i put a 1993 jeep 4.0 engine out of a cherokee into a cj5 and the engine is running way too rich. it also has low intake manifold vacuum pressure at an idle and it will only stay running when i hold the butterfly vavle slightly open. any ideas on what could be wrong? thanks a lot

Could be a number of things. Try and pull a code first. Could be the MAP sensor, O2 sensor, ect....

Your crank position sensor may be off. You could try using less fuel. Prices these days ya know.

hmmm, I have been fighting this similar problem for 2 months now with my YJ 2.5L. I replaced the o2 sensor, crank position sensor, throttle position sensor, and MAP sensor, and nothing worked. The engine was a 97TJ engine that went in my 95YJ. Everything minus the block is back to 95YJ parts, and it still runs rich as hell. I also replaced the ECU (it was bad when I got the Jeep.)

Try your basics first. Remove each vacuum line one at a time and either wash them in a parts washer, or shoot some brake parts cleaner through them. My buddy's Jeep was running real rough, and he bought a new map sensor only to find out that the line to the sensor was clogged.

Do a fuel pressure test and check for the correct spec (31 is correct for a 2.5L, I don't know about a 4.0L) in a Hayne's manual. If the pressure is too high, your fuel pressure regulator on the fuel rail could be clogged.

As LUVMYTJ said, pull a code too.

Try all this stuff then let me know what you find out.