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2018 Charger R/T Hemi Misfire and Rough Idle - P0304 Code

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Mark S. · rep 0 · May 22, 5:38 AM · 1 replies · solved · Question

Vehicle: 2018 Dodge Charger R/T

Hey everyone,

My 2018 Dodge Charger R/T, with the 5.7L Hemi, has been acting up lately. I've noticed a really rough idle, especially when I'm stopped at a light. It feels like the engine is shaking more than usual.

The check engine light came on, and when I scanned it, I got a P0304 code, which points to a misfire on cylinder 4. I'm wondering if anyone else has experienced this with their Hemi and what the common culprits might be.

Could it be spark plugs, coil packs, or something more serious? The car has about 70,000 miles on it. Any advice on where to start troubleshooting would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 5:38 AM

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ClearTheCode admin answer:

A P0304 code indicates a misfire detected in cylinder 4. This is a common issue that can stem from several sources in a Hemi engine. Here's a breakdown of potential causes and steps to diagnose:

Common Causes:

  1. Ignition System:
    • Spark Plug: Worn, fouled, or incorrectly gapped spark plugs are a primary cause of misfires. Hemi engines can be sensitive to plug condition.
    • Ignition Coil: A faulty ignition coil for cylinder 4 would prevent proper spark.
    • Spark Plug Wire (if applicable): Though less common on modern engines, a damaged wire could be the culprit.
  2. Fuel System:
    • Fuel Injector: A clogged, dirty, or failing fuel injector for cylinder 4 would not deliver enough fuel, leading to a lean misfire.
    • Fuel Pressure: Low overall fuel pressure could cause misfires across multiple cylinders, but if isolated to one, focus on the injector.
  3. Engine Mechanical:
    • Compression Issue: Low compression in cylinder 4 due to worn piston rings, a faulty valve, or a head gasket leak. This is less common but serious.
    • Valve Train: Issues with lifters or camshaft lobes can affect valve operation, leading to misfires.
  4. Air/Vacuum Leaks: While less likely to cause a single-cylinder misfire, a significant vacuum leak near cylinder 4 could contribute.

Diagnostic Steps:

  1. Inspect Spark Plug & Coil: Start by removing and inspecting the spark plug from cylinder 4. Look for signs of fouling, wear, or damage. Consider swapping the ignition coil from cylinder 4 with an adjacent cylinder (e.g., cylinder 2). If the misfire moves to cylinder 2 (P0302), the coil is bad.
  2. Check Fuel Injector: Use a stethoscope to listen for the clicking sound of the injector on cylinder 4. You can also swap the injector with another cylinder to see if the misfire follows.
  3. Compression Test: If ignition and fuel systems check out, perform a compression test on cylinder 4 and compare it to others. A leak-down test can further pinpoint mechanical issues.
  4. Scan Tool Data: Monitor live data with an advanced scan tool, focusing on misfire counters for all cylinders, fuel trims, and O2 sensor readings.

When to See a Shop:

If basic troubleshooting (spark plug/coil swap) doesn't resolve the issue, or if you suspect a fuel injector or mechanical problem, it's best to take your Charger to a qualified mechanic. They have specialized tools for fuel pressure testing, injector flow testing, and advanced diagnostics to accurately pinpoint the root cause without causing further damage.

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