Weird Rough Idle Problem 2000 Grand Marquis
My 2000 Grand Marquis has a rough idle. I am getting about 19-20 mpg highway, which is below average for this car. The car has 90k miles on it. I just replaced all the coils with no improvement. I then unplugged each coil one by one as it was running, and the idle dropped with each coil, so I know it is firing on all 8 cyls. I have no check engine light nor do I have any codes stored in the ECU. The rough idle is especially noticable with the car in gear. I have also tried new plugs, fuel filter, fuel injector cleaner in the tank. Does anyone have any ideas???