I have a 1991 Cherokee 4.0 with 115000 that is currently off the road because it will backfire and die within a few minutes of startup. Startup takes about 10-12 seconds of cranking, once the engine fires it will idle and rev to 5000 rpm smoothly for 3-4 minutes then idle rough for a minute once temp passes 200. At this point the air intake starts popping when you rev it and you can keep it alive by rapidly pumping the gas but the revs slowly die down no matter what and it stalls. Jeep always has to be jumped because it won't run long enough to charge the battery.

The process is sped up if you try to drive it. Actually if you rev it smoothly it runs really nice but if you rapidly let off the gas and try to rev it again it coughs and stalls.

I've replaced the fuel filter, ignition coil, spark plugs and crankshaft position sensor and nothing has improved. Curiously when I bought the Jeep the hose closest to the back of the engine bay on the valve cover was totally disconnected. I tried running it after connecting that hose and there was immediately a strong gas smell and steam/smoke coming from below the engine. I shut it off after a few seconds because I wasn't prepared for a mess. A new O2 sensor came today so I'll probably get to that on Friday.

Our mechanic friend is suggesting a vacuum leak or bad catalytic converter. No CEL at all. In the interest of full disclosure the Jeep has some kind of improvised brake system to bypass the Bendix ABS, there are a few disconnected brake lines and electrical connectors in the engine bay (near the brake booster) and the power steering works intermittently. Also either the tcase or tranny is leaking a litte. Only electrical issues are the security system goes off when you do basically anything and the power locks don't work. Although new to working on cars, I'm pretty handy and have lots of tools and time so I'm willing to try anything.