Misfire That Follows the Cylinder, Not the Coil
A 2017 1.4 turbocharged direct-injection petrol car sets P0302 (cylinder 2 misfire); the MIL flashes when the customer climbs a long hill, and there is a noticeable shake at idle. The workshop has already swapped the ignition coil and spark plug from cylinder 2 to cylinder 1, and the misfire stayed on cylinder 2. Compression is 12.4–12.6 bar on all four cylinders and leak-down is 6–8 % on all. Injector resistance is 12.1–12.3 Ω on all four. Scan data shows long-term fuel trim +4 % at idle and +18 % at 2500 rpm, and the cylinder 2 misfire counter climbs to 180 counts in 30 seconds at 2000 rpm. Available equipment: a two-channel scope, a 20 A current clamp, a low-amp probe, a secondary ignition probe and a noid light. The customer needs the car for a 300 km trip tomorrow morning.
1. What do the swap, the compression/leak-down results and the fuel trim already tell you?
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Coil and plug swap: the fault stayed on cylinder 2, so both components are excluded - this is the single most valuable test on a COP engine and it has already been done correctly.
Compression 12.4–12.6 bar with 6–8 % leak-down on all four: even and within range. This makes rings, valves, head gasket and a badly worn cam lobe unlikely (not impossible - a valve-train fault that only appears hot or under load can still hide, but it is low on the list).
Fuel trim: +4 % long-term at idle is near normal (±10 % is generally acceptable). +18 % at 2500 rpm is the interesting number - the ECU is adding fuel under load, i.e. the mixture is going lean as airflow rises. A single-cylinder lean misfire drags the whole bank trim positive, so this is consistent with cylinder 2 not delivering its share of fuel.
Synthesis: coil and plug eliminated, base engine mechanically sound, and the engine is lean under load with the misfire locked to one cylinder. The remaining candidates are: a restricted or sticking injector on cylinder 2, the injector circuit (wiring/ECU driver), or an intake-port/air-path issue specific to that cylinder. Injector electrical resistance being equal at 12 Ω only proves the solenoid winding is intact - it says nothing about flow or spray pattern.
2. Using the scope, how would you separate an injector fault from an ignition secondary fault on cylinder 2? What waveform features would you compare?
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Channel plan: channel 1 = injector current with the low-amp clamp around the cylinder 2 injector feed (or injector voltage), channel 2 = secondary ignition for cylinder 2 via the probe on the coil. Capture the same pair on a known-good cylinder (1 or 3) for reference, at the same rpm and load.
What to look at on the injector current ramp:
- Peak current at opening (typically around 1 A) followed by a drop to a lower hold current (~0.5 A) - the step shows the pintle has opened.
- The 'hump' during the hold phase is the pintle movement; compare its size and timing with the reference cylinder.
- Total injector on-time as commanded by the ECU. If cylinder 2's on-time is longer than the others at the same load, the ECU is compensating for a weak contribution.
Critical judgement: the current clamp proves the electrics - driver, wiring, solenoid. It cannot prove flow. A clogged injector with perfect electrics will produce a perfectly normal current ramp. So a normal ramp does not clear the injector.
What to look at on the secondary: firing kV and burn time. A lean cylinder (starved of fuel) needs a higher voltage to jump the gap and shows a shorter burn time and higher firing line than its neighbours; a rich or fouled cylinder shows the opposite. Correlate the scope events with the misfire counter, and check whether the misfire happens on the injector pulse or on the spark event.
3. State your most probable cause, the confirmation test before any parts are fitted, and what you tell the customer about tomorrow's trip.
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Most probable cause: a restricted/sticking injector on cylinder 2, delivering less fuel than the others (lean, hence the +18 % trim under load and the misfire that worsens on a hill). Second candidate: the cylinder 2 injector circuit or ECU driver - but only if the current ramp is abnormal.
Confirmation before fitting parts: move the suspect injector to another cylinder and re-check. If the misfire counter and P0302 follow the injector, the injector is proven. On a direct-injection engine this means new combustion-seal rings and correct torque, so it is not a five-minute swap - but it is still cheaper than an unnecessary set of injectors. If the fault stays on cylinder 2 after the move, look at the cylinder 2 driver circuit and the intake port. A flow/balance test or pressure-drop test on the injectors is an acceptable alternative if the workshop has the rig. Re-check trims and the misfire counter after the repair; the long-term trim should come back towards single figures.
Customer advice: do not take the 300 km trip. A flashing MIL means a catalyst-damaging misfire - unburnt fuel is being pushed into the catalytic converter, which can overheat and be destroyed (a far bigger bill than an injector), and fuel washing the cylinder bore causes bore wear. If the car must move, it should be driven gently to a nearby repairer, not on a long high-load trip.