What causes a cylinder to misfire?

Basically, it’s one of three things:

  • Loss of spark
  • The air/fuel mixture is too far out of balance to ignite
  • Loss of compression

Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug.

Causes include,

  • Worn, fouled or damaged spark plugs
  • Bad plug wires or even a cracked distributor cap
  • A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder.

Using a Scan Tool

What will a scan tool tell you about misfire? Not much unless the vehicle is equipped with OBDII (1996 or newer). When the OBDII system detects a misfire that exceeds “normal” limits, it illuminates the Check Engine light and sets a P-code that corresponds to the misfiring cylinder. The last number in a P0300 series code tells you which cylinder is misfiring. A code P0304, for example, says cylinder number four is misfiring. If you also find a P0204 code (P0200 series codes cover the injectors), you’d know the misfire was probably caused by a bad injector.

If you find a P0300 code, it means the misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine’s air/fuel mixture, such as a major vacuum leak, leaky EGR valve or unusually low fuel pressure (weak pump or faulty pressure regulator). There’s really no magic bullet for finding misfires. It takes a certain amount of detective work to isolate the fault and determine the underlying cause.