Quick Check – Ignition System

QC – Random / Single-Cylinder Misfire (Ignition)

Tools: Spark plug socket + feeler gauge, spark tester, multimeter, 12V test light, scan tool (helpful for cylinder ID).

Safety: Let exhaust cool before plug work. Keep hands clear of belts/fan. Do not pull plug wires with engine running. Use insulated tools.

0) Identify the affected cylinder(s)

You cannot diagnose a random misfire without knowing which cylinder is involved.

1. Scan tool (if available): Check misfire counters or codes to identify the cylinder.
2. No scan tool: Use plug inspection and temperature comparison (cold header tube or plug after run).

1) Spark plug condition and gap (primary gate)

The spark plug is the most common single-cylinder failure point.

1. Remove and inspect the plug from the misfiring cylinder. Look for:
   • Cracked porcelain
   • Carbon tracking
   • Oil or fuel fouling
   • Burned or rounded electrodes
2. Measure plug gap.
   Result:
   • Plug damaged, fouled, or gapped wide: replace/correct and retest.
   • Plug looks normal and gap is correct: continue.
   
   Proof: QTS – Single-Cylinder Plug Inspection Gate (coming)

2) Coil or plug wire isolation (swap test)

If the plug passes, the next most likely cause is the coil or wire feeding that cylinder.

1. Swap the coil or plug wire with another cylinder.
2. Re-test.
   Result:
   • Misfire follows the component: coil or wire confirmed faulty.
   • Misfire stays on the same cylinder: continue.
   
   Proof: QTS – Coil/Wire Swap Misfire Isolation (coming)

3) Secondary leakage localized to one cylinder

Single-cylinder misfires often come from insulation breakdown near that cylinder.

1. Inspect boot, wire routing, and nearby metal. Look for arcing paths, melted boots, tight bends, or contact with headers.
2. Night or dark-garage test (if safe). Look for visible arcing while cranking.

Proof: QTS – Localized Secondary Leakage Check (coming)

4) Injector vs ignition split (quick sanity check)

Before condemning ignition further, confirm this is not a fuel injector issue masquerading as ignition.

1. Disable injector for the suspect cylinder (if possible). Note RPM drop.
2. Disable injector on a known-good cylinder. Compare RPM drop.
   Result:
   • Suspect cylinder drop is much smaller: injector/fuel issue likely. Exit to Fuel QC.
   • Drops are similar: ignition/mechanical still suspect.
   
   Proof: QTS – Injector vs Ignition Cylinder Balance Check (coming)

5) Mechanical sanity gate (last stop)

If ignition components swap clean and no leakage is found, confirm the cylinder can actually fire.

1. Compression or leakdown quick check.
2. Valve train inspection if needed.

Proof: QTS – Single-Cylinder Mechanical Sanity Check (coming)

6) Decision closure

• Plug fails: replace/correct plug.
• Misfire follows coil or wire: component confirmed faulty.
• Secondary leakage found: repair insulation/boot/wire routing.
• Injector imbalance: route to Fuel QC.
• Mechanical fault confirmed: repair mechanical issue.

Next links: Ignition Troubleshooting Hub | QC – Misfire Under Load | QC – High RPM Misfire | QC – Intermittent No Spark