i have a 2006 expedition with the 5.4 3v. my list of mods: volant cai, magnaflow high-flow cats, magnaflow cat back, troyer e-fans, troyer pulleys, bbk throttle body, tb spacer, saleen dual core radiator, x cal 3 with vmp tunes. after getting the plugs changed i am having a missfire when wot usually around 85mph when it is shifting from 3rd to 4th. i was getting a misfire on cyl #4, changed the plug and the coil, fixed the problem for about 1 hour and it started doing it again, the engine light would flash and then stay on. so i changed all the plugs again with ford racing 2 degree colder platinum plugs and this seemed to fix the problem for a month now the problem is back and it will trigger the engine light but 50% of the time the light doesnt stay on. all the mods have been on the truck for almost a year nothing new to trigger the problem. only thing was the plugs were changed and new cam shaft covers were installed. parts that were changed- all injectors, all coils, all plugs, new mafs. also the truck is getting a 400 code that is saying a bad cat but there are two brand new hi-flow cats and dummy sensors. any help would be great, i have a new ecu on the way but any other opionions? thanks

 

mileage? you dont need to mess with the O2 sensors. you had 2 cats to begin with right? why are you replacing all this stuff? did you test any of it? why 2* colder plugs? did the engine ever get wet? how old is the fuel filter? valve cover gaskets/seals were replaced but were they installed correctly? when you took the plug out what did it look like? did you get a tune for the correct size(3", 3.5", etc) intake? was there any kind of liquid at all down in the spark plug holes?