Comprehensive diagnostic guide for OBD-II code P0308 Cylinder 8 Misfire Detected

Overview

• What P0308 means: P0308 is a powertrain diagnostic trouble code indicating a misfire detected specifically in cylinder 8. In other words, the engine control module (ECM/PCM) has detected that cylinder 8 is not firing properly.
  • Source context: Open Source OBD-II code definitions list (Cylinder 8 ignition fault detected), aligning with the standard interpretation of P0308.
  • General context: OBD-II uses powertrain codes to monitor and report misfires and other engine-related faults. Wikipedia describes that misfire and other powertrain codes fall under the OBD-II diagnostic trouble code framework.

Symptoms

• MIL (Check Engine Light) illuminated and a rough engine feel, especially at idle.
• Engine hesitation or reduced power during acceleration when trying to drive away or climb grades.
• Noticeable vibration or shaking due to one cylinder not producing combustion.
• Degraded engine performance and potentially increased fuel consumption.
  Note: If P0308 occurs in isolation, symptoms may be subtler; if accompanied by P0300 or other P030x codes, multiple cylinders may be involved or a broader system fault may be present.

Typical causes (grouped by system) and expected likelihood
These are common, experience-based likelihoods and should be treated as informed estimates rather than guarantees. Exact probabilities vary by vehicle, age, maintenance history, and usage.

• Ignition system (most common first checks)

  • Faulty ignition coil or coil-on-plug for cylinder 8 (including internal coil failure or damaged coil boot/wiring)
  • Worn or fouled spark plug in cylinder 8 (bad gap, fouling from oil/fuel mixture, or electrode wear)
  • Wiring/connector issues to coil-on-plug for cylinder 8 (loose, corroded, or damaged connector)
  • Likelihood range: high (roughly 40-50% as a practical first-check category)

• Fuel system

  • Faulty or restricted fuel injector for cylinder 8 (sticking injector or injector circuit fault)
  • Insufficient fuel pressure affecting cylinder 8
  • Injector wiring circuit or driver issues (PCM output to injector)
  • Likelihood range: moderate (roughly 15-25%)

• Air and induction

  • Vacuum leak or intake manifold issue that preferentially affects cylinder 8 (less common for a single-cylinder leak but possible with a loose runner/gasket issue)
  • Mismatched MAF/MAP readings or dirty intake air path contributing to misfire perception
  • Likelihood range: low to moderate (roughly 5-15%)

• Engine mechanical and internal faults

  • Low compression in cylinder 8 (valve issue, piston ring, or cylinder wall problem)
  • Camshaft/valve timing issues could manifest as misfire under certain conditions
  • Likelihood range: low to moderate (roughly 5-15%)

• Electrical/PCM (less common but possible)

  • PCM/ECU misbehavior or intermittent drive signal to coil or injector for cylinder 8
  • Wiring harness damage or grounding issue affecting coil/injector circuit
  • Likelihood range: low to moderate (roughly 5-7%)

• External contamination or combustion-related

  • Oil/fuel contamination fouling the spark plug in cylinder 8
  • Contaminated fuel or abnormal combustion environment affecting only cylinder 8
  • Likelihood range: low (roughly 5-10%)

What to inspect first (stepwise diagnostic approach)

1) Verify the fault and collect data

• Confirm P0308 is current/persistent and review any pending codes (if present: P0300, P0301-P0307).
• Review freeze-frame data: RPM, throttle position, engine load, LTFT/STFT at the time of misfire.
• Note engine temperature and running condition (idle vs. under load).

2) Perform a focused visual and physical inspection

• Inspect cylinder 8 ignition components: spark plug condition (gap, fouling), coil-on-plug boot, and 8-cylinder wiring if applicable.
• Check for obvious vacuum leaks or intake gasket issues near cylinder 8 region.
• Inspect the injector 8 electrical connector and wiring harness for damage or corrosion.

3) Ignition system test (cylinder 8)

• Spark plug: remove and inspect; replace if worn or fouled. Compare gap to specifications.
• Coil/coil-on-plug: perform a swap or swap test (see step 9 below) or use a known-good coil for cylinder 8 to see if misfire follows.
• Check coil primary/secondary resistance if service manual provides specs; verify with the proper test procedure.
• Note: If the misfire moves with swapping components, the faulty part is identified.

4) Fuel system test (cylinder 8)

• Check injector 8 circuit resistance and verify injector driver signal with a scan tool or oscilloscope.
• Test fuel pressure to ensure adequate supply; compare to spec and observe whether fuel trims (LTFT/STFT) indicate lean or rich conditions.
• If injector 8 is suspected, perform a flow/functional test or swap injectors to verify whether the misfire follows.

5) Compression and mechanical inspection

• Perform a compression test on cylinder 8 and compare with adjacent cylinders; consider a leak-down test if indicated.
• If compression is low, investigate potential valve seating issues, piston ring wear, or head gasket concerns.

6) Data monitoring while the engine runs

• Monitor misfire counters for cylinder 8 (via scan tool).
• Monitor long-term and short-term fuel trims (LTFT/STFT) to identify lean/ruel conditions around idle and WOT.
• Check manifold pressure, MAF or MAP sensor readings for abnormal air/fuel mixture indications.
• Observe crank/cam sensor readings if available; abnormalities can influence ignition timing and fuel delivery.

7) If no obvious electrical or mechanical fault is found

• Consider wiring/ECU integrity: inspect for harness chafing, corrosion, or poor grounds related to cylinder 8 circuits.
• If all components appear sound, consider PCM/ECU driver issues as a last resort.

8) Cylinder-to-cylinder test techniques

• Component swapping approach (diagnostic quick checks):
  • Swap the spark plug from cylinder 8 with another cylinder's plug (or swap the coil 8 with a known-good coil in another cylinder if feasible) to see if the misfire follows the component.
  • If the misfire moves to the swapped cylinder, the problem component is identified (ignition coil, plug, etc.).
  • Similarly, swap injector 8 with another cylinder's injector and observe misfire behavior.
  • Important: Only swap components within the limit of safe service and with proper torque specs to avoid damage.

9) Final checks and repair plan

• Replace the root cause identified (e.g., ignition coil, spark plug, injector, wiring harness).
• If compression is low on cylinder 8, plan mechanical diagnosis by a qualified tech (valve/seat, piston, rings).
• After repair, clear codes and drive to recheck for reoccurrence; recheck data (misfire counters, fuel trims) to confirm resolution.

Safety Considerations

• Energized fuel system hazards: relieve fuel rail pressure before disconnecting any fuel lines or connectors; keep away from sparks during diagnostic steps.
• Electrical safety: disconnect battery when performing electrical connector inspection or when wiring repairs are planned to avoid short circuits.
• Mechanical safety: use proper PPE; secure the vehicle; avoid hot engine components when performing plug and ignition work.
• Follow vehicle-specific service information for torque specs, harness routing, and component replacement procedures.

Representative diagnostic workflow (concise checklist)

• Confirm P0308 with freeze-frame data and misfire counters.
• Inspect ignition components for cylinder 8 (spark plug, coil, wiring).
• Test/verify spark at cylinder 8; replace or swap as needed.
• Inspect fuel delivery to cylinder 8 (injector, fuel pressure, and injector signal).
• Check compression in cylinder 8; perform leak-down if needed.
• Inspect for vacuum leaks or air-path issues affecting cylinder 8.
• Validate electrical harnesses and PCM driver circuits for cylinder 8.
• If needed, perform component swaps to identify the failing part.
• After repair, clear codes and verify fix with road test and data monitoring.

This diagnostic guide was generated using verified reference data:

• Wikipedia Technical Articles: OBD-II
• Open-Source OBD2 Data: N/A (MIT)

Content synthesized from these sources to provide accurate, real-world diagnostic guidance.

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