This is for an 86 z28 with 20k miles , TPI 305 . This car was sitting for some time before I got my hands on it. I was told that plugs and wires were changed along with an oil change . I thought for sure the plugs need to be changed since you hear a miss when the car idles. Car starts but will stall out quick , I have to kinda give it gas but not too much to keep it started. If I give it too much it will not rev up and stall ( vacuum leak? ) after about 2 mins car will run but with a very rough idle , and the oil gauge is going all over the place . When driving car is fine , stop at a light and it gets rough with very low rpms and almost stalls. something I thought was an exhaust leak I have . Before the cat the small u shape pipe (about 8 inchs) has a huge crack. It actually split open at the seam . Can my idle issue be just too much air going in causing the car to run too rich ? This is my first post so I hope its in the correct spot and any help will be great !

Have you changed the plugs & wires ? have you checked the fuel pressure ?
did you drain out all of the old gas ?

fuel filter??? i would add some race fuel to the old gas. if it has 1/2 tank now go get 6-7 gallons and fill her up!
If you personally didn't change the plugs and wires... do believe it unless you pull a plug out. wires you can tell looking at them if they aren't aged.
but at idle it gets rough... look at the vacuum hoses.