whats up im new to this site srry if this is the wrong spot but i have a 1998 chevy silverado 5.7 L 1500 and its backfiring. while in park or nuetural it doest backfire but when its in reverse or drive the idle goes down to 500 rpms and backfires i changed the spark plugs and wires the timing is right i checked the cap and rotor they look good im thinking its the fuel filter or fuels injectors i just wanted to see what you guys think before i go buy them plz anything will help.

 

Take the valve covers off and check your valve springs. Sounds like one is broken.