I've got a 5.9 magnum that's been sitting for about two months, when I first got it, I had to give it some gas to get it it start, after driving it for a couple weeks it started backfiring and dropping rpms when at red lights, then it would catch itself and run fine, almost immediately after that problem it wouldn't start without idling rough and sucking air like it's not getting enough gas, and it backfires like crazy. I've had the fuel pump replaced, all throttle body sensors, new spark plugs, pcv valve, crankcase breather. It doesn't have a cat because it was clogged and cut off when the fuel pump was replaced, and I know the EVAP canister is bad, it made a constant ticking noise when the truck was started.

 

Compression test.

 

How many miles on this 5.9 engine? Has the timing chain ever been replaced?

 

Sucking air leads me to plenum….but I’d agree with compression check

 

Well it seems there's gas getting in the oil somehow, what could be causing that?

 

Worn rings, As was mentioned do a compression test.