I have a 1993 Mercury Sable station wagon 3.8 eng. I had the tranmission replaced 11 months ago, waterpump 2 years ago. Recently had a complete tuneup, replaced fuel filter, starter, air filter, and TPS. The car rides quite smoothly except when stopped at an idle. Then, it acts like it's possessed. The car starts choking like it wants to cut off but is fighting it(sometimes it does cut off), the dashboard lights do a slow flicker, and it shakes like a jalopy, like the spark plug wires are connected in the wrong order(but they aren't). If I put the car in neutral or park, all the symptoms stop. When I resume moving, there is a slight thrust which seems to get worse after most every stop. Then, depending how long the ride is, the car feels like it's taken flight. Smoothly, but a weird feel.(Hard to explain) If I stop, turn off the engine for awhile, (like to go in a store), everything starts all over again when I re-start the engine. There's no trouble starting, even when cold (35degrees). Also, if I rev up the engine until the exhaust blows white smoke, it seem like it doesn't begin it's symptoms as soon. I've used Lucas and Seafoam fuel treatment but it didn't help much. I tried a higher octane and better gas, but that didn't help. Also, my gas mileage is poor. Maybe 16 mpg. I'm thinking it's the MAF and/or the fuel pressure regulator, or the cam sensor, or vacuum leak, but I don't want to just replace parts without knowing. I've been told it's hard to get a good diagnostic test because it's a 1993(old). Oh, also, there's a strong gasoline smell sometimes while driving, and always after the ride is over and engine is off.
Sorry this is long, but it's the best I could explain.
Thank you for your help.
Loretta: The first thing I would check is for a fuel leak as well as fuel pressure and the fuel pressure regulator.
If you refer to the "Do it Yourself" site at the top of this web page, go to the second page and it will provide directions for how to check the fuel pressure and the regulator. You will need a fuel pressure gauge, but most parts stores will lend or rent them to you.
Do that and let me know what you find. Also, pay attention to fuel pressure drop off. That is when you shut the key. The system should maintain pressure for some time. If it drops to 0psi very quickly, then you have a leak or other problem.