Accepted Answer

Hi XXXXXXX. Thank you for asking your question on JustAnswer.

You say your mechanic replaced the coils and plugs. Did he replace the spark plug wires? The wires themselves are usually the cause of backfiring problems (if it is ignition related). Make sure the wires are routed properly, all retainers are in place and there is wire loom around the wires where they pass between the cylinder head and the engine lift ring by the ignition coils. The factory loom in this area usually is falling apart due to heat and needs to be replaced. The loom is just a piece of split plastic covering that covers many kinds of wires used in car wire harnesses and is available at any auto parts store. With all the backfires you have had, you need to check the induction system to be sure you haven't blown out the air filter, or broken the cover to the air cleaner box. Remove the screws holding the cover in place and check the cover for cracks. Also check the body of the air cleaner where the screws thread in to make sure there are no cracks around them. If something is broken, any late model air cleaner will replace the CNG unit (they are the same). Also inspect the rest of the hoses from the air cleaner box to the throttle body on the back of the engine. Make sure the hoses and clamps are in place and tight. Any leaks in the induction system will lead to hard starting and backfires. As a final note, if everything is working correctly, "pumping the gas" will not help the car to start. You must leave your foot off the accelerator pedal to allow the proper flow of natural gas into the induction system. Pushing on the pedal in a normal start helps create a lean condition the increases the risk of backfire. If these things don't change or fix the problem---come on back for more advise.

Expert:	Car Dude
Pos. Feedback:	97.4 %
Accepts:
Answered:	7/11/2009

Mechanic

15 Years of troublshooting cars and being a mobile mechanic.