An engine misfire occurs when one or more cylinders in the engine fails, which causes the entire engine to misbehave. A cylinder fails when the air/fuel mixture fails to ignite in the combustion chamber. When the engine is misfiring, it will shake enough that the driver can feel vibrations throughout the vehicle, especially in the steering wheel. Misfires can lead to stalls while idling and can also make it difficult to start the car.
A misfire can be constant, making it hard to ignore, but intermittent misfires can be tricky to diagnose. The occurrence of a misfire can vary depending on the driving conditions such as speed and outside temperature, and it may just happen at random.
It is important to address an engine misfire as soon as possible, as this issue will cause a loss of performance and fuel economy. It will also increase the vehicle's emission levels due to an increasing number of hydrocarbons in the exhaust.
The first step in assessing the damage is to determine which cylinder is problematic, which can be done easily with a diagnostic scanner. The last two digits of the code readout typically indicate the cylinder where the onboard computer detected a misfire, but it is a good idea to consult the owner's manual to make sure you are reading the code properly.
There are four main causes of engine misfires - a loss of spark, an unbalanced air/fuel ratio, a faulty fuel injector and a loss of compression.
The loss of a spark could mean that the spark plug needs to be replaced, which is a relatively simple process. It may also be caused by a faulty spark plug wires. In either case, if the spark plugs or wires are failing in one cylinder, they will need to be replaced. When one spark plug or wire goes bad, there's a good chance the others are not far behind. It is a good idea to replace all of the others when replacing one of these components.
If the mixture of fuel and air is too lean - meaning there is more air than fuel - then it will be unable to ignite, even if the spark plugs are working. There is also a possibility that the mixture is too rich - too much gasoline - even though this tends to be far less common than a lean mixture. A gasoline-heavy ratio of air and fuel can occur if the fuel injector leaks, but this tends to occur in all cylinders, not just one, if it happens.
An improper mixture can be caused by leaks in the fuel lines, a weak fuel pump or a faulty fuel injector. It is important to check the fuel injector on the problematic cylinder. It should spray a fine mist of fuel, but if there are drips, it may indicate that it is clogged, dirty or otherwise damaged.
Low compression can also contribute to engine misfires. Pressure in the combustion chamber is just as important to a healthy cylinder as the fuel mixture and the spark that ignites it. If a chamber is losing compression, it means that the gasoline and air are leaving the cylinder before they can be ignited. This can be caused by leaky vacuum hoses, a busted exhaust valve or a blown head gasket. When two adjacent cylinders are misfiring, chances are that the gasket between them is cracked or damaged.