I have a 2001 jeep cherokee with an engine light on I had it read and I got the codes P0300 for multiple misfire and P0304,P0305, and P0306. So my question is what could be causing it, I had a new rebuilt engine put in it awhile back so it only has 42,071 miles on it everything has been changed out except the fuel injectors and the pcm also keep in mind this is the same codes I've had before my engine was replaced thanks in advance

 

Outside of the coil rail and the injectors, the only other thing I can think of is burnt exhaust valves.

 

Bad Camshaft position sensor can cause misfires, crank position sensor can also but they will usually also cause hard or no starts

 

Question: These are misfire codes, but are you actually FEELING a misfire in the engine? Or just seeing the codes and engine is running great?

I would recommend verifying fuel pressure. The fuel pump is in the tank and would not have been replaced with the engine. A marginal pump can cause random misfires.

For your 01, the fuel pressure spec is 49 psi plus or minus 5 psi. If you don't have a fuel pressure gauge, they can often be rented inexpensively from your local parts store, call around.

Not saying that this is your problem. But I am saying that it would absolutely be one of the first things a tech would verify if it was in their shop. Only takes a few minutes.

 

Injectors can cause a misfire code to be thrown.

jhc7399 here on the forum sells upgraded refurbished ones. I've used 3 or 4 sets from him.

 

I had a friend that would sell used cars, he would heat up a paperclip and poke it through the dash and poke out the CEL. ..