When I start my 91 Cherokee it runs extremely rich black smoke an all then when I start driving it it falls on its face like it has no fuel so I check rail pressure when it does that and there will be no fuel at rail. Turn key on an off a couple times and you have fuel at rail but it still runs rough. The jeep has new plugs wires cap rotor fuel filter

 

Your 91 4.0 needs approximately 31 psi to run properly.

If you are not getting that and it sure sounds like you're not, replace your fuel pump.

 

But why would it run rich at start up?

 

How do you know it's rich? And, sometimes vehicles have more than one unrelated problems.

 

It poors black smoke at start up

 

Check the line from the throttle body to the MAP sensor on the firewall. If it is compromised in any way, and leaks vacuum or is plugged, the MAP tells the ECU that you're demanding alot of fuel. Then the eCU signals the injectors to go max flow.