Misfires

What is a misfire?

An engine misfire happens when you don't get complete combustion of the air/fuel mixture in the cylinder. You really have two types, one that is enough to raise emissions, and worse ones that can actually damage/destroy catalytic converters. With the second type you will get a flashing SES light indicating an active misfire, you should stop/adjust your driving immediately to prevent damage.

What causes a misfire?

You need 3 things for combustion, Air/Fuel/Ignition, below are some common issues that can cause a misfire. Remember, before checking electronics/sensors always check your battery and alternator otherwise you can get false readings.

Air

• Dirty air filter

• Dirty throttle body

• Dirty intake/exhaust valves

• Intake/vacuum leaks in seals/hoses/EGR/PCV

• Blocked exhaust(cat)

• Dirty/failing/bad connection on sensors(MAF/MAP/IAT/TP)

Fuel

• Low fuel pressure, pump worn or not getting proper power/clogged filter/leaks/failed regulator

• Fuel injectors leaking/failed/bad connections/wiring

• Sensors failing or bad power/connections ECT/o2

Ignition

• Timing - Check for Pancake Distributor Disease and general Distributor issues. ICM and Sensors - CMP/CKP/KS

• Spark plug wires - worn out/loose/cracked

• Coils - worn/cracked/connections/power

• It's almost never the PCM, rule out everything else first.

Engine condition

• Worn cylinders(walls/pistons/rings)

• Worn valvetrain pushrods/rockers/springs/rollers/timing chain/sprockets

• Valves dirty/worn

• Loose/damaged spark plugs