cylinder 4 misfire is usually caused by a fault isolated to that cylinder, most often in the ignition system, fuel delivery, air intake or compression. The most common culprits are a faulty spark plug or ignition coil on cylinder 4, a failing injector, a vacuum leak, or reduced compression in that cylinder.
Overview of fault-source categories
Ignition system issues
Problems in the ignition pathway for cylinder 4 can stop that cylinder from firing properly, even if other cylinders are working normally.
- Worn or fouled spark plug in cylinder 4
- Faulty ignition coil or coil-on-plug for cylinder 4
- Incorrect spark plug gap or improper installation
In most cases, replacing the spark plug and testing or swapping the coil can quickly identify whether ignition is the root cause.
Fuel delivery issues
Inadequate or inconsistent fuel delivery to cylinder 4 can produce a misfire even when other cylinders run normally.
- Clogged or failing injector for cylinder 4
- Low fuel pressure affecting injector performance
- Leaking injector O-rings causing leaks or vacuum imbalances
- Contaminated or poor-quality fuel that affects combustion
Fuel delivery concerns often show up with related fuel trims in the scan tool and may require injector testing or fuel-system inspection.
Air intake and vacuum issues
Air-fuel imbalance, often from leaks, can cause cylinder 4 to misfire even with good ignition and fuel delivery.
- Vacuum leak near cylinder 4 intake manifold gasket or hoses
- Cracked or loose vacuum lines feeding the intake or brake booster
- Mass airflow sensor (MAF) or intake leaks affecting cylinder 4’s air supply
- Intake valve issues that restrict air into cylinder 4
Vacuum leaks are often detected with a smoke test or by listening for hissing sounds under engine load.
Compression and mechanical issues
Low or inconsistent compression in cylinder 4 points to internal engine wear or timing problems that prevent proper firing.
- Low compression in cylinder 4 due to worn piston rings or cylinder walls
- Valve timing issues (camshaft or timing chain/ belt problems) affecting cylinder 4
- Burnt valve, leaking head gasket, or damaged valve seat
- Piston damage or broken ring causing poor sealing
Compression or timing problems typically require a mechanical test (compression and leak-down tests) and may demand component replacement.
Electrical/ECU and sensors
Electrical or control-system faults can manifest as a cylinder-specific misfire if the injector or coil driver for cylinder 4 is compromised.
- Wiring harness or connector corrosion to injector 4 or coil 4
- Faulty injector driver or ECM/engine control module behavior
- Sensor glitches (rarely cylinder-specific) that disrupt timing or fueling
Electrical issues often show up in live data and fault codes, and wiring inspections may be required.
Carbon buildup and indirect effects
In some modern engines, carbon buildup on intake valves or ports can cause misfires in certain cylinders, including cylinder 4, particularly in direct-injection engines where air flow is sensitive to deposits.
- Carbon buildup on intake valves reducing air intake
- Deposits around injectors affecting spray pattern or sealing
Addressing carbon buildup may involve cleaning the intake, solvent treatments, or, in severe cases, mechanical cleaning or valve service.
Diagnosis: how to pinpoint the cause
To identify the exact cause of a cylinder 4 misfire, technicians follow a structured diagnostic approach that combines history, data, and test results.
- Check the diagnostic trouble code (P0304) and review freeze frame data from the vehicle’s onboard computer.
- Inspect the spark plug in cylinder 4 for wear, fouling, oil deposits, or physical damage; replace if necessary.
- Test or swap the ignition coil/coil-on-plug for cylinder 4 with another cylinder to see if the misfire follows the component.
- Test the fuel injector for cylinder 4 (resistance check, voltage signal, and injector balance test) and inspect its wiring harness.
- Measure fuel rail pressure and verify that fuel pressure is within specification for the engine.
- Perform a compression and/or leak-down test on cylinder 4 to assess mechanical health and valve timing.
- Check for vacuum leaks around the intake manifold and cylinder 4 region; use a smoke test if available.
- Review live data for ignition coil current, injector duty cycle, and air-fuel trim to identify anomalies.
- Consider carbon buildup assessment if the engine has a history of direct injection and misfires persist after ignition and fuel tests.
Accurate diagnosis may require professional tools and knowledge, and several steps may be needed to confirm the root cause before repair.
Next steps and practical considerations
If cylinder 4 continues to misfire after basic inspections, consider scheduling service with a qualified technician. Provide the service history, including recent spark plug or coil replacements, injector service, or timing work, as these details help isolate the fault more quickly.
Some misfires are intermittent and temperature- or load-dependent; a thorough road test or long-term scan can reveal patterns not visible on a single inspection.
Summary
Cylinder 4 misfires most often stem from ignition issues (spark plug or coil), fuel delivery faults (injector or pressure), air or vacuum leaks, or compression/timing problems. A systematic diagnostic approach—checking codes, inspecting and testing the spark and injector for cylinder 4, evaluating fuel pressure, and assessing compression and leaks—usually leads to the root cause. In modern engines, carbon buildup can also contribute, especially with direct injection. Prompt diagnosis helps prevent further engine damage and narrows repair costs.
How do you fix a misfire cylinder 4?
Repairing P0304
- Replace spark plugs: Worn or fouled spark plugs in cylinder 4 should be replaced.
- Replace ignition coils: Faulty ignition coils should be replaced to ensure proper spark delivery.
- Repair or replace fuel injectors: Malfunctioning fuel injectors in cylinder 4 should be repaired or replaced.
What is the most common cause of cylinder 4 misfire?
Spark plug 4 is fouled, worn, or failed. Spark plugs deteriorate with age (typically 30,000-100,000 miles). This is the most common misfire cause. Modern engines have one coil per cylinder or coil pack.
Can I still drive with cylinder 4 misfire?
Technically, yes, you might still be able to drive – but it's not a good idea. Misfires can cause rough idling, poor acceleration, and reduced fuel efficiency, making driving both uncomfortable and risky. Imagine trying to merge onto a highway or pass another vehicle with sluggish acceleration.
How much does it cost to fix a misfire in cylinder 4?
Depending on the cause, the repair or replacement costs for an engine misfire can range between $100 and $1,000. The intensity of the issue will ultimately determines the repair rates.