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Grand Marquis shuddering

My ‘03 Grand Marquis “shudders” when I accelerate, especially up hills. It feels like it cuts in and out momentarily, shaking backward and forward. Here is what I have had done, based on mechanics’ advice: replace spark plugs, replace fuel filter, clean throttle body, clean mass airflow sensor. It still does it after all this and I am stumped.

Have you changed the transmission fluid and filter?
Please post any stored fault codes.

Usually, a misfire causes the CEL to come on and log a trouble code…Usually it will point out which cylinder(s) are miss-firing…Armed with that knowledge, replace the suspect coils…Have you washed the engine lately? Big no-no on these engines…If it has not yet caused the CEL to come on, next time it happens, maintain hard acceleration until it does, then read the codes…The plug-wells should be bone dry. Is that how you found them?

While it could be a problem with the transmission or other drivetrain components, my first guess, based on what you have replaced, would be that you have a misfire on one or more cylinders.

If that’s the case, try to figure out which cylinder the misfire is on. This car has a Coil-On-Plug system instead of a main ignition coil, so those would be a prime suspect in this case. And, if this car has spark plug wires (I think it does, but really don’t remember. Maybe someone else could correct me) then I would certainly change those every time the plugs are changed.

The car definitely has coil on plug.