ive noticed only in idle the engine idles rough, when i put the car into drive its fine even at lights, what could cause the idle all of a sudden to go bad? the only 3 things ive done to the car is install a K&N filter to the stock airbox, PCV, and install a fuel filter, its an aftermarket one, think the filter is causeing the problem?

same here (no mods though)...

How long have you had the new filter in? Did you pull the MAF screen?

Usual culprits of rough idle:

Clogged injectors.
Misfires (plugs, wires, coils)
MAF sensor
IAC sensor
IAT sensor
vaccum leak

i just installed the filter last week, really cant remember if the idle was bad before the filter install, but its bad enough now to feel the engine rocking, and no i didnt touch the MAF sensor, the car only has a little over 20k on it, so i doubt its the wires or plugs, but you never know. And when you mention "pull the MAF" you mean taking it off right?

yeah i just noticed my car doing that today. right when i started the car, it idled really rough. almost reminds me like a lawnmower starting.

my mom'* 2000 SSEi has done that since we bought it 2 years ago. never did figure out why it did it. my 97 L67 did NOT have the same problem though and it had the same chassis as the 2000+ Bonneville.

i kinda doubt this problem were having is normal, something gotta be wrong, if i cant figure it out on my own its going to the dealer

If its really bad its probably a vac leak. Check the vac line and T that run under the SC snout and behind the motor to underneath the MAP sensor.

I never said it was normal. I just don't know what causes it. It has been bad enough before that the engine has stalled though however. Oh, and my 01' Aurora has a vibration at idle too although I don't know if it'* just the 3.5 that idles a little rougher than the 3800 or what. Seems to be common to the 2000+ H body regardless though.

ill check that line, what about the idle control air sensor by the throttle body? maybe it has carbon buildup, and no its not a bad miss at idle, but i can feel the engine rock