97 2.4 Gas/code problem

[Trunks]

I know this is a long shot, but what would make the gas milage in my 97 2.4 go down, drastically. I have replaced the plugs, fuel pump, fuel pressure reg, injectors, and prolly a few other things i am forgetting. I have taken it to a few shops and they cant seem to find the problem.
The check engine light is on also. Been getting different codes for the last few years. Now the code is that a piston is misfiring. This is getting old.
I know that most will say take it to a shop, but none have been able to help me, just try to stick me with a bill.
Thanks

[AceGAGT]

just an idea but u might try checkin out ur catalytic converter. i just "fixed" (a.k.a. The re-bar method) the one on my s10 and my mpg rose from 12 to 19... or talk to a dealership/specialty shop that deals w LD9's a lot...