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Unread 12-11-2014, 12:11 PM   #1
Pierre_r
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Weird rpm issue 5.2

I have noticed for a few days now that if i am driving at 80km/h (50mph)
and have the rpm between 1500 and 1800 that the rpm sometimes jumps up and down but only between 1500 and 1800.
I can see it on the tachometer/revcounter and i can hear and feel the engine go up and down in its rpm.

It does the same thing when coasting down a hill but then it goes from 8-900 up to 1200 and jumps between those rpm until i brake or hit the gas again.
I also noticed a few months ago when this started to happen that my oilpressure gauge jumps between 40 and the line that is before it, but only does it when its warm and at idle.

Can the IAC or TPS or maybee both cause the engine to jump in revs like that? and is any of that also the cause for my fluctuating oilpressure when idling with a warm engine?
It never fluctuates when i start it then its bang on the line between 40 and 80.

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Unread 12-11-2014, 01:49 PM   #2
Boone96ZJ
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My 5.2 does this going up hill between 2000 and 2200. The exhaust note changes and it goes from 2000 to 2100 or 2200 then back down and I don't even budge the throttle.
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96 5.2 242 3" 32x11.5
http://www.jeepforum.com/forum/f13/1996-grand-cherokee-laredo-5-2-a-2570290/
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Unread 12-12-2014, 02:39 AM   #3
newfieZJ
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First thought : Plenum gasket failed .
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Unread 12-12-2014, 08:06 AM   #4
kg6mov
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Thats not a bad idea newfie. Plenum gasket could cause that in the circumstances. How's the oil level in your intake Pierre?
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1996 ZJ Green Laredo V8, 2.5" OME and Bilsteins, 33's, Holes drilled in the roof.
1998 ZJ Platinum Limited 5.9, Backyard parts fodder
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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Unread 12-12-2014, 11:15 AM   #5
Pierre_r
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I have not looked in my intake for a while, but it could be the plenum since it still is factory and i have 300,000km,s (186000miles) on it.
So its probably time to fix that, i have the kit from hughes but dont have time to fix it.
What i do know is that i loose 1/1.5Litres of oil every 10000km,s (6000miles)
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Unread 01-15-2015, 12:54 PM   #6
Pierre_r
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I cleaned the IAC and throttlebody today and there was almost no change.
The rpm at idle dropped alitte and it responds better to the accelerator.
A weird thing did happen, after i started it i noticed that the accelerator was alittle harder to press on when the engine was running.
When it was turned off it was back to normal, but as soon as i start it it gets alittle harder.

I looked at the oillevel and well i named it "lake castrol" so the plenum is shot.
Unfortunatley i dont have time to fix it untill sometime in April so i guess i have to learn to live with it for now.
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Jeep Cherokee XJ 4.0 -89
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Chevrolet Silverado 2500 6.5TD -94
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Unread 01-15-2015, 02:56 PM   #7
dflewelling
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Could be the torque converter unlocking to compensate for load...
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Unread 01-16-2015, 01:21 PM   #8
Pierre_r
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Could possible be that. I think i know another reason to why it does this and it could be because the tv cable is not attached at the trans.
The arm is gone to so i have nothing to mount it to.
It might fix itself once i find a new arm and can attach and adjust the cable.

Another Q
Where can i buy new vacuum hoses?
I have planned to replace my egr valve once the plenum is off and i carefully felt on them and the were rockhard so they will break once i try to remove them.
The few rubberhoses that goes to the TB and intake are also rockhard so i would need those aswell.
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Jeep Cherokee XJ 4.0 -89
Jeep Cherokee XJ 4.0 -90 (partsjeep)
Jeep Grand Cherokee 5.2 -93
Chevrolet Silverado 2500 6.5TD -94
Chevrolet C20 6.2 -82
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Unread 01-16-2015, 02:02 PM   #9
kg6mov
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You can get new mopar molded hoses from the stealership or an online retailer. Thats what I've been doing for things like the PCV to intake hose. A parts store will sell you bulk hose which will also work.

A lot of the mopar hoses should have the part number on the hose.
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1996 ZJ Green Laredo V8, 2.5" OME and Bilsteins, 33's, Holes drilled in the roof.
1998 ZJ Platinum Limited 5.9, Backyard parts fodder
1987 Mercedes 300D Turbo, the diesel daily, 30mpg in style

Quote:
Originally Posted by Texas ZJ1 View Post
I heard someone yell "mall crawler" from the back porch. I'll go see who said that. Be right back.
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