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2021 Jeep Gladiator JT Misfire P0303 - Rough Idle & Loss of Power

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Mike T. · rep 0 · May 25, 2:20 AM · 1 replies · solved · Question

Vehicle: 2021 Jeep Gladiator Rubicon

Hello fellow Jeepers,

My 2021 Jeep Gladiator Rubicon (3.6L Pentastar) has developed a noticeable misfire. The Check Engine Light is on, and when I scanned it, I got a P0303 code, indicating a misfire on Cylinder 3. I'm also experiencing a rough idle and a definite loss of power, especially under acceleration.

The truck has about 45,000 miles on it. I haven't changed the spark plugs yet, but I'm wondering if it could be something else like an ignition coil or an injector. Has anyone else had a P0303 on their Gladiator or Wrangler with the 3.6L? What was the fix? I'd like to try and diagnose it myself if it's straightforward.

Thanks for the help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:20 AM

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ClearTheCode admin answer:

A P0303 code on your 2021 Jeep Gladiator Rubicon (3.6L Pentastar) means the engine control module (ECM) has detected a misfire specifically in Cylinder 3. This is a common issue and can be caused by problems with fuel, spark, or compression in that cylinder. The rough idle and loss of power are typical symptoms.

Common Causes for P0303 (Cylinder 3 Misfire):

  1. Faulty Spark Plug: A worn, fouled, or damaged spark plug in Cylinder 3 is a very common cause.
  2. Bad Ignition Coil: The ignition coil for Cylinder 3 might be failing, preventing a strong spark.
  3. Clogged or Faulty Fuel Injector: If the fuel injector for Cylinder 3 isn't delivering enough fuel, or is stuck open/closed, it will cause a misfire.
  4. Vacuum Leak: A vacuum leak near Cylinder 3's intake runner could lean out the air-fuel mixture, causing a misfire.
  5. Low Compression: Less common, but internal engine issues like a bad valve, head gasket, or piston ring can lead to low compression in Cylinder 3.
  6. Wiring Issues: Damaged wiring to the ignition coil or fuel injector for Cylinder 3.

Troubleshooting Steps (DIY):

  1. Swap Components (Cylinder 3 is on the passenger side, middle cylinder):
    • Spark Plug: Swap the spark plug from Cylinder 3 with a known good cylinder (e.g., Cylinder 1 or 5). Clear the code and drive. If the misfire moves to the new cylinder (e.g., P0301), the spark plug is the culprit.
    • Ignition Coil: If the spark plug swap doesn't help, swap the ignition coil from Cylinder 3 with another cylinder. Clear the code and drive. If the misfire moves (e.g., P0301), the coil is bad.
  2. Listen for Vacuum Leaks: With the engine running, listen for any hissing sounds around the intake manifold, especially near Cylinder 3. You can also use a can of unlit propane or carb cleaner (carefully!) to spray around suspect areas; if the idle changes, you've found a leak.

When to See a Shop:

  • If swapping spark plugs and coils doesn't identify the issue.
  • If you suspect a fuel injector problem (requires specialized tools for testing or replacement).
  • If you hear unusual engine noises or suspect a compression issue. A shop can perform a compression test to confirm internal engine health.
  • If the misfire persists after basic troubleshooting, as it could indicate a more complex electrical issue or a deeper engine problem that requires professional diagnosis and repair.
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