This isn't what I meant. The owner is not telling the service manager anything other than the complaint; ie, misfiring. What is happening is the service writer takes a stab at the diagnosis, either with or without his mechanic's evaluation, and writes up a work order, listing parts to R & R or adjustments to make or whatever. Then he has the owner sign the work order. The work is performed as written and the owner pays. Even if the problem persists, the owner has little or no comeback to the dealer because the dealer did do the work. What I suggest is after the service manager writes up his work order, the owner has him add the phrase "TO FIX THE PROBLEM OF MISSFIRING (OR WHATEVER). Then, if it turns out to be a misdiagnosis on the part of the all-knowing service writer, its his problem, not the owner's to pay again and again until the problem is fixed or the owner gives up and trades in his car.
The idea is to pay for fixing the problem; not to pay for a laundry list of procedures that don't fix the problem. If your shop uses an amateurish shotgun approach of wholesale parts replacement and refuses to commit itself to a solution on a given work order, then find another shop. If its a dealership, complain to Ford.