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Occasional rough engine/engine misfire

My son has an 04 Ford Focus, auto trans., small engine, he has had a cronic occasional rough engine with warning light. The ODB code is “misfire”. This seems to only be when the A/C is on. With A/C off, engine operates normally and light goes off. Rough operation is at cruise at 40 mph. He has replaced various sensors.

It could be park plugs, coils,or plug wires if not coil-on-plug.

My son replaced the plugs and wires. We don’t understand what effect the A/C has on the engine. The rough operation is only occurs with the A/C on.
I am wondering if there is a timing or other change that is triggered when the A/C is on. Or if there is a circuit that is faulting when the A/C is on.

The AC connection could just be from the extra load the AC puts on. E.g. w/out AC something is marginally off but not so bad as to be noticed. The extra compressor load pushes it over the edge. Those engines are not very powerful.

In many ways the Focus is a reworked Escort with a very similar motor. The late 90s early 00 Escorts had very common ignition coil problems. I’d test the coil pack.

Does that check engine light flash or just go on steady? Which misfire code is it? Specific cylinder (e.g. P0301 is cylinder 1) or random misfire?

How is the fuel filter? Check the fuel pressure under load - e.g. with the engine revved to 2500rpm or so for several minutes and then kick the AC compressor on.

My son has replaced the coil. He also has replaced the fuel filter.
The most recent misfire code is a solid engine light and an odb code indicating misfire. He thinks the code is for cylinder #1.
Previously, he has done a lot of tracking down and fixing many problems starting with the damper on the engine pulley, then to the sensor (at the engine pulley), then a broken EGR tube, then a broken engine mount, then the A/C compressor, and a few other sensors that I can’t recall. As I mentioned he has also replaced the spark plugs, wires, coil, and fuel filter.
He has fixed most of the problems except this last occasional misfire. At one time he thought it may be related to roughness after fueling, but several re-fuelings after that didn’t result in rough running or misfire. As mentioned, it now occurs only when the A/C is on. I have previously believed that the fuel injectors were responsible but eventually the replacement of the various parts cured the problem (except for when in A/C mode).

Before going on with anything else I would check the compression - rule out any problems there before spending more time & $$. Obviously one would esp want to know if the misfiring cylinder is any different.

There’s no real need to assume the worst (yet?), but I would have a look at this:

Having replaced the fuel filter does eliminate one other thing, but the fuel pressure shoudl still be checked.