Diagnostics and repair insights provided by ACDelco/GM Parts Training Specialist, James Rokitski
Are your customers having issues with misfires, a ticking noise or an SES light that has turned on in a GM vehicle? GM specialists say the problem could be related to a collapsed or stuck Active Fuel Management (AFM) lifter. Correctly diagnosing potential AFM lifter problems can be challenging.
If the initial diagnosis does not isolate the cause of this concern, perform the following diagnostic steps and repair procedures as necessary:
- Perform a Cylinder Deactivation (Active Fuel Management) System Compression Test. If the running compression of the misfiring cylinder stays below 25 PSI, regardless of the AFM solenoid being commanded on or off, an AFM lifter is mechanically collapsed/stuck or unlocking as soon as the engine is started.
- If the test in Step 1 isolated a possible AFM lifter concern, perform the Cylinder Deactivation AFM Valve Lifter Oil Manifold Diagnosis (VLOM) and Testing. This will test the VLOM for proper operation. A limited amount of air will leak from the bleed holes and outlet ports even when the solenoids are off. Compare the amount of leakage to verify all 4 solenoids are operating the same. If it isolates a concern with the VLOM, replace it and reevaluate the concern.
- The AFM lifters also can be monitored for proper operation by carefully inspecting the cylinder 1, 4, 6, and 7 rockers and valves while cranking or briefly and safely running the engine with the valve covers removed in the service bay. If the valve(s) of an AFM cylinder stop moving while doing this test, the AFM lifter is causing the concern
If diagnosis or any of the previous steps indicate that any AFM lifters are unlocking or have collapsed, follow the applicable notes below when replacing the VLOM, all AFM lifters and all plastic lifter guides.
– If you are working on a 2008-2009 Full Size Truck/SUV or a 2007-2009 Chevrolet Tahoe with the Police Package, reprogram the ECM with the latest calibrations.
– If you are working on a 2008-2009 Pontiac G8, 2010-2011 Camaro, or 2007-2010 Full Size Trucks with AFM, determine if the AFM pressure relief valve shield (listed in the latest version of 10-06-01-008) has ever been installed. If not, remove the oil pan and install the AFM shield. (If the shield was installed before, disregard this step.)
– On passenger cars with the LS4 engine, determine if the updated oil pan gasket (listed in the latest version of 11-06-01-007) has ever been installed. If not, remove the oil pan and install this updated oil pan gasket. It is also important to thoroughly clean the inside of the oil pan while it is removed. Due to oil pan differences, this shield and gasket will not fit Mid-Size Utility Vehicles with the LH6 Engine.
– Carefully inspect the camshaft lobes through the lifter bores with a pen light or bore scope to ensure that they are not obviously worn. Rotate the crankshaft and camshaft so the entire circumference of all cam lobes can be inspected. Also inspect the lifter bores for any obvious scoring/damage that could be a concern.
Reassembly key points
When reassembling, ensure that the lifters are properly aligned to the new plastic lifter guides before they are installed. If they are not, the plastic lifter guide may be damaged once the vehicle is torqued since this may allow the lifter to turn in the guide.
– Clean out the related lifter control oil passages while the old lifters are removed. Generally, it is only necessary to blow through the passages with shop air. However, if you spot a lot of debris, it may be necessary to flush the passages out with brake cleaner. The oil should be changed after doing this work since the cylinder heads will be off during replacement of the lifters.
– Low oil pressure to the VLOM also can cause AFM lifter damage. As a result, GM recommends checking the Tech2 oil pressure parameter at a hot idle. This is located in the Cylinder Deactivation Data List of the Tech2. Most vehicles usually will have around 25 PSI (172 KPA) or greater oil pressure at a hot idle with new engine oil. As long as there are no oil pressure sensor DTCs, such as a P0522 or P0523, the oil pressure sensor will read fairly accurately.
– If the sensor reads 19 PSI (131 KPA) or less while at a hot idle with new oil, perform the Cylinder Deactivation (AFM) Oil Pressure Relief Valve Diagnosis and Testing as outlined in SI and repair as necessary. In some cases, the new VLOM can correct a low oil pressure concern if there is an internal leak or plugged VLOM filter screen.
A number of issues can lead to the AFM lifters being damaged or stuck. ACDelco Training Specialist James Rokitski says dirty engine oil and low oil levels are a common culprit. He recommends telling customers to regularly check their oil level between oil changes. Also, always use Genuine GM Parts and recommended procedures for every repair.