Backfires

SYMPTOM DEFINITION:
Fuel ignites in intake manifold, or in exhaust system, making a loud popping noise.

PRELIMINARY CHECKS
^ Refer to Diagnostic Notes. Initial Inspection and Diagnostic Overview

IGNITION SYSTEM
Check:
^ Proper ignition coil output voltage with spark tester J 26792 or equivalent (ST 125).
^ Spark plugs. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits. Repair or replace as necessary.
^ Ignition system and ignition timing.
^ For crossfire between spark plugs (distributor cap, spark plug wires, and proper routing of plug wires).

ENGINE MECHANICAL
Check:
^ Compression, valve timing, manifold gaskets, sticking or leaking valves.
^ EGR operation for being open all the time. Refer to EGR System C Chart. System Diagnosis - Exhaust Gas Recirculation (EGR) System
^ Intake and exhaust system for casting flash or other restrictions.