My 4.6 just started misfiring on cyl 5 (code 305).
I hooked up a code reader and it also gave me
(code 453) Evaporative Emission System Pressure Sensor / Switch High
Start by removing spark plug #5. If it not oily but a bit black with carbon, replace it. Does that solve the P0305? Yes? Done. If it doesn’t, replace the coil to #5
If the plug is oily, remove the other 7 and run a compression test. #5 may be on its way out.
The P0453 is a bad pressure sensor for the evap system. Not sure where this sensor is on a Merc. It may be on top of the tank by the fuel pump inlet.