Ignition System Diagnosis and Repair
What the candidate must know
Gasoline ignition systems include the coil(s), distributor or coil-on-plug, spark plugs, wires, and the crank/cam sensors that trigger them. Know how to test for spark, how to read spark plug condition (fouling, gap, electrode wear, insulator color), how to evaluate coil and wire resistance, and how ignition timing is controlled. Understand the difference between a no-spark and a weak-spark condition and how a misfire code maps to a cylinder.
Traps in questions
- Spark plug color is a diagnostic tool: black sooty fouling = rich or weak ignition; oil fouling = rings or guides; white/blistered = lean or overheating; normal tan = good. Questions often swap rich and lean.
- A no-spark condition on one cylinder with good spark elsewhere points to that coil, wire, or plug - not the crank sensor, which would affect all cylinders.
- Crank sensor vs. cam sensor: a failed crank sensor usually causes a no-start or stall; a cam sensor fault can cause a misfire or poor timing but often allows the engine to run. Do not swap these.
- Coil resistance out of spec or an open secondary indicates a bad coil; a coil can also fail only when hot. Test hot and cold when intermittent.
- Spark plug gap and heat range must match the application; the wrong heat range causes fouling or preignition.
- Worn plug wires can cause a misfire under load (moisture, high resistance) that disappears at idle.
Worked example
A misfire on #3 under load only. The plug is black and sooty, the coil tests good cold, and the wire resistance is high. The high-resistance wire is the likely cause under load. Replace the wire and recheck; do not replace the coil first.
Rule of thumb
One-cylinder misfire = plug, wire, or coil. All-cylinder no-spark = crank sensor, ignition module, or power/ground. Read the plug before replacing parts.