Rough Running Grand Marquis



My wife’s Merc developed an intermittent but consistent “shudder” at speeds above 40-50 mph. I took it to my trusted mechanic and they changed spark plugs, plug boots, air filter and fuel filter. The checked the transmission and said it was ok. The car doesn’t shudder anymore, but between 40 and 65 mph it feels like it is misfiring. It starts up fine, runs like

a “scalded dog” when you floor it, and purrs like a kitten at 70 mph and above. But its rough in the 40 to 65 range. Any other ideas?


What year of car and how many miles on it?

I’m a bit undecided here about the symptom.
If it’s an engine performance problem then you should drop by a local AutoZone, Advance Auto, etc. and have them pull the codes. Post any results back here for further discussion.

If this problem appears to be when the transmission shifts into or shortly after shifting into Overdrive, then the problem may be a torque converter shudder.
Usually a fluid change with some high quality ATF will cure this.
This could be dependent upon the year model.