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:
• 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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