Ill start off with the basics I have a 2000 Chevrolet Cavalier Z24. It is giving a P0456. So this all started with a slight hesitation when pressing on the gas to accelerate from a dead stop, lightly pressing. once above 2000-2500 Rpms it wouldn't do it and it ran perfect. So a few weeks later the car is running super rough and the acceleration is horrible and studders a lot and does not want to speed up and when it does studder if I let off the gas and reapply it the car accelerates like normal. Idle is terrible it use to idle at about 700-800ish and now its idling at about 1100-1200. Ive suspected a vacuum leak and told the whole intake manifold off and cleaned it up, removes the whole plastic intake and cleaned where the intake meets the head. The gasket looked fine no breaks in the rubber or plastic. Checked all my vacuum lines and I do not see any broken or rotted lines. Car still runs like crap, checked the Fuel pressure regulator and found that it is leaking, replaced it from my old engine that had a rod knock and that one is leaking at the regulator not the vacuum line on top but in the middle its hard to see. I did change the Fuel Filter and that did not help either. I looked up the code of course before I replaced anything and everywhere says there is no symptoms with that code other than maybe the smell of gas near rear of vehicle. So I'm not sure where to go with this and it's irritating me, i feel like leaning towards fuel pump but money wise i would not want to waste money on a new one and that not be the problem.

Also the engine has about 110,000 miles on it and body is 117,000 replaced engine about a year ago.

I had the same code but none of those symptoms. Mine turned out to be a small crack in the gas filler neck. Replaced it and still no codes. I had that hesitation issue at one point with my 96"Z24 with no check engine light and it was the ignition coil housing cover.

Yea I have the neoncoil swap so thats not it wven replaced it with orginal stuff and its still doing it.

Which engine? 2.2L or 2.4L? If 2.4L, consider replacing the MAP sensor vacuum tube. They don't necessarily have to look bad to be bad. It's only a few pennies and easy to do, so it's a got a good risk/reward ratio.

Any engine temperature dependency? For example, does the problem only happen when the engine is warm? If so, perhaps consider replacing the relatively-cheap, easy-to-replace temperature sensor (not the one under the water pump on the 2.4L). It's possible that the sensor may not have failed completely but may be feeding bad information to the computer causing the computer to make the fuel mixture too rich when the car is already warmed up.

I'm just grasping at straws based on some issues that I've had on my 2.4L engines. Good luck.