Donâ??t quote me on this, but if I remember right two of the plugs on the ztec have a different part number then the other two. Are you installing the right plugs into the right cylinders?
Also, there is a lot more to making a car idle correctly then plugs and wires. Have you checked the following?
TPS
Coil pack
Fuel pressure
IAT sensor
Mass flow
EGR
Have you cleaned your injectors (a real cleaning not the gas tank stuff), throttle plate, IAC and EGR?
Have you checked for vacuum leaks or air bypassing the IAC?
jeff
