1. #1
    lgbent's Avatar
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    91 jeep running rough

    I've had this problem for a while. i have 91 jeep yj 2.5l ,once it warms up . I'll be driving it with high rpms then shift into the new gear it will backfire like a pop sound. It also starts to tick like a lifter sound after it warms up. I replaced the cps sensor the one at the base of the distributor. I don't have any more idea's.

    Thanks:confused:

  2. #2
    TwistedCU's Avatar
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    Re: 91 jeep running rough

    When was the last time you tuned it up?

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    Re: 91 jeep running rough

    i tune it up every spring ,plugs cap, so on. still same problem

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    Re: 91 jeep running rough

    To find out easily if the sparkplugs wires and dist cap are in good shape do this on a manual trans vehicle start of slow in 1 st gear once rolling stick it in 3th or 4th and floor the accelerator if it backfires you have a bad sparkplug or coil wire or one that is not properly seated in other words their is not enough spark voltage to fire one or more sparkplugs this trick works well if you listen to the engine you can hear it drop a cylinder the ticking sound could very well be an arching sparkplug wire or sparkplugwire boot

    Art

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    Re: 91 jeep running rough

    Quote Originally Posted by aw12345 View Post
    To find out easily if the sparkplugs wires and dist cap are in good shape do this on a manual trans vehicle start of slow in 1 st gear once rolling stick it in 3th or 4th and floor the accelerator if it backfires you have a bad sparkplug or coil wire or one that is not properly seated in other words their is not enough spark voltage to fire one or more sparkplugs this trick works well if you listen to the engine you can hear it drop a cylinder the ticking sound could very well be an arching sparkplug wire or sparkplugwire boot

    Art
    Although I'd be a bit leery of lugging an engine like that (in case there are weak parts in the engine that may very well give way), I agree it may be an arcing plug wire. Do you have a radio in the Jeep? If so, do you hear a rhythmic popping/clicking through the speakers? That is a sure fire way to tell if you've got an arcing plug wire.

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    Re: 91 jeep running rough

    i hear nothing in the radio when it backfires.

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    Re: 91 jeep running rough

    Quote Originally Posted by lgbent View Post
    i hear nothing in the radio when it backfires.
    Any ticking through the speakers when you hear the ticking in the engine when it warms up?

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    Re: 91 jeep running rough

    once it warms up and i've drove around for awhile , it will start ticking when idling, if i rev it up a couple times it goes away. But still comes back does tick all the time . that why i thought it could be timing.

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    Re: 91 jeep running rough

    you could have 2 problems one a sticking valve lifter causing the ticking not the source of your backfire. When a sparkplug wire develops high resistance it foes not necesarily cause a ticking sound on the radio but under heavy load it simply stops firing on one or more cylinders hence the lugging thing you lug the engine for a minute or so and listen for it starting to miss and backfiring and you have a bad wire or a plug that is missing a chunck of the electrode I.E very high secondary ignition resistance. its simple and should not hurt it I do this with Gasoline UPS trucks that use Gasoline engine and if it does not kill the engine in a 20.000 pound truck a jeep should be ok. to me its quicker and easier than using a scope that does not always nail it when the engine is hot and under load. start in 1 st rol to 5 or 10 mph stick it in 3th and floor it if in lugs but slowly accelerates without missing or backfiring the ignition system is ok period

    Art

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    Re: 91 jeep running rough

    A weak lifter will start to tick after the engine oil gets hot and thins out. The "pop" may be caused by a wire, plug or a sticking valve. As AW said, an ignition problem will show up under load (heavy acceleration). But a sticking valve may hang on decel (as in changing gears)
    Tee-ing into a manifold vaccuum line with a vaccuum gauge and driving the jeep as you normally would while the gauge is taped to the windsheild, will give you an idea if a valve is hanging up. A sudden rapid drop in vac (the needle will jump) will tip you off. I would not be surprised if the ticking and the popping are related to each other.

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    Re: 91 jeep running rough

    i tried with the engine warm, went for 1 to 3 gear and floored it no backfire. if it is a valve problem how do you fix it? also remove the spacer in the air cleaner box and removed the plate the was in the box also. seems to have a litttle more punch now.

  12. #12
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    Re: 91 jeep running rough

    You would need to remove the cylinder head and send it out to a machine shop. They will re surface the valve seats, grind the valves and knurle guides (if required). They will do seals and springs and plane the head.
    While you are waiting for the head to come back, it would be a good idea to do the lifters.

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