Diagnostic Overview - Variable Valve Lift (VVL) System
PCM DIAGNOSTICS: The PCM performs diagnostics for each VVL Solenoid High Side Driver (HSD) Circuit to detect an open or short circuit. If no circuit faults are detected the PCM runs a rationality diagnostic to detect Rocker Arms that are stuck in the High Lift or Low Lift positions. The PCM monitors for an erratic MAP Sensor signal as well as the Misfire, Knock, and Individual Cylinder Fuel Control (ICFC) monitors to detect a Rocker Arm that is stuck in an incorrect position. A Rocker Arm or arms can stick due to an oil pressure issue, a mechanical failure of a VVL Solenoid (including O-rings) or a faulty Rocker Arm. When a Rocker Arm fault is detected the PCM will begin to record in-field data. If the fault condition goes away during the next drive cycle the data collected will also be cleared.
- Low Lift stick condition: A single Rocker Arm that is stuck in low lift may not be detected and the system will continue to operate normally. However, some misfire and engine roughness may be felt. One Rocker Arm stuck in Low Lift mode will cause a misfire at higher engine speeds. Two or more Rocker Arms stuck in Low Lift mode can cause a misfire at any engine speed, including idle. If more than one rocker arm is stuck, the in-field diagnostic will fail, command all Rocker Arms to Low Lift mode and limit the engine speed to approximately 3000 RPM. The PCM will illuminate the MIL on the first trip.
- High Lift stick condition: If any Rocker Arm is detected as being stuck in High Lift Mode, the in-field diagnostic will fail and quickly default to commanding all of the Rocker Arms to High Lift mode. The engine will have some roughness and misfire may be felt. A single Rocker Arm stuck in high lift mode typically misfires at idle. The PCM will illuminate the MIL on the second trip.
Tech Tip: Since the two rear cylinders are controlled by the rear solenoid on each bank, a VVL issue that affects both rear cylinders on a bank is most likely a common issue for both cylinders. These would include a damaged or missing O-ring, faulty solenoid, oil supply or control circuit issue. A VVL issue that only affects one of the rear cylinders is more likely going to be a failure of a Rocker Arm or pin for that cylinder.
SCAN TOOL DIAGNOSTICS: The scan tool intrusive VVL Service Routine can be used to verify the VVL system operation as well as a likely possible cause if there is a fault. The routine can only run if the misfire monitor is enabled. If there is an active open circuit fault for a solenoid the routine will run but display all zero's for the results. When the routine is initiated the scan tool will check the misfire monitor status. If the monitor is enabled the test will continue. If the monitor is disabled, the routine will abort with a message indicating the test has stopped due to misfire disable condition. There are several other conditions that can disable misfire. Some of the common reasons are fuel level (too high or too low), Cam/Crank TLC not learned or ambient temperature too cold. When the test initiates it will run three tests in an attempt to detect a stuck Rocker Arm condition. The on screen display will describe each test being performed and give relevant data. The test will typically take approximately two minutes. The exception is 2016 and 2017 WK/WD vehicles. The test takes approximately four minutes on these vehicles due to an engine stabilization period after the third forced test. See the graph below for details on the routine.
When the routine completes the results will display in a table showing the misfire percentage result for each cylinder during the three tests that were performed. At the top will be a general description of the possible issue or issues. For a more detailed description of the probable causes compare the misfire percentage data recorded to the table in the diagnostic test below. NOTE: Only count the misfire percentages that are above 75-80% when looking up the results in the table. Any percentage below 75% should not be factored into the result table.