Q: I have a 2003 Grand Marquis and I have jerking and hesitation around 30 to 40 miles per hour

The first thing I would check for is vacuum leaks. Any air that gets into the engine without being measured by the Mass Air Flow meter can lean out the mixture and cause drivability problems. Check all the hoses and rubber parts to be sure that they are in good condition and are properly connected. The parts that you mentioned could cause a problem like you describe, but it’s far better to do a proper diagnosis before buying parts that you may not need. Simply replacing parts until the car works again can get very expensive very quickly. You can have that done at your convenience by contacting YourMechanic. They will send a technician to your home or office to check out your jerking Mercury and let you know what it is going to take to solve the problem.

The problem may not be the engine causing the problem but is caused by the transmission torque convertor overdrive clutch chatter. You may be able to fix or improve the problem by doing a transmission fluid flush and make sure to use the new Mercon 5 or 6 fluid. The chatter is caused by lock up of the convertor clutch. If after servicing shifting does not improve within 100 miles then you may need to replace the torque convertor.