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Engine Backfires

If fuel ignites in intake manifold or exhaust system and makes a loud popping noise, check the following items:

Ensure exhaust system is not leaking.
Check base timing and ensure timing advance system is functional.
Using Spark Tester (ST-125), check output of ignition coils.
Check for intake manifold gasket leak and TBI gasket-to-manifold leak.
Check for proper valve timing.
Check engine for sticking or leaky valves.
Check ECM and ignition system for faulty grounds.
Check throttle body dashpot adjustment (if equipped).
Check throttle position sensor.
Check coolant temperature sensor.
Check air valve, ISC solenoid and idle switch operation.
Check intake air system and throttle body electrical connections.
Ensure fuel system pressure is correct.
Check A/C vacuum solenoid valve (if equipped).