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2018 F-150 3.5L EcoBoost Misfire & P0304 Code After Cold Start

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Heather Y. · rep 0 · May 22, 2:23 AM · 1 replies · solved · Question

Vehicle: 2018 Ford F-150 Lariat

Hey everyone,

My 2018 Ford F-150 Lariat, with the 3.5L EcoBoost, has been acting up lately. Specifically, after a cold start, it runs really rough for about 30 seconds to a minute, almost like it's going to stall. The Check Engine Light (CEL) comes on and flashes during this time, then stays solid.

I hooked up my OBD-II scanner, and it's throwing a P0304 code, indicating a misfire on cylinder 4. Once the engine warms up, it seems to run a bit better, but the power feels down, and the fuel economy has definitely taken a hit. I've heard these EcoBoost engines can have issues with misfires. Any ideas on what I should check first? Plugs, coils, injectors?

Thanks for any help!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 2:23 AM

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ClearTheCode admin answer:

The P0304 code on your 2018 Ford F-150 3.5L EcoBoost indicates a misfire detected in cylinder 4. Given it's worse on a cold start, this often points to a few common culprits.

Here's a diagnostic approach:

  1. Check Ignition Components: The most common cause of misfires. Start by inspecting the spark plug in cylinder 4. Look for signs of wear, fouling, or damage. If it's been a while, consider replacing all spark plugs with OEM-recommended ones, as worn plugs can stress ignition coils. Next, inspect the ignition coil for cylinder 4. You can try swapping it with a coil from a different cylinder (e.g., cylinder 1) and see if the misfire code moves to the new cylinder. If it does, the coil is faulty.
  2. Fuel System Inspection: A clogged or faulty fuel injector for cylinder 4 can also cause a misfire. This is harder to diagnose without specialized tools, but you can sometimes hear a difference in injector clicking or use a noid light to check for pulse. Fuel pressure issues (low pressure) can also cause widespread misfires, but P0304 specifically points to one cylinder.
  3. Compression Test: If ignition and fuel components check out, a compression test on cylinder 4 is crucial. Low compression could indicate internal engine issues like worn piston rings, a faulty valve, or a head gasket problem. This is less common but serious.
  4. Vacuum Leaks: While less likely to cause a single-cylinder misfire, a significant vacuum leak near cylinder 4's intake runner could lean out the mixture enough to cause a misfire. Inspect all vacuum lines and intake manifold gaskets.

Common Causes for EcoBoost Misfires:

  • Worn or incorrect spark plugs (especially if not gapped correctly).
  • Failing ignition coils.
  • Carbon buildup on intake valves (due to direct injection, though less likely to cause a single-cylinder misfire unless severe).
  • Faulty fuel injector.
  • Internal engine issues (least common).

When to See a Shop: If you've checked the spark plug and coil and the issue persists, or if you're not comfortable performing a compression test or diagnosing fuel system components, it's best to take your F-150 to a qualified mechanic. A flashing CEL indicates a severe misfire that can damage the catalytic converter, so addressing this promptly is important.

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