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2017 340i Rough Idle & Misfires - P0301 Code

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Chris P. · rep 0 · May 25, 2:57 AM · 1 replies · solved · Question

Vehicle: 2017 BMW 340i (F30)

Hey everyone,

My 2017 BMW 340i with the B58 engine has suddenly developed a very rough idle and occasional misfires, especially when cold. It feels like the engine is shaking quite a bit.

The check engine light came on, and I scanned it with my OBD-II tool. It's showing a P0301 code, indicating a misfire on cylinder 1. I'm wondering if this is a common issue with these models or if there's a specific diagnostic path I should follow before just swapping parts.

Any advice on what to check first? Plugs, coils, injectors? The car has about 75,000 miles on it.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:57 AM

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ClearTheCode admin answer:

Experiencing a P0301 misfire on your 2017 BMW 340i (B58 engine) is a common diagnostic challenge. Here's a structured approach to diagnose and resolve the issue:

  1. Confirm the Misfire: While P0301 points to cylinder 1, it's good practice to monitor live misfire data for all cylinders using a scan tool. Ensure the misfire is consistently on cylinder 1 and not intermittently jumping.

  2. Swap Components: The easiest diagnostic step is to swap components. Start by swapping the ignition coil from cylinder 1 to an adjacent cylinder (e.g., cylinder 2). Clear the codes and drive the car. If the misfire moves to cylinder 2 (P0302), you've identified a faulty ignition coil.

  3. Spark Plugs: If swapping the coil doesn't change the misfire location, swap the spark plug from cylinder 1 to another cylinder. Again, clear codes and test. If the misfire moves, the spark plug is the culprit. Given your mileage, spark plugs are a common wear item.

  4. Fuel Injector: If coils and plugs are ruled out, the next likely suspect is the fuel injector for cylinder 1. This requires more involved diagnostics, such as checking injector pulse with a noid light or performing a fuel pressure leak-down test specific to the injector. Swapping injectors is possible but more labor-intensive.

  5. Compression Test: As a last resort, if all other components check out, perform a compression test on cylinder 1 to rule out internal engine issues like a faulty valve or head gasket. Low compression would indicate a mechanical problem.

Common Causes for P0301 on B58:

  • Faulty ignition coil (most common)
  • Worn or fouled spark plug
  • Clogged or faulty fuel injector
  • Vacuum leak near cylinder 1 (less common for a single cylinder misfire)
  • Internal engine issue (least common, but possible)

When to See a Shop:

If you're uncomfortable performing component swaps or if the problem persists after trying steps 2 and 3, it's best to take your BMW to a qualified independent BMW specialist or dealership. Fuel injector diagnostics and internal engine checks require specialized tools and expertise.

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