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Unread 12-13-2008, 10:28 AM   #1
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1996 XJ Cherokee 
Join Date: Nov 2008
Location: Kentucky
Posts: 9
96 Cherokee rough idle lacks power. 4.0 engine.

I need help figuring out what to do next to get my engine running right.

My 1996 Jeep Cherokee Country with 4.0 engine has a rough idle and lack of power. I get engine missfire codes in cylinders #1 and #2, and sometimes #3. The engine has 150,000 miles and no major engine work has ever been done on it. It does not accelerate smoothly. It started several months ago with only a rough idle and now it has progressed to missfiring during acceleration.

Here is what I have done so far:

Replaced the spark plugs twice (Autolite the first time, OEM Champion the second time). Gapped them both times at 0.035.

The second time I replaced the spark plugs (after 1,000 miles) I looked at the removed plugs and the color on all 6 plugs looked a light tan. The insulator was still white. Other than the electrodes being a light tan they still looked new.

Replaced plug wires, rotor, and distributor cap.

Replaced fuel filter.

Replaced Throttle Position Sensor and Idle Air Control valve.

Removed the throttle body and cleaned every nook and cranny.

Checked fuel pressure at the fuel rail. 38-40psi with engine running. drops about 2 psi when throttled to 2500rpm. When I first turn the key on with no pressure the gauge moves to about 25-30psi. It takes a second turn of the key to get it to 40psi. I'm not sure if that is normal. I did a bleed down test and the pressure dropped about 15psi in 5 minutes.

I checked all six cylinders for compression:

#1 - 140
#2 - 130
#3 - 145
#4 - 152
#5 - 148
#6 - 150

I replaced all 6 injectors with brand new OEM injectors.

The engine still idles rough, gets missfire codes in #1, and #2, and lacks power when accelerating.

I know the fuel pressure is a little lower than spec's. I am hesitant to replace the fuel pump assemble (it also contains the regulator) because the missfire codes are for only two cylinders. Maybe someone out there has experience with fuel pumps and can tell me if this could be an issue.

Can anyone help?

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Unread 12-13-2008, 10:30 AM   #2
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1996 XJ Cherokee 
Join Date: Nov 2008
Location: Kentucky
Posts: 9
Correction to miles on 96 Cherokee

The engine has 250,000 miles, not 150,000 as in my previous post.
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Unread 12-13-2008, 03:56 PM   #3
It's the crank sensor!
tjwalker's Avatar
1999 XJ Cherokee 
Join Date: Nov 2007
Location: Central Minnesota, MN
Posts: 8,432
My first gut instinct is that you have a vacuum leak somewhere, possibly in the intake manifold?? I've seen where a leak there can be responsible for random cylinder misfires.

I tend to think you have a "common" hardware cause like this, rather than coincidentally having something wrong with the hardware on 3 of your cylinders at the same time. You've pretty much ruled that out anyways by replacing plugs/wires/cap/rotor and testing compression.

If it were mine, I would start by doing some vacuum testing and if that didn't show anything, possibly an engine analyzer may help. You can tell a lot about how an engine is running by testing vauum.
99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
Without "data", all you have is an opinion!
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Unread 12-29-2008, 08:35 AM   #4
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1996 XJ Cherokee 
Join Date: Nov 2008
Location: Kentucky
Posts: 9
Just replaced the fuel pump and MAP sensor. Got my power back. No longer lacks power when accelerating. Checked vacuum and no leaks. Still runs rough and missfires at idle. Engine codes indicate all cylinders are missfiring. I don't have any garages around here I can trust not to rip me off. Anyone know what else I should look at?
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Unread 12-29-2008, 09:09 AM   #5
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1988 XJ Cherokee 
Join Date: Jun 2008
Location: NorCal
Posts: 398
i would get a can of staring fluid and start the jeep, while running lightly spray the starting fluid around were the intake manifold connects to the block up and down it. if the idle increases at all while doing this. the gasket may need to be either tightened or replaced also i would try disconnecting the vaccum canister from the intake, see if it improves at all, also try spraying the starting fluid around hoses that look bad, good luck
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