1999 Jeep Grand Cherokee missing on cylinders 3 and 4, have replaces plugs, coil pack, injectors, cam sensor, crank sensor and checked compression and fuel pressure nothing seems to help......

Check for the computer codes. It can point to the cylinder that is miss firing. Could be a bad injector. Do a cylinder balance.

Check fault codes and replace faulty sensor

TO REPLACE SPARK PLUGS YOU REMOVE COIL PACK MOUNTING BOLTS THEN REMOVE THE COIL PACK WITH BOOT OVER THE SPARK PLUG.YOU HAVE 6 INDIVIDUAL COIL PACK TO EACH CYLINDER. YOU NEED A 3/8 RATCHET WRENCH - LONG 3/8 EXTENSION -SPARK PLUG SOCKET - SPARK PLUG GAUGE.AND TORQUE WRENCH.THE 2000 MODEL JEEPS FUEL FILTER INSIDE THE TANK.1993 - 1996 MODEL JEEPS FUEL FILTER LOCATED LEFT SIDE OF THE VECHICLE NEAR THE FUEL TANK.

How about plugs wires.

A misfire may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer

If you notice symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors.

1.Plugs
2. Vacuum leak
3. Injector fault
4. High or low fuel pressure.
Thats the list of things to check.
This is a #3 cylinder misfire.
There is also TSB about #3 injector needing added heat protection. This could definitely be the cause. I start at plugs and go from there.
Hope this helps.
Thank you,
Lee

I would be thinking that there is a good chance that you need to be checking out replacing the computer, if you aren't getting any communication with the proper scan tool then the computer isn't communicating with anything in the vehicle...... I would have guessed first off that the ignition control module might be bad but that is obviously part of the computer also, I would be looking in that direction.

Did the engine light go on, and you got a miss-fire code? If so, 1st check spark plugs (which u did), then plug wires, if that's not it, the injectors would be next. Let me know how u make out
Curt

You say you changed the cam sensor. A1391 code is a crank sensor on the top of the trans. bell housing.

Check the cylinders Commpression. over 115 psi is good, 120 psi is normal.