Single-Cylinder Misfire — Ignition & Fuel Ruled Out, Compression Fault Confirmed

Vehicle / system context

Educational case: 4-cylinder naturally-aspirated petrol. Synthetic/demo scenario for misfire methodology training.

Complaint

Rough idle, MIL flashing under load, P0303 stored (cylinder 3 misfire).

Initial evidence

Scan: P0303 (Cylinder 3 misfire). Per-cylinder counters: cylinder 3 high, others zero — single-cylinder misfire. Fuel trims balanced. No other codes.

Working differential

Ranked by evidence (single cylinder):

1. Ignition — coil/plug on cylinder 3 (leading single-cylinder cause)
2. Injector — restricted/leaking on cylinder 3
3. Compression — low compression / valve issue on cylinder 3
4. Wiring — coil trigger/injector command to cylinder 3
5. Mechanical — burnt valve, valve seat

Tests performed

1. Swap test (coil): moved cylinder 3 coil to cylinder 1. Misfire stayed on cylinder 3 — coil ruled out.
2. Swap test (plug): moved cylinder 3 plug to cylinder 1. Misfire stayed on cylinder 3 — plug ruled out.
3. Injector command (noid/scope): injector pulse present on cylinder 3 — injector command OK.
4. Compression test: cylinder 3 compression significantly below cylinders 1/2/4.
5. Leak-down test: cylinder 3 leakage high, audible leak at exhaust — exhaust valve not seating.

Evidence updates

Swap tests ruled out ignition. Injector command present ruled out injector driver. Compression + leak-down confirmed mechanical — exhaust valve not seating. Hypothesis collapsed from ignition to mechanical valve fault.

Confirmed root cause

Burnt exhaust valve on cylinder 3 not seating, causing low compression and the single-cylinder misfire. Ignition and fuel were ruled out by swap and command tests before any mechanical test.

Repair summary

Cylinder head removed; burnt exhaust valve on cylinder 3 replaced and seat re-cut. Head reinstalled with new head gasket.

Before / after

Before: P0303, cylinder 3 compression low, misfire counters high on cylinder 3 only. After: cylinder 3 compression matches others; misfire counters zero; P0303 cleared.

Verification

Compression test: all cylinders within spec. Road test under load: no misfire, no flashing MIL, P0303 did not return.

Lessons for technicians

1. Single-cylinder misfire -> swap-test ignition before replacing anything.
2. Swap tests isolate ignition from injector/compression in minutes.
3. Confirm injector command before condemning the injector.
4. Compression + leak-down find mechanical faults swap tests cannot.
5. A burnt valve produces the same P0303 as a bad coil — only the test sequence separates them.