01 Durango R/T 5.9L dropping 1 cylinders, runs rough

[N00b-un-2]

My wife purchased this vehicle while I was deployed and when I returned I came home to a vehicle that ran like crap, and it steadily got worse over a few months. It rattles and runs kind of rough and my fuel economy dropped from about 15.5 mpg to around 11 currently. two times, the check engine light has come on and I had it read out at Autozone, both times it popped with the code "open gas cap" and both times the gas cap was on securely. just to be sure I changed the gas cap the first time, so I know that that is not the real problem. it leads me to believe that the problem is a fuel delivery problem, but it never stalls out.
After taking it to a mechanic for diagnosis, I was told that it is dropping 1 or two cylinders and that I should start with a full tune up and then work forward from there. So I took it home and performed the tuneup myself, replacing the PCV valve, air cleaner, spark plugs, distributor cap and rotor, spark plug cables, cleaned the batter terminals and with a wire brush and sprayed them with corrosion prevention compound, ran a full can of throttle plate cleaner through the throttle plate, ran a can of fuel system treatment through the gas tank and used top end cylinder treatment, the kind that you plug in to a vacuum line and run through the intake manifold. I even flushed the coolant and brake fluid while I was at it. That was about six tanks of gas ago, and after burning through all of that fuel injector cleaner and stuff, the next time I filled it everything ran perfectly. The engine was running very smooth and made LOTS more power, and my fuel economy jumped back up to around 15. Then all of a sudden one day about a week ago, it started running like crap again, exhibiting all of the same behavior.
I narrowed it down to it being a misfire/failure to fire on the cylinder closest to the firewall on the drivers side, by disconnecting the spark plug cable from the spark plug and starting the engine with no perceived change.
Any ideas? I'm deploying again in the next few weeks and I need to be assured my wife won't have any vehicle problems while I'm deployed.
Also, I've noticed that sometimes the PCV valve makes a weird rattling noise that can be heard inside the car.

[n8zdogg]

Check your plug wire routing. It is posted in this section somewhere. I guess crossfire has been a known problem with certain Dodges (not a suprise since the rest of the electrical has a mind of its own) Its not logical either, like running them parralel and dropping them down to each plug -- Other problems could be typical mores serious "miss" causes like a bad valve, bad injector, bent push rod, low compression etc

[N8ECH]

NOOB:

I know this is an old post, but the 5.9 motors have a plenum gasket on the bottom of the intake manifold that goes bad.

When this happens, engine runs rough and oil consumption goes up.

Way to diagnose this is to remove breather hat from throttle body and look down inside with a flashlight. IF you see wetness and alot of black stuff, you have the failed gasket.

Don