91 3.1 runs rough at idle sometimes

ok here is the deal i have a 1991 lumina with a 3.1 109,000 miles i bought the car about 3 years ago and it never really idled good , but only sometimes and with the a/c off with it on it runs like a top. i have no codes. changed plugs, fuel filter,o2 sencer, put in a test pipe, cleaned the intake, i also drove around with my snap on scanner and know that the egr is working right but i cant seem to find the reason for this weird idle it sounds like a miss but the car does not stall out. any 1 have any ideas.

Things you need to check:

1. Take the vacuum hoses and check them individually with no exceptions!!! Very important!!! One of them and the most notorious is the one that goes around the back of the engine and up to the very front of the car to a vacuum reserve chamber on the drivers side in the bumper!
Look under the car at the very front drivers side and you will see a big sphere lookin thing with no wires just a vacuum hose comming out of it! Take the host off and suck on it to see if it holds a vacuum.

2. Check your intake hose from the filter to the throttly body. I can almost be certain that you have a hole in it. Chevy mechanics have a passion for over tightening it. Take out the hose completely and inspect it.

3. Check your ignition wires. One by one see if there are and cracks.

4. Check your coils. one may have a crack or be damaged.

Good luck