When diagnosing misfires, it’s important to use tools that allow you to go beyond a simple code reader that ­displays a P03XX. Even an OEM-level scan tool can’t tell you what the firing voltage is or what the ignition pattern looks like. Nor can it tell you if the ­serial data is accurate or correct.

 

For that kind of information you need a DVOM or a graphing multimeter/oscilloscope that can look at sensor voltages directly and display primary and secondary ignition patterns. If the vehicle has a distributorless (DIS) or coil-over-plug (COP) ignition system, you’ll also need the appropriate inductive pickups to get a good ignition pattern signal from the coils. Mode $06 data via a scan tool is also helpful on some vehicles for determining which cylinders are misfiring, as well as the­ ­severity of any misfires.

Misfire causes can be categorized into four possible areas:

• Ignition System

– Fouled/worn spark plugs

– Shorts, leaks or open ­circuits in secondary ignition wiring/components

– Defective ignition coil(s) (COP or DIS)

– Intermittent ignition ­module/coil driver

– Low system voltage

• Mechanical

– Carbon deposits

– Worn valve train/burnt valves

– Loose timing belt or chain/valve timing

– Head gasket failure

– Worn rings, piston mechanical damage

• Air/Fuel System

– Low fuel level/poor fuel quality

– False air/vacuum leaks

– Clogged fuel filter

– Low fuel pressure/volume

– Open or shorted injector coil

– Damaged injector driver

– Injector ground/power supply ­circuit problems

– Clogged injectors

– Stuck (open) EGR valve

– Low system voltage

• False Misfires

– Rough road

– Heavy acceleration when cold

– Loose motor or transmission mounts

– Transmission/torque converter ­mechanical faults

 

Courtesy Delphi Product & Service Solutions.