ok so i figure ill move the post over here, mayb e someon can help....plug gap is set at .45 , all exhaust is tightened, timing has been advanced from 0 -2-4-6-8-10-12, nothing helps , whole ignition system is new, coil cap dist rotor ignition coil plugs and wires. the backfire only happens on a de-celleration , lets say after i go 60-100 mph on the highway....when i start de-cellerating, itll backfire...liek a shot gun. there are no leaks on any hoses anywhere, ....--every sensor is new as well....i have no power loss at all. any ideas?

 

Check the diverter valve on the air injection system. When decelerating the F/A mixture gets richer, the diverter routes the air from the pump away from the exhaust manifold to eliminate backfiring. On GM's there should also be a couple of check valves. Basically you don't want air going to the tubes that feed it into the exhaust during deceleration. On most TG's the diverter is electrically driven and located on the pass side.

 

where exactly is it, what does it look like and how do i go about fixing it lol ...

 

My 89 5.7 TPI has the diverter located on the left side looking at the engine from the front. It is a square plastic box with a removable cover. The diverter is electrically driven by the ECM. It reroutes the air from the air injection pump(I assume you are still using the AIR system with the headers)during deceleration. Inspect the hoses etc. The rear of the box has 2 hoses which if you follow go to check valves and then to the preformed tubes that attach to the exhaust manifold or headers. Remove the 2 hoses and try it then. The check valves should prevent back feeding of exhaust gases so you should not have to plug the hoses. See if that cures the backfiring. If it does post and we'll get deeper, no pun intended.