Pontiac Repair: 2003 pontiac sunfire wont rev, pontiac sunfire, 2003 pontiac sunfire

Question
we have a 2003 pontiac sunfire that wont rev past 4000 without misfiring or take the gas on hard acceleration.we have replaced the coil pack that improved the idle and the sparkplugs as well as the fuel filter. seems to run smooth at idle and upto 3000 rpm under light acceleration but hesitates under full throttle. any ideas.

Answer
Hi Randy:

will if the engine will not rev higher then it's not breathing correctly I would start at looking for a restriction in the exhaust. possably the catalitic converter is getting restricted. unbolt the front exhaust pipe at the exhaust manifold and drive it and see what happens.. If it runs good you found the problem.. If you still have issues then I would take a look at possably a fuel pump problem where at higher sppeds it's loosing it's pressure/volume to keep the engine running at higher speeds. put a pressure gauge on it and see what ahppens at higher engine speeds. The pressure should be the same or a bit higher then it is at idel. Lastly take a look at the pulg wires possably breaking down and leaking. You replaced both ends of the spark path coil and plugs but not the wires. Use ONLY AC Delco wires.
Good luck :)