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2020 911 Carrera S Rough Idle and Misfire - CEL On

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Mike T. · rep 0 · May 25, 4:49 AM · 1 replies · solved · Question

Vehicle: 2020 Porsche 911 Carrera S

Hi everyone, I'm experiencing an issue with my 2020 Porsche 911 Carrera S (992 generation) that's really bothering me. The car has developed a noticeable rough idle, especially when cold, and I can feel slight vibrations through the cabin. The check engine light (CEL) came on yesterday and is now solid.

I managed to pull a code with my basic OBD-II scanner, and it's showing P0304, indicating a misfire on cylinder 4. I'm hoping it's something relatively simple like a spark plug or ignition coil, but I'm worried it could be something more serious.

Has anyone with a 992 Carrera S or similar Porsche experienced this? What was the culprit? Any advice on troubleshooting or what to expect at the dealer would be great. The car only has about 25,000 miles on it.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:49 AM

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ClearTheCode admin answer:

A rough idle and a P0304 misfire code on your 2020 911 Carrera S point directly to an issue with combustion in cylinder 4. This is a common problem across many modern engines, and while it can be simple, proper diagnosis is key.

Common Causes for Misfire (P0304):

  1. Ignition System:

    • Spark Plug: A fouled, worn, or cracked spark plug in cylinder 4 is a very common cause. Modern direct-injection engines can be harder on plugs.
    • Ignition Coil: A failing ignition coil for cylinder 4 is another frequent culprit. Coils can degrade over time, leading to intermittent or complete misfires. This is often the most common fix.

  2. Fuel System:

    • Fuel Injector: A clogged, dirty, or faulty fuel injector in cylinder 4 can prevent the cylinder from receiving adequate fuel, leading to a misfire. This is less common than ignition issues but still possible.
    • Low Fuel Pressure: While less likely to affect only one cylinder, overall low fuel pressure could exacerbate an existing issue.

  3. Air/Compression Issues:

    • Vacuum Leak: A vacuum leak near cylinder 4 could lean out the air/fuel mixture, causing a misfire. This is less common for a single cylinder code.
    • Compression Loss: In rare cases, a mechanical issue like a faulty valve, worn piston rings, or a head gasket leak could cause low compression in cylinder 4, leading to a misfire. This is usually accompanied by other symptoms like smoke or coolant loss.

Troubleshooting Steps (for a qualified technician):

  • Swap Components: A common diagnostic step is to swap the spark plug and then the ignition coil from cylinder 4 with an adjacent cylinder (e.g., cylinder 1). If the misfire code moves to the new cylinder (e.g., P0301), then the swapped component (plug or coil) is the issue.
  • Fuel Injector Testing: If swapping doesn't isolate the problem, fuel injector diagnostics (flow testing, resistance checks) would be next.
  • Compression Test: If all else fails, a compression test on cylinder 4 would be performed to rule out mechanical engine problems.

When to See a Shop:

  • Immediately: With a solid CEL and a noticeable misfire, it's best to get your 911 Carrera S to a Porsche specialist or dealership soon. A persistent misfire can lead to unburnt fuel entering the catalytic converter, potentially causing damage that is far more expensive to repair than the initial misfire cause. Driving with a flashing CEL indicates a severe misfire that is actively damaging the catalytic converter, and should be addressed immediately.
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