Missfiring and backfiring on 89 cherokee 4.0

[richpinkham]

I am hopping someone may know what I should look for or try next.
Brief rundown on what happing. New engine w/plugs,wires, distributer, pickup, cap and rotor. New crank senser, throtle pos senser, oxy senser, injectors, and MAP senser. Prolbem use to be missing and backfireing while driving at speed 60+ under cruise conditions would clear up when accelerating or slowing down, Now with new engine can not run over 1000 RPM or misses and backfires out intake and exaust. When idleing it smell like running very rich but runs great. Starts right up and idles like sopposed too. Checked ECU and ICM units and they are good. No converter (test pipe) and new muffler. Have spent lots of money trying to figure out this prolbem and talking to dealers have been no help. Has anyone had this prolbem?
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[jaarcher]

you may or may not have a timing problem most likely its not right, i was having a similar problem and it turned out to be that i had the wrong distributor, they where almost identical but apparently not, so check that and have the timing checked, unfortunaly you cannot do it yourself becasue it is supposed to be "automatic" haha but they can advance or retard it with a scan tool at the dealer, hope this helps

[davidparkwalker]

I had the same problem w/ "smelling fuel" after a tune up. The problem was I had misconnected plug wires. This usually occurs with the farthest cylinders from distrib cap / the longest plug wires.
After switching wires the car ran like a charm and the fuel smell instantly went away. A dumb F@#K mistake I guess but I learned from it and it didn't damage the engine.

[davidparkwalker]

Ooops, left something out. My Cherokee was an 8 cyl, I'm not sure if you can even accidently cross wire a 6 cyl cap and plugs

[richpinkham]

I got an Auto tech to hook it upto a SNAP-ON diagnostic computer and everything is working, all sensers are in range, O2 is saying engine is running rich at idle, put on Scope, and Timing starts jumping all over the place above 900 rpm, thats whats causing missing and backfiring. Still have not found prolbem. Computer would not give up any codes, but displayed all information on sensers. Even changed computers w/ a known good one and still same prolbems and no codes.
I really need help on this one as even the tech is running out of ideas. We checked plug wires, compression, fuel pressure, injectors, dist & module and Crank senser. Need someone really good with jeep 4.0 engine prolbems.

[Glenn Foster]

I have some information for you. check all of your ground wires attached to the block. There should be two connections, one on the drivers side next to the fire wall connecting the engine to the firewall, and one on the passenger side in between the fourth and fifth cylenders. Remove all wires and clean with die-electric connection cleaner, reconnect and ensure that all connections are tight. Test the vehicle if problems still exists, the next thing that I would look at is the wires between the computer and the sensors that you have already replaced. I am currently having to do the exact same thing with my 89 jeep cherokee, I just need a meter to take a few readings.

good luck, Glenn

[richpinkham]

Been there done that :-)... but thanks. I just installed a new engine and have rechecked all the wires 4 times. We put it on a SUN scope and everything looked great till reved past 900 rpm and then the timing went nuts, it jumped all over the place firing every plug 3 or 4 times, maybe more as couldnt tell everything on the scope. We also could not get the computer to give any codes or what the timing was. I am in Las Vegas for the weekend so I am going to check with some 4X4 shops for info. Hope I can find someone that knows what too look for.

[EZ1908]

I had the same problem when I rebuilt the engine in my 89 Jeep Cherokee 4.0L I6. Try looking at the idle stepper motor. The pintle inside is suppose to be around 1 1/8 inches out of its assembly. Usually, the ECM will reset the motor if you criuse at 40mph for a few min. Sounds like the pintle in your idle motor is not unscrewing itself.
Also, if you have an automatic trans., try replacing the speed sensor that attaches to the speedometer cable. It'll be next to the gas pedal if you look under the dash from the floorboard.

[richpinkham]

I guess I didnt make something clear enough about the fact that the truck is not driveable in its present condition. ie. will not run over 900 rpm and even then misses so bad it comes back down to about 400 rpm and then idles perfect, so anything to do with idling or driving speed sencer isnt the prolbem.
But thanks anyway, at least someone is trying to help.

[ron8615]

Try this

Stand on passenger side of the truck remove the dist cap, rotor. The remove the 2 5/16 screws to remove the sync plate. Observe The screw that secures the Dist sync pickup. It hold down alittle tab that the other end hold the pickup from rotating. The End that holds down the pickup should be to the left of the securing screw. If it is to the right of the screw this throws the timing off about 28%
After setting to the correct position you should index the dist. an Easy way to do this will cost you a dist cap. You cut out a slot so you can view the #1 post install dist cap have Eng on TDC, rotate the dist so that the rotor just passes the #1 post in the rotation the rotor turns. Spec are 0.20 from the edge of the rotor to the #1 post on the cap. Lock down Dist. Let me know how you make out!!