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Thread: 4.0 Runs like Crap Need Help!

  1. #1
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    4.0 Runs like Crap Need Help!

    Heres the deal

    Blew up my 90 cherokee, had a factory rebuild 4.0 HO sitting in the shop, tore it down all new gaskets, plugs, wires, rotor, cap, water pump, thermostat, alt, and starter. Put it back in the jeep. Wont run worth a shit. The new motor is the same year. From the same make and model. I put a new TPS sensor in it this week.

    So here is some of the problems. Will run great for 30 seconds, then it boggs down and stalls, idles fine for any length of time. Once you hit the gas, revs come up sometimes, other times its just bogs down and stalls.

    To me it seems like a fuel issue, but i really dont know. I have a 10 gallon cell, with an intank fuel pump. Brand new, and im using the old factory fuel filter.

    The jeep sits a long time inbetween runs (trail pig) and can loose its prime. Easy fix, releave the pressure on the fuel rail, then cycle the key a few times, and it fires right up.

    Also blows a shit load of black smoke when you give it gas, so its running really rich.

    Any info would be much appreciated.

    Luke
    1990 Cherokee 2dr, 38's, toyota axles, 488's, welded front and rear, D300, Square driveshafts, Riding on leaves.

  2. #2
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    sound like the fuel filter is pluged it will idle fine but when it demands more fuel the dirt in the filter will move and clog the remaining passageways and choke out get a pressure gage to tell
    you pump will also do this if it's shot ....

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    Thanks, i will get a new fuel filter and give it a try. The fuel pump is brand new. So im not worried about it.

    Luke

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    Get some throttle body cleaner and clean out the Idle Air Controller (IAC or IACV) it's on the TB and from the inside of the TB there is an offshoot for air to flow through thoroughly clean this out because if the brass valve pin thing (think its called a pinnacle or something like that) gets clogged up it wont idle worth crap and bog out

  5. #5
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    thanks! i will give that a try

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    you can remove it from the TB entirely as well for a more through cleaning which is what I did with my YJ and XJ the YJ had a K&N oiled filter and after a few weeks of having it in it wouldn't idle right then started stalling. After cleaning the IAC and running a paper filter never had a problem again

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    i will take a look at it tonight hopefully

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    you might have an o ring stuck in the fuel line when you pulled the old line off the rail and hooked up to the new rail.
    just a thought, it happened to me once.
    i think there are 3 o rings. 2 black and an orange if i recall. orange in the middle...
    proud OF4WD member #3875

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    I thought i had the same problem. I only have an orange o ring. When i put the line back on, there was on orange one in the fuel rail, i took it out, thought there was only supposed to be one o ring

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    Well i changed the fuel filter, and cleaned out the sensor. and still runs the same. a little better. However still running really rich and hesitating some times. No longer stalling out

    Anyone have some more ideas>?

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    Pretty sure a 90 XJ had the Renix MPI system so not a High Out 4.0L

    I would check the CPS ( Crank Positioning Sensor) on the bell housing of the tranny. if it got wacked during R&R or is to far away from the sensor ring it can cause problems like this.

    S.

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    Sean,

    I replaced the cps from the donor jeep when the motor was replaced. I dont think it is the problem, unless they arent compatible?

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    Just a idea, check the fireing order and plugs. Sounds like a misfire

  14. #14
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    i double and tripple checked the entire ignition system. Not the problem

  15. #15
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    it still sounds like a fuel issue you said it got better when you changed the filter . i think you not done looking at the fuel system yet .did you blow out the lines and the fuel rail it still sound like you r olny getting enough pressure and flow to run at idle and low rpm's but still not enough to feed higher rpm's or load ,check fuel pressure and flow .

  16. #16
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    Record a video of the engine idling and trying to rev then upload it to youtube and post the link here would help a lot.

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