Why does the engine continuously backfire when accelerating ? Idle is perfect

POSSIBLY YOUR TIMING BELT JUMPED TIME. PULL THE UPPER TIMING COVER AND CK CAM AND CRANK TIMING

I hate changing any parts without proving for sure they are bad! It becomes very expensive. Has the oil been changed frequently, could have a variable cam issue or many other things hard to say without being able to do basic diagnoses tests. Start again with the basics. There is a professional service called Identafix. http://www.identifix.com/info/about_us.htm They only deal with professional repair shops. They are in the Minnesota 651-633-8007 begin_of_the_skype_highlighting 651-633-8007 end_of_the_skype_highlighting. Your auto repair shop do all their diagnosis tests, and discuss your problem with a team of professional technicians great service cheap and very bright people. If the repair shop that are repairing your car are stumped this is a great service. This company only deal with automotive repair shops and don't answer questions for the general public but are an amazing knowledge base. A professional grade scan tool reads stored trouble codes, pending trouble codes, looks at live sensor data so you can look at a sensors performance and output signals without having to do intensive testing at each sensor, (cheaper scan tools have limited capabilities and may only read the trouble code stored). As with most things automotive it's best to check the basics first, check for vacuum leaks, fuel pressure and delivery volume. Verify engine mechanical integrity (even compression on all cylinders sufficient intake manifold vacuum at idle, ect....) before looking for complex problems check that the basics are sound.