90 lasabre engine stumble at idle

[shadetree3]

this same car i have previously wrote about also has an engine stumble that can only be felt when the car is at rest idling. every few seconds a strong vibration can be felt in the floorboard and in the seat. very annoying.. i have been having engine problems here in the past year costing me big bucks at the local shop. tune up with new plugs and wires. one of the coil packs was replaced. and the computer was replaced. it runs great now but.. after all the money i have sunk into this car it seems to be idling rougher now than it ever has. now, this is a 3.8 v6 with 86k miles. i have been babying this car for the past 10 years. regular oil and air filter changes. then the battery went bad one summer and it has never been the same. like a chain reaction of problems.. so, what do you think? intake leak? bad head gaskets? is there such a scan tool that can tell exactly what the problem would be? not just to read trouble codes.. like, what would be the difference between a regular tuneup and a computerized tuneup? can the electrical system be tweaked to make the engine purr?

[buickmastermind]

I would check to be sure that other coils haven't gone bad, or that the fuel injectors are all working fine. When I replaced the coolant temp sensor on my buick, it caused two or the coils to go bad, and the EGR solinoid to not work. You can find programs for a laptop computer online that use the USB on the computer to hook up the the connector under the dash for reading codes, and it will tell you almost everything. I have considered getting a laptop just to constantly mearsure everything on mine, so I might get a heads up when things are going bad, but haven't done anything about it yet. If the motor seems to be starved for power or fuel, especially when accelerating heavily, then I would definatly check the coils. If not, the injectors. unless you are getting a SES light, it isn't likely that it is a sensor.
Just so I don't get confused, when you say what motor you are running, please call it the 3800, b/c the 3.8 is a different motor used in the 86-87 buicks. Good Luck!