Relative Misfires…
This weeks tip comes to you in the form of misfire diagnoses. When you have a vehicle with a misfire, the very first tool you should break out is your ears. Crank the vehicle over in clear flood mode(accelerator pedal to the floor) or disable the ignition system and crank over. When doing this test you want to listen to the cranking sound and cadence. The cadence should sound uniformed, with no noticeable change in tone. This will help give you an indication if your misfire is due to loss of compression, with out even hooking up any tools to the vehicle. This past week I was called out to three different service calls with complaints of a misfire, all of which had different parts installed in an attempt to correct the issue. All three failed the hearing check, due to loss of compression. Once the vehicle has failed the hearing test, your next step should be to break out your oscilloscope and preform a relative compression test, to confirm you initial diagnoses. The relative compression test can be performed in may different fashions. My recommendation would be to perform a relative compression check, a cylinder sync (secondary ignition or command signal), and a pressure sensor in the intake manifold. Doing these three checks will show you which cylinder(s) low, and if it is a possible valve sealing issue all with in 5 minutes of diagnosing. Below I will attach a link to a YouTube channel Super Mario Diagnostics, the video I am going to attach shows a bad cranking cadence followed up by a relative compression test to confirm the initial diagnoses. Also I have attached some links to tools that will assist you in this diagnoses.
Relative compression in action.
ATS eSCAN Elite (the eSCAN Elite is a diagnostic scan tool, that has a relative compression function built in)
