I have a '98 Expo w/5.4 and 128K miles which developed a misfire. The only previous misfire was corrected 8 mos. ago w/a new coil. I retrieved the vehicle y'day from a very reliable and well equipped shop who stated that they were not able to diagnose the problem. The correct plugs were replaced as was a coil that was damaged during plug swap. The results of all findings are, all coils and injectors ok, all sensors check out and all required pulses occur in the proper sequences. What they cannot diagnose is why the engine misfires in a random sequence, ie.,The video scanner shows a misfire on, lets say, #1 for 10 seconds, then will jump to#7 for 10 seconds, and then to #3 for 10 seconds and so on until all plugs misfire and the cycle repeats, actually the cycle doesn't repeat, it just never stops and all misfires are in the 9-10 second range per plug. Any opinions or similar happenings out there?

 

When we get something unusual or not easily explained, it's time to check connections and grounds. Also, something simple that can be tried is to unplug the alternator plug and drive the vehicle. It could be throwing voltage spikes into the system and the PCM is affected by that type fluctuation.