2004 Mercury Marquis Car Shakes

Did you get this resolved? I have two identical 2004 Gran Marquis'. They both have vibrations at about 45-50 MPH. One is worse then the other.

The worse one is a steady shaking feeling at this speed, but it is not a wobble. It feels like the engine is running rough.

My technician has found several cylinders mis-firing. He suspected the individual ignition coils were defective on these cylinders (they showed mis-firing on the scanner tool). He then moved these original coils to other cylinders to make sure the problems followed, and they did. He then replaced these coils with new one. 4 new coils. No further mis-firing observed.

However, the vibration is still there and twice it almost stalled out while stopped at a traffic light. Had to put in in neutral and rev the engine to prevent stalling. My technician then replaced all 8 spark plugs and did a fuel injection cleaning service as a long shot. The vibration still remains but no further stalling has been observed. Too soon to tell however.

The roughness /vibration is a subtle feeling, not extremely pronounced, but still noticable.

I do not believe this issue is resolved.

Any other owners with a 2004 out there who have experienced rough running or a vibration feeling at 45-50 mph? The tires are only 3 months old on both cars.

Had the same issue. 2004 grand marquis and had my rear bearings replaced. Boom vibrations gone.