I have a 2004 GT, 5 speed. So far I've added Steeda CAI, Accufab 70mm throttle body, BBK shorty headers, Borla cat back, 3:73 gears, short throw. Recently I noticed what I believed to be a misfire, mostly at speed, 70 mph +. I thought it may be plugs or coil packs so I replaced both, NGK iridium ad Accel coil packs. The "miss" is still there. Any idea what else it could be?
first and i hope that i am WRONG...what weight of engine oil are you running???? there were issues a long time ago... about thicker than spec oil... holding the lifters all the way up and floating the valves...next... do you own an OBD2 scan tool that displays life data stream and graphs some of the lines of data???look at the short term and long term fuel trims... see if they are within 10% plus or minus.. look at them at idle... look at them at normal cruising speed.. look at them at high speed..most of the time.. you can set the scan tool up to take a snap shot of the data stream.. then you can go thru and review the data...with the mods you have done.. you could actually need more fuel ... larger flow injectors.. but looking at how far off they are and also examining the oxygen sensor voltage graphs at 70s.. with the person sitting in the passenger seat looking at the data.. not the driver.. if you don't have a scan tool... slip over to harbor freight.. pick up a 98614.. obd2 scan toolits really hard to diagnose anything on 96 and newer cars without live data... and graphing the sensor data.. have you see the OBD2 info from the motorcraftservice dot com site... its a pdf file that explains how the system things... i see that you done a bunch of mods..
Probably something simple....Plug wire loose, terminal broken, or bad plug wires. Take those brand x plugs out and replace them with the Ford spec sparkplugs. Try a scan that most auto parts stores can do for you at no cost and it might give a code that can help. Geoff at Need for Speed