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2018 F-150 3.5L EcoBoost Check Engine Light - Rough Idle & Misfire

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Mark S. · rep 0 · May 22, 2:21 AM · 1 replies · solved · Question

Vehicle: 2018 Ford F-150 Lariat

Hey everyone, I'm hoping someone can help me out. My 2018 Ford F-150 Lariat with the 3.5L EcoBoost engine just threw a Check Engine Light this morning. It's been running a bit rough lately, especially at idle, and I can feel a slight hesitation when accelerating.

I hooked up my OBD-II scanner and it's showing a P0303 code, which indicates a misfire on cylinder 3. I'm wondering if this is a common issue with these trucks and what the usual culprits are. Could it be a bad spark plug, ignition coil, or something more serious?

Any advice on troubleshooting steps I can take before heading to a shop would be greatly appreciated. I'm comfortable doing some basic DIY work.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 2:21 AM

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ClearTheCode admin answer: A P0303 code on your 2018 Ford F-150 3.5L EcoBoost indicates a misfire detected in cylinder 3. This is a relatively common diagnostic trouble code, and several factors can cause it.

Common Causes for P0303:

  • Worn Spark Plug: The most frequent cause. Spark plugs wear out over time, reducing their ability to ignite the fuel-air mixture effectively.
  • Faulty Ignition Coil: The ignition coil provides the high voltage to the spark plug. A failing coil won't deliver enough spark.
  • Clogged Fuel Injector: A dirty or failing fuel injector can prevent cylinder 3 from getting enough fuel, leading to a lean misfire.
  • Vacuum Leak: A vacuum leak near cylinder 3 could cause an air-fuel imbalance.
  • Low Compression: Less common, but internal engine issues like a bad valve or head gasket can cause low compression in a cylinder, leading to misfires.

Troubleshooting Steps:

  1. Inspect/Replace Spark Plug: Start by removing the spark plug from cylinder 3. Look for signs of wear, fouling, or damage. If it looks bad, replace it. It's often a good idea to replace all spark plugs if they're due for maintenance.
  2. Swap Ignition Coil: To confirm if the coil is bad, swap the ignition coil from cylinder 3 with a coil from a different cylinder (e.g., cylinder 1). Clear the code and drive the vehicle. If the misfire code changes to P0301 (misfire in cylinder 1), you've identified a faulty coil.
  3. Check Fuel Injector: If the spark plug and coil are good, consider testing the fuel injector. This can be done with a noid light to check for pulse or by having it professionally cleaned/tested.
  4. Inspect for Vacuum Leaks: Listen for hissing sounds around the intake manifold and vacuum lines. You can also use a smoke machine if available.

When to See a Shop: If after checking and potentially replacing the spark plug and coil, the misfire persists, or if you suspect a fuel injector issue, vacuum leak, or low compression, it's best to consult a qualified mechanic. They have specialized diagnostic tools to perform compression tests, fuel pressure tests, and more in-depth electrical diagnostics to pinpoint the exact cause without guesswork. Driving with a persistent misfire can damage your catalytic converter over time.

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