I have a 4.9l engine that has had 3 injecters replaced i've replaced the plugs, ignition wires, dist. Cap and butten and the car still runs rough has reduced power and horrible gas mileage. Do you have any idea what might be the problem
REAL common problem that affects mostly GM products. Chrysler is one manufacturer that buys their injectors from Bosch in flow-matched sets and problems with them are just about unheard of. Don't know about other manufacturers but GM, on the other hand, has a huge bin full of injectors that flow all different rates. They grab a handful on the assembly line and stuff them in. You typically don't notice the slight lack of performance or fuel mileage when the engine is new but after many miles and a little varnish buildup, or the fuel pump gets a little weak, and symptoms start to show up. The biggest red flag is you only replaced three injectors. That should never be done.
Depending on the year, model, and engine size, the injectors are all fired with the same "pulse width" or all of them on one side of the engine are fired the same amount of time. The time is the same for every injector, the fuel pressure to them is the same, but their flow rates are different. The one that flows the least will leave that cylinder a little lean, and that extra unburned oxygen will be detected by the oxygen sensor and reported to the Engine Computer. He doesn't know only one cylinder is affected and will respond by increasing the pulse width to command more fuel to either all of the injectors or all of them on that side of the engine. Most cars built before 1996 only had one oxygen sensor so all the injectors are pulsed the same.
To overcome what it thinks is a lean condition, the computer commands more fuel from all the injectors. That dumps unneeded fuel into the engine where it is wasted. In severe cases, that lean cylinder may still be lean even after more fuel is requested, and no matter how much raw fuel is going out the tail pipe, the oxygen sensor will still be detecting that unburned oxygen. (They don't detect unburned fuel).
A similar problem can occur when a cylinder misfires due to a spark problem. Raw unburned fuel and oxygen go into the exhaust system where only the oxygen is detected. The computer will try to correct that by adding more fuel to all the cylinders but there will still be that unburned oxygen from the misfire.
The fix for this is to buy a flow-matched set of rebuilt injectors. Do an internet search for "Jim Linder" in Indianapolis. He puts on real high level and expensive schools all over the country for engine performance specialists. His company rebuilds injectors and sells them in matched sets. He sells more injector sets for GM vehicles than all other manufacturers combined. Mechanics love them too because so many owners say their cars never ran so smoothly even when they were new. They buy used injectors too because they can't get enough to meet the demand.
To be fair, GM may have improved how they choose injectors on the assembly line in recent years, but they don't do anything unless they can make money on it.