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Check for ticks: LS engine misfire

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Check for ticks: LS engine misfire

Some customers may complain of an SES light, engine misfire and/or ticking 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.

This applies to V8 engines and AFM (Active Fuel Management) versions L76, LC9, LFA, LH6, LMG, LS4 and LY5. Vehicles include 2008-2009 Buick Lacrosse and Allure, 2007-2009 Cadillac Escalade, 2007-2009 Chevy Avalanche, Silverado, Suburban and Tahoe, 2006-2009 Chevy Trailblazer, Impala SS, GMC Envoy and Saab 97x, 2005-2008 Pontiac Grand Prix GXP, 2006-2007 Chevy Monte Carlo SS and 2007-2009 GMC Sierra and Yukon.

If an AFM lifter unlocks as soon as the engine is started, a SES light and DTC P0300 will be experienced, with engine misfires on cylinder 1, 4, 6 or 7, but it is unlikely that any noise will be experienced. If an AFM lifter is mechanically collapsed or stuck, a consistent valvetrain tick noise, SES light and DTC P0300 will be experienced with engine misfire on cylinder 1, 4, 6 or 7.

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 or stuck, or is unlocking as soon as the engine is started. The AFM lifters can also be monitored by carefully inspecting the rockers/valves for proper movement while cranking the engine with the valve cover removed.

* 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 that all 4 solenoids are operating the same. If it isolates a concern with the VLOM, replace it and re-evaluate the concern.

If the tests above isolate an AFM lifter concern, follow SI repair procedures to replace the oil pump, oil pump pickup tube and its O-ring, and the pair of AFM lifters for the related cylinder. Before installing new lifters, blow out the related lifter control oil passages with the old lifters removed. Also, inspect the VLOM oil filter under the oil pressure sending unit for debris/sludge and clean or replace it as necessary.

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