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Thread: Cherokee engine surging/missing?

  1. #1
    Meat-Hippy Brett's Avatar
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    Cherokee engine surging/missing?

    1989 Jeep Cherokee 4.0L automatic
    No engine mods

    This started yesterday and did it again today. When under load and warmed up, the engine seems to act like it suddenly has no spark and lacks all power. It comes back in a second and everything is normal, but it will keep doing this if it's under a load (acceleration.) If I am just mainiting speed it doesn't do anything odd that I can notice. Idle is fine as well, no weird fluctuations at all. All of this is ONLY when the motor has run for a while, up at operating temps. When it's cold, it's fine!

    Can the CPS cause anything like this to happen or should I be looking for a bad vacuum line? Bad gas maybe?
    Brett Matthews

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  3. #2
    Registered User Bart's Avatar
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    I hear a broken door lock cylinder will cause this.

    Sorry, couldn't help it. Yes, a CPS could possibly cause this, as could a TPS. Also a plugged cat or muffler come to mind.
    Too many projects, too little time.

  4. #3
    too poor to wheel... :( gijohn40's Avatar
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    my old cj has been kind of acting like this... and I think the cat is plugged on mine... how does your exhaust look? just a thought...
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.

  5. #4
    Meat-Hippy Brett's Avatar
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    Cat...who knows how old that is. The muffler is only a couple years old. The rest of it looks fine to me, but I couldn't guess at if it's plugged.
    Brett Matthews

  6. #5
    Meat-Hippy Brett's Avatar
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    2 hours of cooling down in the parking lot and it still does it. I tried spraying the vacuum lines (while running) with carb cleaner, didn't do anything out of the normal.

    I'll try unplugging the TPS on the next break.....
    Brett Matthews

  7. #6
    Off the wall here but I am thinking this might be a Knock sensor? I just read through the factory fuel injection manual and this kinda sounds like your problem.

    http://www.pirate4x4.com/tech/billav...ion_manual.pdf

  8. #7
    Meat-Hippy Brett's Avatar
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    Quote Originally Posted by jeepspeedtj View Post
    Off the wall here but I am thinking this might be a Knock sensor? I just read through the factory fuel injection manual and this kinda sounds like your problem.

    http://www.pirate4x4.com/tech/billav...ion_manual.pdf
    Just read through the section on the knock sensor, but it sounds like it'd be a much slower reaction. The surging I am having is taking place within a couple of seconds, while the knock sensor would take 10-15 seconds to cycle through that.....

    It's a good guess though......another sensor they got rid of on the newer 4.0's I did clean up the connection on the CPS sensor, that might make a difference. It felt better when I ran it around the parking lot and the block. No surging.....we'll see how it is when I head home in 20 minutes.
    Brett Matthews

  9. #8
    Meat-Hippy Brett's Avatar
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    So, last night on my 20 mile trip home and to the gym, it ran just fine. No surging at all.

    Did it again today, after I'd been driving for about 20 minutes. This time it actually shut down as well on the freeway, but started up just fine and idled normally. The weird part is that I just floored (mad) and once it went past 3k RPM, it ran perfectly.

    I'd considered the fuel pump, but with it at WOT it should have run out of gas, but it ran better. Could it possibly be the cat? Get the RPM's high enough and it's okay? It acts like it's running out of gas, but I can hear the pump running and it doesn't sound odd.

    I'm confused
    Brett Matthews

  10. #9
    too poor to wheel... :( gijohn40's Avatar
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    ok its your fuel pump... my son's 91 xj had the same problems... ran like sh## until it hit about 3k and then took off like no other... replaced the fuel pump and it ran fine from there on out...

    get a fuel tester from harbor freight or I could loan you mine and test the fuel rail idling with vacuum to the fuel regulator and then without and it has to meet certain guidelines otherwise the pump is bad... I have it all somewhere what the numbers are suppose to be and such.... I'll start digging into this..
    IF YOU CAN'T FIX IT WITH A HAMMER, YOU'VE GOT AN ELECTRICAL PROBLEM.

  11. #10
    Meat-Hippy Brett's Avatar
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    Quote Originally Posted by gijohn40 View Post
    ok its your fuel pump... my son's 91 xj had the same problems... ran like sh## until it hit about 3k and then took off like no other... replaced the fuel pump and it ran fine from there on out...

    get a fuel tester from harbor freight or I could loan you mine and test the fuel rail idling with vacuum to the fuel regulator and then without and it has to meet certain guidelines otherwise the pump is bad... I have it all somewhere what the numbers are suppose to be and such.... I'll start digging into this..
    If it was the pump, wouldn't it be doing this all the time though? That's what I'm confused on
    Brett Matthews

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