Engine Controls - Trouble Shooting - No Codes: Symptoms: Backfire

Check ignition timing.
Check ignition coil output.
Check for engine vacuum leaks and/or engine not tuned to specifications.
Check secondary air injection system.
Check for faulty spark plugs.
Check EGR system.
Check for fuel or water in vacuum hose to MAP sensor. Also check for restricted hose.
Check fuel injectors. Perform INJECTOR BALANCE TEST in SYSTEM & COMPONENT TESTING article.
Check for crossfire between spark plugs.
Check for sticking or leaking valves.
Check for improper or worn camshaft.
Check engine compression.
Check fuel pressure.
Check for correct valve timing.
Check harmonic balancer interrupter rings for missing, broken or bent vanes.
Check for cracks in exhaust and intake manifold.