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2020 Acadia Denali Rough Idle and Check Engine Light (P030x)

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Mike T. · rep 0 · May 25, 2:36 AM · 1 replies · solved · Question

Vehicle: 2020 GMC Acadia Denali

Hey GMC community,

I'm having a frustrating issue with my 2020 GMC Acadia Denali, which has the 3.6L V6 engine. For the past few days, the engine has been running really rough, especially at idle. It feels like it's shaking more than usual, and there's a noticeable vibration throughout the vehicle.

Yesterday, the check engine light came on. I used my OBD-II scanner and pulled a P0304 code, indicating a misfire on cylinder 4. I've also noticed a slight decrease in power when accelerating, and the fuel economy seems to have dipped a bit. Before I start throwing parts at it, I wanted to see if anyone else has experienced this with their Acadia or a similar GMC model. What was the culprit for you?

Could it be just a spark plug or coil pack, or something more serious like an injector? Any advice on troubleshooting steps would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:36 AM

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ClearTheCode admin answer:

A P0304 code on your 2020 GMC Acadia Denali (3.6L V6) indicates a misfire detected in cylinder 4. This can indeed cause a rough idle, reduced power, and decreased fuel economy. Here's a systematic approach to diagnosing and resolving the issue:

  1. Spark Plug Inspection/Replacement:

    • Cause: A worn, fouled, or damaged spark plug is the most common cause of a misfire. Over time, spark plugs degrade and can't ignite the air/fuel mixture efficiently.
    • Diagnostics: Start by inspecting the spark plug in cylinder 4. If it looks worn, fouled, or damaged, replace it. It's often recommended to replace all spark plugs if they are near their service interval.

  2. Ignition Coil Check:

    • Cause: A faulty ignition coil for cylinder 4 would prevent the spark plug from receiving the necessary high voltage to create a spark.
    • Diagnostics: You can swap the ignition coil from cylinder 4 with a known good cylinder (e.g., cylinder 2). Clear the codes and drive the vehicle. If the misfire code changes to P0302 (misfire on cylinder 2), then the coil pack is the culprit.

  3. Fuel Injector Testing:

    • Cause: A clogged or malfunctioning fuel injector in cylinder 4 would not deliver the correct amount of fuel, leading to a lean misfire.
    • Diagnostics: This is harder to diagnose without specialized tools. A mechanic can test the injector's resistance and spray pattern. Sometimes, a fuel system cleaner can help with minor clogs, but a faulty injector usually requires replacement.

  4. Compression Test:

    • Cause: Low compression in cylinder 4 due to issues like a bad valve, head gasket leak, or piston ring wear can cause a misfire.
    • Diagnostics: This is a more serious issue and requires a compression test to confirm. If compression is low, further engine diagnostics are needed.

  5. Vacuum Leaks:

    • Cause: A vacuum leak near cylinder 4 could lean out the air/fuel mixture, causing a misfire.
    • Diagnostics: Visually inspect vacuum lines and intake manifold gaskets for cracks or signs of leaks.

When to See a Shop:

  • If you've tried swapping spark plugs and coils and the misfire persists or moves to a different cylinder.
  • If you're uncomfortable performing these diagnostic steps yourself.
  • If the check engine light is flashing, which indicates a severe misfire that could damage the catalytic converter. Drive as little as possible in this scenario.
  • If the issue is suspected to be a fuel injector or an internal engine problem, professional diagnosis is essential.
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