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2018 Discovery HSE Si6 - Persistent Misfire P0303 & Rough Idle

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Mike T. · rep 0 · May 25, 4:41 AM · 1 replies · solved · Question

Vehicle: 2018 Land Rover Discovery HSE Si6

Hey everyone,

I'm having a persistent issue with my 2018 Land Rover Discovery HSE Si6. The check engine light came on last week, and when I scanned it, I got a P0303 code, indicating a misfire on cylinder 3. The engine also feels a bit rough at idle, especially when cold, and sometimes I notice a slight hesitation during acceleration.

I've tried clearing the code, but it comes back after a day or two of driving. I'm wondering if anyone else with this engine has experienced something similar? Could it be a bad spark plug, coil pack, or something more serious like an injector?

Any advice on what to check first or common culprits for this specific model and engine would be greatly appreciated. Thanks!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 4:41 AM

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ClearTheCode admin answer:

A P0303 code indicates a misfire detected in cylinder 3. While a misfire can have several causes, for your 2018 Land Rover Discovery HSE Si6, here's a structured approach to diagnosis and potential solutions:

  1. Basic Checks First:

    • Spark Plug: The most common cause. A worn, fouled, or improperly gapped spark plug in cylinder 3 can cause a misfire. Consider replacing all spark plugs if they are due for service.
    • Ignition Coil: A failing ignition coil for cylinder 3 is another frequent culprit. You can often swap the coil pack from cylinder 3 to another cylinder (e.g., cylinder 1) and see if the misfire code follows the coil (e.g., P0301).
    • Fuel Injector: A clogged or faulty fuel injector for cylinder 3 can lead to insufficient fuel delivery. This is harder to diagnose without specialized tools but can be indicated by fuel trim readings.
    • Vacuum Leaks: Less common for a single-cylinder misfire, but a significant vacuum leak could affect engine performance broadly.
    • Compression Test: In rare cases, low compression in cylinder 3 due to valve issues or piston ring wear can cause a misfire. This is a more involved diagnostic step.

  2. Diagnostic Steps:

    • Scan Tool Data: Use a good OBD-II scanner to monitor live data. Look at fuel trims (short and long term), O2 sensor readings, and misfire counts for individual cylinders. High positive fuel trims might suggest a lean condition (vacuum leak, dirty injector), while negative trims might suggest too much fuel.
    • Visual Inspection: Check wiring harnesses and connectors to the spark plug and coil pack for any signs of damage or corrosion.
    • Fuel Pressure Test: If you suspect fuel delivery issues, a fuel pressure test can rule out a failing fuel pump or clogged fuel filter.

  3. When to See a Shop:

    • If basic component replacement (spark plugs, coil packs) doesn't resolve the issue, or if the problem persists after initial troubleshooting.
    • If you lack the tools or expertise to perform more advanced diagnostics like compression tests or fuel system checks.
    • If you notice any unusual noises from the engine, significant loss of power, or if the check engine light is flashing (indicating a severe misfire that could damage the catalytic converter).

Addressing a misfire promptly is important to prevent potential long-term damage to your catalytic converter.

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