RE: 318 backfiring bad
Could be the,map sensor, or an injector that's stuck open. Backfire running on an efi vehicle is likely coming from gas being thrown into the manifold. When you say "runs fine to a certain point", are you saying it runs fine until it warms up a bit? The pcm has probably got youleaning back at idle and heating your manifolds/headers up real good, then when you throw fuel in there at low rpm's it goes right out the exhaust valve and ignites in the exhaust. As the rpms get higher the pcm balances out and the exhaust velocity is higher so you don't get the back fire.You need to put a scanner on it (cheap one - free to use at Autozone, etc.) and look at your fuel trims. Bad coils will do the shutter/ backfire thing, but usually won't run well at high rpms.