and thus camshaft oscillation. Different valve lifting curves lead to different oscillations.

If the change of the camshaft oscillation amplitude before and after the valve lift change request is smaller than a threshold, i.e. no change of the oscillation, then a failure (valve lift change not possible) is detected.

The algorithm is the same for each cylinder bank.

11.3.3 Flowchart