OBD II Misfire And Diagnosis

July 30, 2014

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:

This scope pattern of a normal secondary ignition event indicates the condition of the spark plug, plug wire, ignition coil, air/fuel mixture and even the amount of air turbulence in the cylinder.

• 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

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