I just recently bought and installed a new fuel pump for my 280zx. Along with that, I flushed the fuel lines and took out the fuel rail and flushed it out and cleaned all the injectors and reinstalled them. After all that work, I thought for sure my car would run a little smoother, but it actually runs worse!! It idles verrrrry roughly and slowly. When you rev it up, it runs better, but when you let off the gas it slows to almost a stall and sometimes even does stall and shut off. Why would this be? I also replaced my cold start valve, about 4-5 vacuum lines, and a bit of fuel line coming from the cold start valve. Could I have a vacuum leak, or maybe injectors don't work now or maybe I hooked up the spark plugs wrong?? Please help!!

Check your wiring. The injector connectors and the wires inside them get brittle from 30 plus years of heat soak, often it's as simple as that. Also be sure to check the connectors at the chts located on the thermostat housing and the O2 sensor, either of those not connected properly will cause a rich condition that could cause the car to idle poorly or even stall.

Evildky, I thought the same thing, but when I had the car running, I revved it up slightly and unplugged then plugged back in the injector connectors. There was a noticeable difference in how the engine ran, so I know the connectors are good. Also, I'm only 17 so I don't even know what the abbreviations you're using stand for. "Chts" ???

LOL, Cylinder Head Temperature Sensor, It's the sensor in the bottom of the thermostat housing that looks like an injector plug. If Fuel isn't the issue the next suspect is spark. Have you checked the spark plugs? Plug wires? Distributor cap? Rotor button? They are all pretty cheap and worth replacing if they don't look new.