2008 GMC Sierra - P0300 and ticking sound (AFM Failure)

Question

My 08 Sierra has 140k miles and has developed a very distinct 'ticking' sound from the driver's side of the engine. It sounds like a sewing machine. Along with the noise, I'm getting a P0300 random misfire code and the traction control light keeps coming on and off.

I've replaced the plugs, wires, and even the coil packs on that side, but the tick stays. It seems to get louder as the engine warms up. I'm really hoping it's just an exhaust manifold leak, but it sounds more internal. The oil pressure also seems a bit lower than it used to be (around 20psi at idle).

OvernightOscar · Answer

✅ Confirmed fix: You have a collapsed AFM (Active Fuel Management) lifter on cylinder 1 or 7. This is the 'death tick' for these engines. The lifter internal locking pin fails, and it no longer opens the valve, causing the P0300 misfire.

I had to pull the cylinder head and replace the lifters and the VLOM (Valley Cover). To prevent this from happening again, I installed an AFM disabler to keep it in V8 mode all the time. If the camshaft lobe is wiped, you'll need a full cam/lifter replacement.

AudiAlex22 · Answer

Try running a quart of Marvel Mystery Oil in the crankcase for 500 miles. Sometimes the Active Fuel Management (AFM) lifters get stuck due to sludge, and a good cleaning can free them up without tearing the motor down.

2008 GMC Sierra - P0300 and ticking sound (AFM Failure)

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