Chevrolet Repair Questions? Ask a Mechanic for Answers ASAP
Welcome. Does your truck have the Active fuel management?
Bulletins are not as easily found so I will post it here.
2008 Chevrolet Silverado - 4WD | Sierra, Silverado (VIN C/K) Service Manual | Document ID:(NNN) NNN-NNNN
#PIP4568F: Tick Noise And/Or Misfires On AFM Cylinders 1 4 6 And/Or 7 - keywords possible - (Dec 14, 2009)
This PI was superseded to update Recommendation/Instructions. Please discard PIP4568E.
The following diagnosis might be helpful if the vehicle exhibits the symptom(s) described in this PI.
Some customers may comment on an SES light, engine misfire on cylinder 1, 4, 6, or 7, and/or tick noise. This may be the result of an AFM lifter that unlocks as soon as the engine is started or one that is mechanically collapsed/stuck all of the time.
If an AFM lifter unlocks as soon as the engine is started, low compression will be experienced on that cylinder during an AFM compression test, along with a SES light and DTC P0300 with engine misfires on the related cylinder but it is unlikely that any noise will be experienced.
If an AFM lifter is mechanically collapsed/stuck, low compression will be experienced on that cylinder during an AFM compression test, along with a consistent valve train tick noise, SES light, and DTC P0300 will be experienced with engine misfires on the related cylinder.
If this concern is experienced, perform SI diagnosis. If SI does not isolate the cause of this concern, perform the following tests:
Cylinder Deactivation (Active Fuel Management) System Compression Test in SI. 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.
Cylinder Deactivation (Active Fuel Management) Valve Lifter Oil Manifold Diagnosis and Testing in SI. If the test above isolated a possible AFM lifter concern, it will lead to this test, which tests the VLOM (Valve Lifter Oil Manifold) for proper operation. SI states 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 re-evaluate the concern.
In addition to the SI diagnostic test above, the AFM lifters can also be monitored for proper operation by carefully inspecting the cylinder 1, 4, 6, and 7 rockers and valves while cranking or running the engine with the valve covers removed.
If the tests above isolate an AFM lifter concern, inspect the related lifter to determine if it is physically collapsed/stuck all of the time.
If a collapsed/stuck lifter is found, it may be the result of the lifter ingesting air through the oil pump and/or pick up tube oring, which can damage the lifters. If a collapsed lifter is found, follow SI repair procedures to replace the oil pump, pick up tube o-ring, and all AFM lifters.
If a collapsed/stuck lifter is not found but the related lifter(s) is unlocking upon start up causing low running compression and/or no rocker/valve movement, there may be a concern with the related lifter and VLOM. If this concern is encountered, replace the VLOM and all AFM lifters.