#ignition probe

Why is the Vehicle Engine Misfiring?

When you encounter a rough running engine caused by a misfire, one of the best ways to quickly find out what is causing the motor to miss is to scope test the ignition system.

In this article we will discuss what ignition patterns look like when there is a lack of fuel causing the misfire.

Ignition waveform characteristics of a lean misfire.

Let’s first look at a good coil on plug ignition waveform pattern.

                    Notice in the above known good coil on plug secondary waveform that the Spark Line also called the Burn Line is relatively flat-with upward slope starting at about 75% of the total burn time and has a small nose at the end. The Burn Time equals the length of the Spark Line, and it’s duration  should be about 1.1 milliseconds to about 1.5ms long.

Rarely will you be able to detect a lean misfire while scope testing just at idle, you are best off to place the engine under a slight load, or perform multiple light snap throttle events while viewing your scope.

Here is a screen capture during a slightly lean condition.

                    Notice how as the air-fuel mixture in the cylinder is leaning out that the height of the nose rises, the nose gets a lot larger and that the upward slope of the spark line is starting  at about 60% across the total burn time. Another thing that is happening during this lean condition is the total burn time is getting shorter, if you compare both images above.

Here is what an extremely lean ignition pattern looks like.

                    When the air/fuel ratio is extremely lean as in the image above, the spark line slope is starting it’s upward slope almost immediately. The spark line slope and length (in time) plays a huge role in oscilloscope diagnostics of engine performance issues.

The Wyzeprobe coil-on-plug-probe was used to capture the ignition patterns from a 2008 Honda Fit 1.5L engine.