First, have new spark plugs put in the car and a set of ultra-premium plug wires put on also.
If the car still shudders around 35mph or as it goes into overdrive, then add 2oz of friction modifier to the transmission. Buy this at the Ford dealership in a 4oz bottle for about $4.00 or so. Be sure to tell Ford that you're going to pour it in your trans so they will give you the right one.
These cars are excellent car that often go 300,000-miles with few problems. The do usually start to consume a lot of oil around 140,000-miles, but as long as you keep it full, you can drive the car for a lot longer.
Also the engine should have a carbon cleaning, induction flush, done every 75,000-miles as well as the EGR valve ports cleaned out every 100,000-miles.
If you do these things you should get 300,000-miles with only normal items to fail. Like Alternators every 150,000-miles and maybe a water pump every 150,000-miles. Maybe the A/C compressor may go out at 200,000-miles. Power window regulators usually break with heavy use after about 8-years.
The Crown Vic, Town Car, and Grand Marquis are excellent quality cars that have plenty of room and lots of safety.
City 17mpg Highway at 70mph is 28mpg.
I have a '94 Grand Marquis with a 4.6L engine. I have a problem where the car sounds like a sewing machine at times. I am not talking about fuel pinging. I am talking about a sound like it has no oil. This happens even in city driving when the car is warmed up and I have been driving about 15-20 minutes. I have just had the oil and filter replaced. I have been told its the oil pump. I have also been told it is not the oil pump, but something is causing the "sump" to lose oil after the car is warm and has been driven awhile. Also, the check engine light comes on here and there, but I have been told that when a certain sensor detects noise it will trip the light. I am confused on exactly what to do from here?
Crown Vics and Mercury Grand Marquis between 1993 and 1996 had problems with some torque converters that after about 25,000 miles caused a shudder when shifting between 35 and 45 mph and worsened if not repaired. In some cases the converter had to be replaced, particularly the 93 models, and in following years service at every 30,000 miles is recommended. This consists of changing the transmission fluid and filter.