Why Does an Engine Misfire?

[DelRichard]

Engine misfiring can be caused by: 1. Severe burning of the contacts in the distributor cap of the ignition system, leading to spark jumping; 2. The ignition coil failing to produce high voltage; 3. Reduced radial width and weakened elasticity of the piston rings; 4. Excessive carbon buildup on the spark plugs; 5. Carbon accumulation in the fuel injector, clogged injector filter screen, resulting in imprecise injection and atomization. Symptoms of engine misfiring include: 1. Noticeable body vibration at idle; 2. Obvious intermittent exhaust flow with severe shaking of the exhaust pipe; 3. Significant body vibration at low RPMs around 2000, accompanied by weak acceleration, with reduced vibration sensation at higher RPMs.

[DeMiguel]

I had this issue last time, the car was shaking like a tractor. A misfire simply means one cylinder is slacking off. Most likely it's the spark plugs or ignition coils that are faulty—one is responsible for sparking and the other for supplying power. A clogged fuel injector can also starve the cylinder; if gasoline can't get in, it just shuts down. For older cars, you should also check the cylinder compression—if the piston rings or valves are leaking and the compression pressure is insufficient, it’ll stop working too. Once, I took in a car with a misfire code, and it turned out the intake carbon buildup had completely clogged the fuel injectors—after cleaning, it ran smoothly. A dirty electronic throttle can also cause trouble; if the signal is inaccurate, the ECU will mess up the fuel injection. The easiest way to fix it is to read the data stream with a computer—it directly reveals which cylinder is slacking off.

[AlejandroLynn]

Misfire issues depend on whether they're persistent or occasional. The most common culprit is ignition system problems—cracked ignition coils leaking electricity can cause intermittent misfires, especially noticeable at highway speeds. Weak spark from excessive spark plug gap or cracked ceramic insulators can lead to sudden failures. Sticky fuel injectors result in poor fuel atomization and inaccurate fuel delivery measurements. Sensor failures are particularly troublesome; a faulty camshaft position sensor makes the ECU miscalculate ignition timing. Last time I fixed a car with consistent cold-start misfires, pulling the connector revealed aged spark plug wires with excessive resistance—replacing the wiring harness immediately solved the shaking. If multiple cylinders misfire simultaneously, check the timing belt; skipped teeth throw everything off. Damaged hydraulic lifters failing to open valves properly is another sneaky cause.

[Leroy]

Cylinder misfire can be divided into three situations. Complete misfire means the cylinder completely stops working, usually caused by spark plug breakdown or fuel injector being completely stuck. Partial misfire can still manage to combust, but with obvious power loss and soaring fuel consumption, mostly due to aging ignition coils or fuel injection deviation. Intermittent misfire is the most troublesome, such as occurring only when going over speed bumps, requiring inspection of loose wiring harness connectors. I usually start with swap diagnosis, moving the suspected ignition coils and spark plugs to other cylinders - if the fault follows, it's confirmed. Insufficient cylinder pressure needs measurement; if below 8 bar, consider valve sealing or piston ring issues. Special reminder for vehicles converted to natural gas - their ignition systems bear greater loads and are more prone to cylinder misfires.

[DelBrielle]

Four cylinders are like a choir, and a misfire is like someone singing out of tune. There are five common causes: ignition issues (spark plug carbon buildup/electrode wear), insufficient ignition energy (coil aging/excessive cylinder wire resistance), fuel problems (clogged injectors/low fuel pressure), sealing failures (valve leakage/piston ring wear), and control malfunctions (ECU signal errors/sensor drift). Once, a BMW N52 engine was brought back for repair due to a misfire in the third cylinder, which turned out to be caused by an intake manifold leak leading to an overly lean mixture that couldn't ignite. Turbocharged cars require extra attention to the intake piping, as even tiny cracks can cause the corresponding cylinder to fail.

[OGianna]

Misfiring is essentially a combustion interruption. When dealing with it, I follow the three principles of fuel, ignition, and air systems. For the ignition system: Pull out the ignition coil to observe spark intensity, measure the resistance of the secondary coil (normal range 4-6 kΩ), and check for cracks in the spark plug ceramic body. For the fuel system: Listen for the clicking sound of the injectors, measure fuel rail pressure (around 3 bar for standard EFI vehicles), and disassemble and clean the injectors to inspect the spray cone angle. For the mechanical aspect, observe backfiring and popping phenomena: Backfiring indicates premature ignition of the air-fuel mixture, while popping is unburned fuel exploding in the exhaust pipe. Cylinder compression testing is crucial; a variance exceeding 15% between cylinders indicates an issue. Excessive crankcase pressure suggests piston ring blow-by, which is often overlooked.