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'88 4.0; intermittant high idle?


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I have an '88 MJ with the 4.0...Sometimes on startup it revvs real high, like usually 2500-3000rpms; sometimes for just a few seconds and then drops to normal idle, sometimes for a long time. Blipping the throttle or bogging it with the clutch does not seem to effect it; sometimes turning it off and re-starting it does it. I noticed it was particularly bad rescently when we moved, just two blocks down the road. Almost every time it was started it did it, where as it is usually its every few times, so maybe the short drives vs. regular driving has an effect? Any ideas on what this may be? :???: Thanks!

 

James

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Clean your IAC. That's probably either not sealing properly or sticking open.

 

Your best bet would be to take the throttle body off, take the IAC off the TB(leave the TPS on or you'll have to re-adjust it), and spray the hell out of everything with throttle body cleaner.

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If cleaning the IAC doesn't help mine 87 was doing the same thing was driving me nuts I ended up swapping out all the fuel injectors to the 4 port was more of an upgrade, but it idles like a dream now and never ever idle high again no more blipping the throttle. :clapping:

 

So I say your injectors need cleaning or replacement they are sticking run some fuel injector cleaner in your tank.

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If cleaning the IAC doesn't help mine 87 was doing the same thing was driving me nuts I ended up swapping out all the fuel injectors to the 4 port was more of an upgrade, but it idles like a dream now and never ever idle high again no more blipping the throttle. :clapping:

 

So I say your injectors need cleaning or replacement they are sticking run some fuel injector cleaner in your tank.

 

Does this apply to an idle that is really really high in P or N, but less high (still higher than normal) in D, 3, 1-2? Not intermittently, but every single time?

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Every single time is more likely to be the IAC.

 

I thought so too, until I replaced it and the TPS and it didn't work. I'm going to double-check the manifold bolts on the bottom of the manifold. It's not just a high idle; this thing revvs up super high, I"m guessing ~3k RPM in P and N and doesn't come back down.

 

I covered the IACV hole with my finger and the truck finally came back down to normal idle. This makes me think I have a vacuum leak somewhere.

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Trust me guys if all else fails swap out the injectors it will be like driving a new MJ. I did it for more power and better mileage and this crazy symptoms being driving me crazy for years! Now it just idles perfectly well behave. No more embarrassing moments at the stop light (Super high idle) :rotf:

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Trust me guys if all else fails swap out the injectors it will be like driving a new MJ. I did it for more power and better mileage and this crazy symptoms being driving me crazy for years! Now it just idles perfectly well behave. No more embarrassing moments at the stop light (Super high idle) :rotf:

 

Are you talking about going from a TBI 4 cylinder to an MPFI 4 cylinder?

 

Cause if you are, it doesn't apply here. He's got a 6 cylinder. All 4.0s are MPFI.

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Trust me guys if all else fails swap out the injectors it will be like driving a new MJ. I did it for more power and better mileage and this crazy symptoms being driving me crazy for years! Now it just idles perfectly well behave. No more embarrassing moments at the stop light (Super high idle) :rotf:

 

Are you talking about going from a TBI 4 cylinder to an MPFI 4 cylinder?

 

Cause if you are, it doesn't apply here. He's got a 6 cylinder. All 4.0s are MPFI.

 

WTF? :fs1:

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i second the intake bolts.even after loctite mine would work loose and cause a vacuum leak and a very high idle.if replacing injectors fixed a high idle it was because the lower o-rings were bad causing a vacuum leak,not the injectors themselves.i would use some water in a squirt bottle or some carb cleaner to look for vacuum leaks.

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It is part of my routine on Renix 4.0L to check those intake manifold bolts every oil change. There is usually at least one or two which have started to back off. It is usually the number 5 or 6 cylinder ones because they are the hardest to access. I have been wanting to pull each of them and locktite them but have just not had the time.

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  • 1 month later...

So since all high idle issues seem to be a little different for everyone, i guess ill post mine up here to see if i can work this one out. mine is not intermittant, it happens every time i turn my truck on, but only after a few minutes weather I'm driving or its parked makes no differance, and it idles VERY high, if i shut down, then restart it will happen the same way, only a little quicker. my intake bolts are tight and my IAC and TB are spotless. it started after i cleaned my TB and IAC. i went back and made sure the o rings were still good and everything was tight, so I'm a bit lost. i did however remove my TPS, could that be out of adjustment?

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if it happened after you cleaned the iac then check the throttle stop on the side of the tb.when i was a tech at a jeep/dodge dealership i had a 4.0 jeep that wouldn't idle very well.i cleaned the iac then it idles high as hell.i checked and someone had adjusted the idle stop to compensate for a dirty iac.the girl later told me that her b/f had adjusted the idle up but it didn't help any.

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if it happened after you cleaned the iac then check the throttle stop on the side of the tb.when i was a tech at a jeep/dodge dealership i had a 4.0 jeep that wouldn't idle very well.i cleaned the iac then it idles high as hell.i checked and someone had adjusted the idle stop to compensate for a dirty iac.the girl later told me that her b/f had adjusted the idle up but it didn't help any.

 

care to elaborate on the idle stop? is it a screw somewhere?

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on the side of the tb there is a little screw that stops the throttle from completely closing .i think it's either got a torx or allen head in the screw to adjust it.it's very small but it's on the side where the throttle cable is at.

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Well, for what its worth to those of you still chasing it, I took the advise of many (thanks to all who advised) and removed my throttle body assy and let almost an entire can of carb/choke cleaner go on it; into all the nookes/crannies, holes, sensors, etc...problem solved. After the cleaning, the truck consistantly idled at 1,200 at that point, reguardless of conditions so I bent that little arm at the throttle cable housing a little to set the idle at about 500...it always sits at 500 now, smoothly too...and its hard to tell just yet, but I think my gas mileage boosted quite a bit (from about 17 average to 20 my last tank, same driving conditions) :eek: ...but I want to run a few more tanks for consistancy to that claim :cheers:

 

Anyway, thanks again!!!

 

James

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Well, for what its worth to those of you still chasing it, I took the advise of many (thanks to all who advised) and removed my throttle body assy and let almost an entire can of carb/choke cleaner go on it; into all the nookes/crannies, holes, sensors, etc...problem solved. After the cleaning, the truck consistantly idled at 1,200 at that point, reguardless of conditions so I bent that little arm at the throttle cable housing a little to set the idle at about 500...it always sits at 500 now, smoothly too...and its hard to tell just yet, but I think my gas mileage boosted quite a bit (from about 17 average to 20 my last tank, same driving conditions) :eek: ...but I want to run a few more tanks for consistancy to that claim :cheers:

 

Anyway, thanks again!!!

 

James

 

Sounds bad for your o2 sensor. I bet if you replaced that, your mileage may bump up a little.

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