Hello I am new here so not really sure how this works. I have a 2003 5.7l hemi and it was running fine then it dropped off like it is running on 6 cyl. I have changed all the plugs, wires, and even 8 new coils. Checked for power to coils and fuel injectors and that was good. I am at a loss.

 

What vehicle?

 

1500 pick up 4x4

 

Are you using the copper champion plugs, and gapped correctly? Has the truck been overheated?
Rec check head gasket by removing radiator cap then starting truck. Let truck idle until thermostat opens. You should not see any bubbles in the radiator filler neck.
Dirty fuel line or faulty fuel pump is another possibility.

 

Run a compression test. All the numbers should be fairly close. Any high or low cylinders, pull the valve covers and look at the valve gear on them.

 

Wrong plugs could be a problem as @jrsick states, and I agree with @magnoom about checking compression (if you're running copper plugs). Leak down testers can tell you what is leaking (compression wise) w/o disassembly. Do you have any CEL codes?

 

Yes I am using the copper plugs. I will do a compression check. Yes it has overheated before. And it does bubble in the filler reservoir.

 

That is most likely head gasket failure, I'd have the head checked and corrected for flatness - and have your valve seats checked too. You can buy a test to make sure, it detects combustion gases in the coolant...

 

It's called a block tester and you can rent it (free when you return it) from an auto parts store. The blue solution is less than 10 bucks. Videos on youtube.