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Rough idle and P0303 misfire code on 2018 Tacoma V6

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

Vehicle: 2018 Toyota Tacoma TRD Off-Road

Hey Tacoma owners,

My 2018 Toyota Tacoma TRD Off-Road with the 3.5L V6 (78,000 miles) just started acting up. The engine has a noticeable rough idle, especially when cold, and I'm feeling some vibrations through the steering wheel. To top it off, the Check Engine Light came on, and when I scanned it, I got a P0303 code, which points to a misfire on cylinder 3.

I haven't noticed any significant loss of power while driving, but the rough idle is definitely annoying. I'm wondering if this is a common issue for this engine and what the usual culprits are. Should I start with spark plugs, or could it be something more serious like an injector or coil pack?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 1:25 AM

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ClearTheCode admin answer:

A P0303 code indicating a misfire on cylinder 3 in your 2018 Toyota Tacoma's 3.5L V6 is a common diagnostic starting point for rough idle issues. Here's a systematic approach to diagnosing and resolving it:

Common Causes for P0303 (Cylinder 3 Misfire):

  1. Ignition System:

    • Spark Plug: A worn, fouled, or incorrectly gapped spark plug is a very common cause. Given your mileage, it's a strong candidate.
    • Ignition Coil: The coil pack for cylinder 3 could be failing. This is also a frequent culprit. You can often swap the coil with an adjacent cylinder to see if the misfire code follows the coil (e.g., P0304 if you swap with cylinder 4).

  2. Fuel System:

    • Fuel Injector: A clogged, dirty, or failing fuel injector for cylinder 3 will prevent adequate fuel delivery, causing a misfire.
    • Fuel Pressure: While less likely to cause a single-cylinder misfire, overall low fuel pressure can contribute.

  3. Air/Vacuum Leaks:

    • Vacuum Leak: A vacuum leak near cylinder 3 or a general intake manifold gasket leak could lean out the fuel mixture in that cylinder, causing a misfire.
    • MAF Sensor: A dirty Mass Air Flow (MAF) sensor can cause general running issues, but usually not a single-cylinder misfire unless it's severely affecting the overall air-fuel ratio.

  4. Engine Mechanical Issues (Less Common but Possible):

    • Low Compression: Worn piston rings, valve issues, or a head gasket leak could lead to low compression in cylinder 3, causing a misfire. This is typically diagnosed with a compression test.

Recommended Diagnostic Steps:

  1. Check Spark Plug: Remove and inspect the spark plug from cylinder 3. Look for signs of wear, fouling, or damage. Replace if necessary. Consider replacing all plugs if they're due for maintenance.
  2. Test Ignition Coil: As mentioned, swap the coil pack from cylinder 3 with another cylinder (e.g., cylinder 1). Clear the code and drive. If the code changes to P0301, you've identified a bad coil pack.
  3. Fuel Injector Test: If the coil and plug are good, consider having the fuel injector for cylinder 3 tested. This might require professional equipment.
  4. Compression Test: If all else fails, a compression test on cylinder 3 will rule out major internal engine issues.

When to See a Shop:

  • If you're uncomfortable performing these diagnostics yourself.
  • If replacing the spark plug and swapping coils doesn't resolve the issue.
  • If the misfire worsens or causes significant drivability problems.
  • A professional can perform more advanced diagnostics, including oscilloscope testing of ignition signals and fuel injector pulse tests.
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