Engine: Misfire

What it Does

“Misfire” refers to a condition in which the engine fails, partially or fully, to develop power by burning the air/fuel mixture in one or more cylinders.

Why This Matters

Engine misfires reduce engine power, risk further mechanical damage, increase engine emissions, and cause the engine to run roughly. In rare cases, the engine may even stall or be difficult to start.

Possible Problems or Concerns

Loss of spark:

Loss of spark will cause a cylinder’s air/fuel mixture not to ignite. This results in unburned fuel going through the exhaust system, which can contribute to catalytic converter damage.

Loss of fuel:

The cylinder(s) cannot produce power if fuel is not present for the air/fuel mixture. If fuel is present only in limited quantity, the cylinder may run too lean, with excessive in-cylinder heat resulting in internal engine damage.

Loss of compression:

Compression refers to the “squeeze” affect caused by the piston moving upward in its bore and compressing the air/fuel mixture above it. Piston and valve problems allow compression pressure to leak out of the cylinder. This reduces the engine power, and if compression loss is severe enough, lack of mixture combustion will result in a “dead” (no power generated) cylinder.

Loss of engine balance:

A misfiring engine runs roughly. Depending on engine design, number of cylinders, and misfire severity, this roughness may be felt as shaking the engine or vehicle, or it may only be perceived as mild vibration (while idling, cruising, or accelerating) or occasional blips while idling. The engine running out of balance poses risk of mechanical damage due to excessive vibration.

Stalling or starting problems:

Severe misfires on multiple cylinders can cause an engine to stop running or be difficult or impossible to start.

Catalytic converter damage:

Misfires induced by lack of spark or lack of compression result in unburned fuel going down the exhaust. This unburned fuel can contribute to overheating of the catalytic converter. In severe cases, the converter may actually come apart and block the exhaust system. This can result in severe loss of engine power. Even if the catalytic converter stays intact, it’s efficiency in cleaning exhaust gases can still be affected by misfire contamination.

Cylinder wash-down

Unburned fuel in the cylinder will wash engine oil off the cylinder walls. This reduces lubrication in the cylinder and could result in wear damage to piston rings or cylinder walls.

Repair Notes

Requisite repairs can range all the way from a relatively simple ignition coil replacement to replacing a broken piston ring (this is a major internal engine repair procedure.) Repair costs cannot be accurately estimated or quoted without first confirming the failure.