Need some help!!!! 99 f150 4.6 code came up .. missfire on 8. So i was told to replace plugs ..did so, still misses on start up seems to be when it is cold or below 60. Checked it again after new plugs went fine for three days and came back again. Was told again.. Coil pack is bad replace it. OK!!!! more money
Well it did not help. can some one out there help me.
It comes and gos
Thanks

 

Check in back of the throttle plate area for the PCV elbow hose.
This hose has a bad reputation for leaking, sometimes it might feel mushy or you may feel a hole in it.
Try swapping the ign coils with number 7, also the number 8 and number 7 injector. I've seen this problem come up on IATN.
There's also the infamous water leak that tends to find its way down the side of the coil boot to the area the plug lives in. It can cause the spark to pass through the boot to ground bypassing the plug.
Make sure the connectors on the coil and injector for that cylinder are NOT corroded or loose. This last part can be tricky. The connector wires are very small. I saw this happen at the Ford Training Center in La Mirada Ca. As we were diagnosing a "bug" the instructor put in the system we had to remove the coil and connector. After repeated removals, the wires at the connector went open. If you held the wires up/down/right/left (you get the picture) we could get the system to work properly. If the engine misfires repeatedly on that cylinder, the PCM will shut off the injector until the ign is switched off then on again.
Hope this works.
Mike

 

Forgot one other thing. The EGR valve can also stick partially open. On the older GM vehicles (ya I know this is a Ford board, but please read on) the PCM would momentarily turn on the EGR when you throttle up in Park or Neutral, then turn it off. As they aged, the solenoid for the EGR valve would stick keeping the valve on, causing an extreme lean condition and even stalling.
HTH
Mike