An engine misfire occurs when one or more of the cylinders inside the engine doesn't fire correctly. It is caused by an interruption of the air-to-fuel ratio inside the combustion chamber in the cylinder.
An engine misfire causes the power to drop and reduces gas mileage. It also increases the vehicle's emissions and can cause it to fail an emissions test. A severe misfire can cause the driver to feel a jerking action while driving. Misfires can happen constantly or intermittently. Misfires can be grouped into three categories: fuel, ignition and engine mechanical. Common causes are an issue with the spark plugs, plug wires, the coils or the fuel-delivery system. A scanner can be used to determine which cylinder is misfiring.
The most effective way to prevent engine misfires is to follow the maintenance schedule outlined in the car's owners manual. The cost to repair an engine that is misfiring varies based on the cause of the misfire. Issues with the spark plugs cost about $100 to $300 to repair. Bad fuel delivery or a vacuum leak can cost between $100 to $1000. A mechanical breakdown, such as a burnt valve or a blown cylinder head gasket, varies significantly and can cost up to $10,000.