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1991 Cherokee rough idle.

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Old 04-25-2010, 10:49 PM   #1
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Default 1991 Cherokee rough idle.

Hello, as you can tell this is my first post here. I must say that after many hours of web searching your forums seem to be the most comprehensive and has many knowledgeable user. That aside, the reason I'm posting today is try and get pointed in the right directions on some repair for my jeep.

1991 Jeep Cherokee
4.0L Inline 6 MPFI
Automatic Trans

This jeep has been sitting for a year or so. I've just now started to work on it in hopes of having an every day driver. After, sitting for an hour or more it starts fine. Idles OK for 1-2min then RPM drops to or below 1,000rps and starts to "Surge", not violently, but to the point it wants to die. Most of the time it doesn't die while in park and idling but while driving it will only run 5mins, give or take, before it quits. It has "power" while on the gas and throttle is responsive. After, dying it will not start again until it has set for an hour or so. Also, after dying it has a gas smell.

What I've done so far:
Fuel Pump
Fuel Filter
Fuel pressure regulator
Head gasket
Intake/Exhaust gasket
Crank position sensor
Throttle position sensor
Idle air control valve
O2 sensor (up)
Ignition coil
Engine temperature sensor
Distributor (cap, rotor, pick up coil, wires, plugs*, the distributor it's self)
Check all vacuum line (many time over again)
Check fuel pressure (31 PSI give or take a few pounds)
* = 4 ground spark plugs, thinking these might be a problem so I'm going back to the good ole single ground tomorrow.

My Thoughts:
To me it seems to be flooding it's self out but to the best of my knowledge that's what the IAC does it control the amount of gas/air during idle. I know that just because I have replaced these parts that doesn't mean they are good, but I was hoping to get a few other thoughts before I start re-buying parts. One other thing I've noticed is that my Fuel pump ballast is just a little off. It has 1.1ohm stamped on it and it is reading 1.5-1.8 but even if I bypass this ballast the problem seems the same. As for, the remark about the spark plugs above it that maybe, the 4 ground or 4 prong are not gaped correctly and are unable to be gaped so I've going back to the "old school" style with a .035 gap, lets hope that helps.

So if there are any super duper master jeep techs out there that has seen this problem or can give me any help in a next step I will be forever in your debt. Thanks in advance , Michael

Last edited by laidoff; 04-25-2010 at 10:53 PM.
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Old 04-26-2010, 10:47 AM   #2
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Well, I've replaced the spark plugs and that seem to have help my idle problem. But, I am still having a hard re-start. It does start now and has not died all day. So, who know it might have been the jeep didn't like me putting fancy German plugs in it. I am still getting a fuel smell, any ideas on why it might be running rich?
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Old 04-26-2010, 10:55 AM   #3
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2003 Grand Cherokee

your injectors might be going out. letting to much gas in.


Mud donkeez offroad club
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Old 04-26-2010, 11:01 AM   #4
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Sound reasonable I'll pull the fuel rail and give them a look. Thanks
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Old 04-26-2010, 11:01 AM
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