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2000 Ford Expy 5.4 L cyl misfire

I started feeling a slightly rough idle three days ago and yesterday the check engine light illuminated. When we read the OBD, it indicated a Cyl #1 misfire. Question is this. Can I replace a single coil pack (1/cyl), or do I ahve to replace all 8 as a set and why?

One at a time.
Check the plug pigtail there too.

I would first change the plug and carefully examine the COP, especially the boot and resistor and spring inside…If the new plug cures it, change all the plugs. A new COP should always get a new plug too.

Moisture intrusion is the kiss of death for both COPs and plugs.

I would just add that along with the plug change recommendation that you do 3 things.
One is to just snug the new plugs up when installing them. Overtightening can create a huge headache.

Two would be to check the plug gaps and adjust as necessary. It’s often assumed that plugs are pre-gapped and this is not the case. If the gaps are correct then it’s because you just lucked out. If the gaps must be adjusted then it should be done very carefully without prying on the center electrode.

Three is use anti-seize on the plug threads and dialectric grease on the plug boots.

If you know the problem is spark, you can replace the offending coil and plug one at a time.

These coils seem to have a fairly high failure rate.

If you have an air compressor, after removing the coil but before removing the spark plug, blow air in the spark plug tube to remove any dirt and debris that may have found its way down there.