The largest online repair guide library for every car

About us
Contact support
FAQ
Sign up
Sign in

2018 Sierra 5.3L Rough Idle & Misfire - AFM Issue?

0

Ryan M. · rep 0 · May 22, 3:38 AM · 1 replies · solved · Question

My 2018 GMC Sierra 1500 SLT, with the 5.3L V8, has been acting up lately. I've noticed a rough idle, especially when cold, and sometimes it feels like it's misfiring under light acceleration. The Check Engine Light came on, and a quick scan showed P0301 and P0303, indicating misfires on cylinders 1 and 3.

I've heard a lot about Active Fuel Management (AFM) lifter problems on these engines, leading to similar symptoms. Could this be what's happening with my truck? It has about 85,000 miles on it, and I've been pretty diligent with oil changes.

What are the typical signs of an AFM lifter failure, and is there anything I can check myself before taking it to a shop? I'm hoping it's not a major repair.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:38 AM

0

ClearTheCode guidance (Gmc · Engine & drivetrain)

Summary: For 2018 Sierra 5.3L Rough Idle & Misfire - AFM Issue?, work through these steps on your 2016+ vehicle:

  1. Confirm the symptom — note when it happens (cold start, highway, after fuel fill, etc.).
  2. Scan codes — save all pending/stored DTCs; don't clear until you've recorded them.
  3. Check TSBs & recalls — look up your VIN for open campaigns and known fixes.
  4. Inspect the obvious — related fuses, connectors, fluid level, and recent repairs.
  5. Test the common fix — follow model-specific service procedures; one change at a time.
  6. Escalate if needed — burning smell, no brakes, overheating, or stalling in traffic → shop visit.

Owner reported: My 2018 GMC Sierra 1500 SLT, with the 5.3L V8, has been acting up lately. I've noticed a rough idle, especially when cold, and sometimes it feels like it's misfiring under light acceleration. The Check Engine Light came on, and a quick scan showed P0301 and P0303, indicating misfires on cylinders 1 and 3.

I've heard a lot about Active Fuel Management (AFM) lifter problems on these engines, leadin…

This is educational guidance, not a substitute for hands-on diagnosis.

Menu

Help & account