Load-Dependent Misfire on a Four-Cylinder Engine
A 1.6-litre four-cylinder petrol engine with a distributor ignition system misfires under acceleration and when climbing a hill. At idle it is smooth. A scan tool shows a flashing misfire code for cylinder 3 and a long-term fuel trim of +9% under load. Compression is 12 bar on all cylinders. The No. 3 spark plug has a gap of 1.6 mm against a specification of 1.1 mm, and its electrode is rounded. The No. 3 high-tension lead measures 25 kΩ; the other leads measure 9–11 kΩ, and the specification is 12 kΩ maximum. The coil secondary measures 9.8 kΩ and the primary 1.1 Ω, both within specification. When the No. 3 plug and lead are moved to cylinder 2, the misfire moves to cylinder 2. The fuel injector resistance and injector balance are normal.
1. Explain why the fault appears under load but not at idle, using the relationship between cylinder pressure, spark voltage demand and the condition of the plug and lead.
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Under load, cylinder pressure rises, so the voltage required to jump the spark plug gap also rises. A 1.6 mm gap instead of 1.1 mm increases the gap by about 45%, and a rounded electrode further raises the demand. The 25 kΩ lead adds resistance in series with the secondary circuit. The coil must therefore produce a higher voltage to push current across the gap.
At idle, cylinder pressure is low and the coil has reserve voltage, so the cylinder fires. Under acceleration or hill climbing, the required voltage may exceed what the coil can deliver through the faulty lead, so cylinder 3 misfires. The other cylinders have normal leads and gaps, so they continue to fire. The fault is load-dependent because the breakdown voltage demand is load-dependent.
2. Interpret the swap test and the fuel trim, and state whether the +9% trim indicates a separate fuel fault.
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Moving the No. 3 plug and lead to cylinder 2 and seeing the misfire move to cylinder 2 proves the fault is in those components, not in cylinder 3's injector, compression or wiring. The ignition secondary is therefore confirmed as the cause.
The +9% long-term trim is most likely a consequence of the misfire: unburned oxygen reaches the oxygen sensor and the ECU adds fuel. It is not proof of a separate fuel fault. After repairing the ignition, clear the codes, road test and watch the trims. If the trim remains high under load with no misfire, then investigate fuel delivery or air leaks.
3. List the repair and the checks you would make before releasing the vehicle, including a check of the distributor cap and rotor.
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Repair
Replace all spark plugs as a set with the correct 1.1 mm gap. Replace the high-tension leads as a set. Check the coil for carbon tracking or cracks.
Distributor checks
Inspect the cap and rotor for cracks, carbon paths, burned terminals and correct air gap. Replace them if worn or tracked. Confirm the ignition timing.
Before release
Clear the codes, road-test under load and confirm the misfire counter stays at zero. Verify fuel trim returns near normal, check for vacuum leaks, and ensure the plug boots are clean and fully seated. Confirm no pending misfire code remains before handing the vehicle back.