Backfires

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

IMPORTANT PRELIMINARY CHECKS
Before using this section you should have performed the "On-Board Diagnostic System Check" - Powertrain/On-Board Diagnostic System Check and determined that:
1. The control module and MIL (Service Engine Soon) are operating correctly.
2. There are no diagnostic trouble codes stored, or there is a diagnostic code but no MIL (Service Engine Soon).
^ Several of the following symptom procedures call for a careful visual/physical check. The importance of visual/physical checks cannot be over stressed. because they can lead to correcting a problem without further checks and can save valuable time.

VISUAL / PHYSICAL CHECK
This check should include:
^ Control module grounds for being clean, tight, and in their proper location.
^ Vacuum hoses for splits, kinks and proper connections, as shown on "Vehicle Emission Control Information" label Check thoroughly for any type of leak or restriction.
^ Air leaks at all mounting areas of the intake manifold sealing surfaces.
^ Ignition wires for cracking, hardness, proper routing, or carbon tracking.
^ Wiring for proper connections, pinches, and cuts.
^ The following symptom charts contain groups of possible causes for each symptom and cover several engines. These procedures are not necessarily meant to be done in consecutive order. If Tech 1 scan tool readings do not indicate the problems, then proceed in a logical order, easiest to check or most likely cause first. To determine if a particular system or component is used on a specific vehicle, refer to the "Electronic/Powertrain Control Module (ECM/PCM)" Wiring Diagrams for application. Electronic/Powertrain Control Module (ECM/PCM)

IGNITION SYSTEM
^ CHECK: Proper ignition coil output voltage with spark tester J 26792 or equivalent (ST-125).
^ CHECK: Spark plugs. Remove spark plugs, check for wet plugs, cracks, wear, improper gap, burned electrodes, or heavy deposits Repair or replace as necessary.
^ CHECK: Ignition system and ignition timing. Ignition Timing
^ CHECK: 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.
^ CHECK: EGR operation for being open all the time.
^ CHECK: Intake and exhaust system for casting flash or other restrictions