I have a 2001 mercury grand marquis with 99k miles on it. Its missing on cyli dear 4 and 7 according to the computer. I have changed the spark plugs, intake manifold, fuel filter, fuel pump, and drilled holes before the cat converters. Yet it still is misfiring. Any ideas on why?

 

Something you might try, switch the coils from 4 & 7 with say 1 & 5 they might be easier to access. See if the misfires move with the coil changes.