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Misfire or Ticking Noise on 2005-Later GM V8 Engines With AFM

Engine Misfire or Ticking Noises on 2005-Later GM V8 Engines With AFM


Engine Misfires or ticking noises have been reported to be affecting cylinders 1, 4, 6 and or 7 on some 2005-Later GM V8 engines with “Active Fuel Management” (AFM).

The misfire or ticking noise condition may be the result of an AFM lifter that has unlocked as soon as the engine has started or is mechanically collapsed or has stuck permanently.

If an AFM lifter unlocks as soon as the engine is started, low compression will be detected on that cylinder during an AFM compression test, along with the SES (Service Engine Soon) light illuminated, a DTC (Diagnostic Trouble Code) PO300 observed along with engine misfires on the related cylinders, however it is unlikely that any noise will be detected.

If an AFM lifter is mechanically collapsed/stuck, low compression will be detected on that cylinder during an AFM compression test and a consistent valve train noise or ticking will be observed. The same SES  light will be illuminated, as well as a DTC PO300 observed along with engine misfires on the related cylinders.

The lifter concerns may be the result of internal lifter locking pin damage, which can occur if the response time of an AFM lifter unlocking event is:

  • Decreased due to low oil pressure
  • Oil aeration
  • Internal engine sludge
  • VLOM (Valve Lifter Oil Manifold) wear
  • Plastic lifter guide wear
  • Lifter Bore wear
  • Camshaft lobe wear

GM is offering the following diagnostic steps in determining if this is the problem:

  1. Perform a cylinder deactivation (AFM) system compression test If the running compression stays below 25 PSI regardless of the AFM solenoid being commanded on or off, and the AFM lifter is mechanically collapsed/stuck or unlocking as soon as the engine is started.
  2. Perform a cylinder deactivation (AFM) Valve Lifter Oil Manifold (VLOM) diagnosis and test. If the test in procedure #1 isolated a possible AFM lifter concern, it will lead to this test which tests the VLOM for proper operation. GM 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 functioning the same. If this isolates a concern with the VLOM, replace it and reevaluate the concern.
  3. The AFM lifters can also be monitored for proper operation by carefully inspecting cylinder #’s 1, 4, 6 & 7 rockers and valves while cranking or briefly and safely running the engine with the valve covers removed. If the valve(s) of an AFM cylinder stop moving while performing this test, the AFM lifter is causing the concern.


  • Carefully inspect the camshaft lobes to ensure that they are not worn as well as the lifter bores for any scoring or damage that could be of concern.
  • When reassembling the engine, ensure that the lifters are properly aligned with the new plastic lifter guides before they are installed. If they are not properly aligned, it may cause damage to the plastic lifter guide once it is torqued which may allow the lifter to turn in the guide.
  • Low oil pressure to the VLOM can also cause an AFM lifter damage. Generally most known good vehicles will have around 25 PSI or greater oil pressure at hot idle with new engine oil. If oil pressure is 21 PSI or below with new engine oil, then damage could occur to the AFM lifters.
  • It is always recommended when purchasing the replacement components, have your VIN available to ensure you are purchasing the correct and latest version of each component. This will ensure you are installing any component that may have been updated to resolve any previous issue with the component.


   This information is gathered form the best available sources at the time of this post.

                         Parts Counter Guru assumes no responsibility and cannot be held liable for any results or consequences regarding the use of this information.

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