92 Dakota 3.9..Starts but "sputters" or skips.Driving has low power on take off and skips even worse.Did a full compression test and getting between 150-180psi from each cylinder.Getting fuel and has spark.After pulling all the plugs,they are black with 2 that are not even firing.Has new plugs wires cap and button.Could this be valves? Want to confirm before I start the tear down.Also,if its something more than this I have considered putting an 8 cylinder in it.Seems to be alot of problems with the 3.9. My other ? would be what 8cylinder would bolt up to my transmission? Thank you

 

So it is not your injector that is failing or both? If a spark plug is not firing you have an ignition problem.

If your worried on a leaky valve, then do a leak down test.

 

if you pulled all the plugs and 4 are black and 2 are not, this is a classic example of a bad fuel injector(s) because the oxygen sensor sees a lean signal, it tells the computer to dump more fuel to compensate and make the other 4 work harder. If you replace one injector, you will need to replace all 6 because another one could go bad.

If you plan on putting a V8, the transmission in your truck now will bolt up but will not last long as it can't handle to torque of a V8. Your best option is to find a 1992-1995 Dodge Dakota V8, Dodge Ram truck or van and swap it over to your truck. As far as engines, you can use either a Magnum 5.2 or 5.9. The only difference between a 5.2 & a 5.9 is that 5.2's are internally balanced and 5.9's are externally balanced, meaning they use different torque converters and harmonic balancers.