My wife has a grand chreokee with a 318 also and it had the same problem and it turned out to be a dirty throttle body. I cleaned it and has been fine ever since.

 

I was wondering with the tach if its erratic or not.. If it goes wonky it might be a crank sensor going bad too and cause the engine to stumble.

Is it worse under load or no load?

 

I will double check the tach. The truck can be driven, but the symtoms are similar to stationary operation. Once you get above 1500-1800 it stumbles and if you try to accelerate it stumbles also. It will idle forward in 1st but it is very laggy if going up an incline. It does not die though. While in gear you can slowly accelerate, but only up to the 1500-1800 threshold.

 

This actually sounds like a fuel delivery issue also. These have the "Intank" fuel filter that is supposed to be good for 80k miles (IIRC) Do you know if it has been changed?

 

Chad, no idea if it has been changed. Michael told me his fuel economy is horrible too. I'm thinking if we can't figure out how to get the code read, the old girl might just have to get retired. I was really hoping for a simple fix so he could sell it, he is moving out in a month and needs to pad his wallet. Dang!!!

Anyone know of a key trick on this thing to get the codes?

 

My first thought was the IAC as well, with the throttle body needing cleaning. If you take the throttle body off (gaskets are about $2) you can clean out the IAC reservoir from behind as well as the underside of the throttle body bore. Is there a PCV valve on that year? Cehck all the vacuums. Does cruise work ok? It's usually vacuum operated. Might replace the inline fuel filter if it has one. Besides being plugged, some of the chrysler/jeep filters have a regulating valve inside as well and when it quits working right, the mileage and performance drops. With all that going on, plugs and O2 sensors are probably cruddy now too.

 

I have to say that with the rapid onset of drive-ability issues and the fuel mileage drop, looking in the tank before continuing on is probably the logical path. It really sounds like filter and/or pump.

On edit, you might be able to confirm this very simply by disconnecting the fuel supply line and giving it a couple blasts of compressed air back to the tank. If there's an immediate improvement in the engine performance, proceed to the filter. Remember to remove the gas cap before you hit it with the air and if you get any improvement, it may be very short lived, pay attention when you fire it up.