1991 cherokee runs rough sometimes dies backfires,hestitates ect.

[tweety1017]

Please help i don't know much about jeeps i have a 1991 jeep cherokee 4.0 it is fuel injected it will start most of the time when started the engine will rev all on its own from 2500 down to 500 or less if goingto die. When driving it will start to backfire when theres a load on it loss of power. I can put my foot all the way down on the gas and it will sluggishly move forward and then sometimes accelerate fine for a very little while. then back to hesitating ect.
I have replaced the spark plugs, spark plug wires, fuel filter, cap. roter, coil, distributor,and fuel pressure regulater. I have tested the injectors but they read different from the book but same as 4 yrs ago so i am assuming they are good, i tested the crank shaft position sensor but it seemed to read wrong to different from book, i tested the map sensor and the first half of the test was the same as the book and good but the second half of the test failed, i think i should replace the map sensor but i am leary becouse the other readings had been off too.if i disconnect the vacuum line it dies and won't start. i don't have the money to put on scope or replace more than about one more part. if anyone has any info that can be helpful please help (has the book been wrong on the readings before or what should they really be) is there anything i can cross off the list or that i haven't thought of. thank you for your time

[AlohaBra]

Sorry no one has replied to you yet. I was hesitant because of the older model year of the 4.0L.
But a couple of things came to mind:
* possible clogged exhaust
* contaminated gas

What book (or manual) are you using?
I sent you a PM (private message) also.

[fredjacksonsan]

I'd agree on the possible clogged exhaust, but it also might be the crankshaft position sensor (cps)

How long has it been doing it....did it suddently happen, or has it been gradually getting worse?

[tweety1017]

thank you for a response i am using a chilton's manual. One day i started it up and it would hesitate between 25 and 40 mph. 3 days later it was drastically worse overnight. It no longer matters what speed any time there is a load on it boggs down if i push the gas down it starts popping and sometimes accelerates slowly the rpms get to about 4500 and it will either have a short burst (10 more mph)of acceleration and then hestitate some more or accelerates till up to speed then boggs down and hestitates once theres a load again. when in park it will rev up all on its own (the 1st 3 days it didn't) Thank you for any help you may have.

[xj31]

I would check the fuel pressure or like Aloha said,possible clogged exhaust

[tweety1017]

i replaced the fuel pressure regulator and fuel filter

[xj31]

Originally Posted by tweety1017

i replaced the fuel pressure regulator and fuel filter

Thats fine,but what if the fuel pump is not producing enough pressure?