Common causes include ignition system failures (spark plug or coil), a faulty fuel injector or fuel delivery issue, air leaks around the cylinder, or mechanical problems that affect compression or timing. These factors can cause cylinder 4 to fail to ignite properly and trigger a misfire diagnostic code such as P0304.
In automotive diagnostics, a misfire in cylinder 4 means the air-fuel mixture in that specific cylinder is not igniting as intended. Cylinder 4 is simply the fourth cylinder in the engine’s firing order, and its misfire can stem from issues localized to its spark, fuel delivery, air intake, sealing, or timing. A methodical approach—often starting with onboard codes and moving through component tests—helps pinpoint the root cause.
Ignition system problems
Ignition issues are among the most common reasons a single cylinder, including cylinder 4, misfires. The following items are typical culprits and how they manifest.
- Worn or fouled spark plug in cylinder 4
- Defective ignition coil on cylinder 4 (coil-on-plug failure)
- Damaged spark plug boot or high‑voltage lead to cylinder 4
- Corroded or loose electrical connector to the coil pack
- Weak or inconsistent spark due to a failing coil pack or wiring
Repair or replacement of the faulty component—often a spark plug and/or coil on cylinder 4—typically resolves ignition-related misfires. Wiring or connector issues are addressed by cleaning or replacing damaged connections.
Fuel delivery problems
Fuel delivery problems restrict or unevenly supply fuel to cylinder 4, producing a misfire that may appear under load or at idle.
- Clogged or sticking injector for cylinder 4
- Electrical fault or driver issue to injector 4 (PWM signal)
- Low fuel pressure affecting injector spray
- Contaminated fuel or failing fuel pump
- Leaking injector causing a lean or rich condition
Diagnosing fuel issues often involves testing injector operation, verifying fuel pressure, and confirming proper fueling Trim. Replacing a faulty injector or repairing the fuel delivery path usually corrects the problem.
Air intake and vacuum leaks
Air leaks around cylinder 4 disturb the air-fuel ratio and can cause a misfire, particularly at idle or during deceleration.
- Vacuum leak at the intake manifold runner for cylinder 4
- Collapsed or cracked intake hose affecting cylinder 4
- Intake manifold gasket leak around cylinder 4
- Faulty mass air flow sensor (contributes to lean condition, not cylinder-specific but aggravating)
Repair typically involves replacing gaskets or hoses, and testing or cleaning sensors. Correcting a vacuum leak often resolves the misfire caused by an irregular air supply.
Compression and mechanical problems
Low compression or mechanical faults within cylinder 4 can produce persistent misfires and require a deeper inspection.
- Low compression due to worn piston rings or cylinder wall damage
- Valve issue such as a burnt valve or bad valve seat on cylinder 4
- Head gasket issue causing compression loss or cross‑flow
- Timing components (timing belt/chain, tensioner) out of sync affecting cylinder 4
Diagnosis typically includes a compression or leak-down test on cylinder 4, followed by appropriate mechanical repair or component replacement.
Electrical/wiring or sensor issues
Electrical problems and sensing faults can manifest as a cylinder-specific misfire, especially if control signals or timing data are affected.
- Damaged wiring or connector to coil or injector for cylinder 4
- Faulty crankshaft or camshaft position sensor affecting timing signals
- ECU/PCM software or calibration issues
Repair involves wiring repairs, sensor replacement, or software updates as needed to restore proper timing and fueling.
Diagnostic steps to identify the cause
To pinpoint the root cause of cylinder 4 misfire, follow a structured approach that confirms or rules out each major system. The steps below are commonly used by technicians.
- Read OBD-II codes with a scanner and note P0304 (misfire in cylinder 4) along with any related codes (cam/crank, fuel trim, etc.).
- Perform a spark check: swap the cylinder 4 coil with another cylinder or swap the spark plug with a known-good plug from another cylinder; observe whether the misfire follows the component.
- Inspect the spark plug in cylinder 4 for wear, fouling, or oil contamination; replace if necessary.
- Test fuel delivery to injector 4: listen for injector operation, measure resistance, and perform a fuel-pressure check. Swap injectors if feasible to see if the misfire follows.
- Check for vacuum leaks around cylinder 4: inspect intake manifold gasket, runner hoses, and the PCV system; a smoke test can help locate leaks.
- Conduct a compression test or leak-down test on cylinder 4 to assess mechanical health and identify possible valve or piston issues.
- Inspect wiring and connectors to coil 4 and injector 4 for damage, corrosion, or loose connections.
- Consider timing components if multiple cylinders are affected or if ignition/fuel paths appear normal; misfires can result from timing drift.
Accurate diagnosis reduces unnecessary repairs. If you are unsure, seek a qualified technician who can perform professional tests and, if needed, scope tests of ignition and injection signals.
Summary
A cylinder 4 misfire can originate from ignition, fuel delivery, air intake/vacuum, compression, or timing/electrical faults. A methodical approach—checking codes, swapping components, testing fuel and vacuum, and confirming compression—helps identify the exact cause and guide effective repair. Prompt attention to misfires protects the catalytic converter and minimizes potential engine damage.