rough idle in gear/ and when accelerating

My 1995 STS has viberation when I idle in drive, or when I accelerate in the 1to 3 RPM range. I have used fuel injector, and fuel port cleaner to solve this problem. I also filled up with premium gas but nothing helps. Can this be muffer system problem or a fuel injector problem or something else? does anyone have any history with these problems with the northstar motors The mileage on the car is at 64000. I am also new to the Cadilac experice.

hunter,
welcome to AF.
so, by reading your post;
"...I also filled up with premium..."
does this mean you haven't been using premium all the time?
could be slightly clogged fuel filter, this is least expensive starting point.
could still be injectors, if you have been running low grade for awhile, or a partially clogged cat converter.
it might not be this extreme yet, because you don't have an engine light; or didn't mention it.
is the Cadi mostly "in town" or stop and go traffic (no 50, 60, 70+ mph)?
if so, your carbon build up could be more severe than one tank of gas.
change the filter, and whenever possible get onto the highway and run it HARD/FAST, and see if it blows smoke for a mile and clears up.
if the idle/miss/stumble is gone, good deal.

hope this helps,
chris
nrt racing

you do have a new air filter right?

and you could also take a little bit of carb cleaner, and spray into the MAF, and allow to dry before closing up the air intake hose.
this may be it, as, if you are in a high dirt or humid area, Mass Air Flows can build up flow restrictions, and cause off-idle and poor running conditions.

hope this helps,
chris

Yes I have used the premium gas, but I just purchased the car a month ago. the car was owned by and older couple who more than likely drove it in the city traffic. they also owned a newer van that they used to travel in. I have taken the car out of the city and drove it hard for about an hour of steady speed of 100 kph. but there was no change in the accelleration it is still slugish in the 1to 2 RPM range. I will check the other advice that you guys have given.......... Thanks for the help.

if its missin or tremoring i know cadilac had a problem with there engine back around that year. Which caused the engine to go down to 4 cylinders at a time. It was an engine fawl with the car that they really dont know how to fix. That could be the problem i know when i had my seville that was it i ended up sellin it 3 weeks later :) .

My first thought is that it has nothing to do with your fuel delivery system, but it is probably the spark plugs and wires. Be careful with fuel addatives, they can and will eat away at the insulation on the fuel pump and the injector windings.