XJ Cold Start= rough idle, then fine after 5mins - JeepForum.com

 
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post #1 of 9 Old 12-26-2010, 12:45 AM Thread Starter
sfsutro
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1998 XJ Cherokee 
 
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Location: san francisco
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XJ Cold Start= rough idle, then fine after 5mins

My 98' Cherokee sport( 4wd, manual trans., 4.0L, 142k miles) has the strangest idle symptoms.
When I start the car when the engine is cold, it kicks right over. It idles like normal for about 45secs, then at about 800rpm, the engine goes into this stumbling idle. It does this rough idle for about 4mins, and then once i give it the throttle, it runs fine. If i try to drive the car before the 4min mark, it stumbles down the street like it's lacking power or starving for gas. The exhaust smells like raw fuel during the 4 min warm up stage. Once the car is warmed up, it drives normal and never reoccurs until I let the engine get cold again.

Here's my dilemma: I have had 2 different mechanics look at it and run diagnostics and functional Tests.
We have cleaned the throttle body, and IAC. Checked fuel injectors for leaks, checked plugs, coils, and sensors. Fuel Pump is good. CPS was replaced 6months ago. TPS registers good on Diagnostics.

Mechanics went on to:
Then replace the IAC motor, Coolant temp. Sensor, Intake air temp sensor, and distributor plate. It still does the same thing as before.


I'm really at a lose.... Someone please help us before i end up going broke dumping parts into it for no reason and paying mechanics.

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post #2 of 9 Old 12-26-2010, 04:43 AM
tjwalker
It's the crank sensor!
 
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1. I'd be sure that there were FRESH champion copper plugs, new plug wires, new distributor cap and rotor in there. Don't want to chase your tail over tuneup parts.

2. Did the mechanics find any stored codes? Did they look for any? That is a must, even if you have never seen a check engine light.

3. Sounds like you tested the MAT, but did you test the MAP sensor too?

4. Did you put a fuel pressure gauge on the fuel rail and watch it closely during this 4 minutes of symptoms to ensure that fuel pressure/delivery is consistent?

99 Cherokee, 4.0 AW4, NP242
Past Jeeps: 49 Willys, 81 Scrambler, 88 Comanche
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post #3 of 9 Old 12-26-2010, 09:07 AM
xjokee
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I would suspect that you're pre-cat O2 sensor is not heating. Replace and enjoy.
Mike

~ When in doubt........Throttle out! ~
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post #4 of 9 Old 12-26-2010, 12:33 PM Thread Starter
sfsutro
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Location: san francisco
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All of the sensors were diagnostically checked... All the plugs and distributor parts are new. There were no codes or check engine light on. Why would the
O2 sensor on the cat cause a rough idle?
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post #5 of 9 Old 12-26-2010, 12:59 PM
xjokee
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1998 XJ Cherokee 
 
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Location: Southbury, Conn.
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It's your "upstream" O2 sensor. They have a heating element that triggers during cold start warmup and adjusts fuel accordingly. When the element does not function properly, the computer is fooled into thinking the engine is warm and runs rich as if it were. There's a technical term for the routine.....closed loop or something, I just don't recall. O2 sensors are relatively inexpensive and the problem you are decribing is just like the one I had on my '98 until I changed that sensor. Good Luck!
Mike

~ When in doubt........Throttle out! ~
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post #6 of 9 Old 01-09-2011, 09:41 PM Thread Starter
sfsutro
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Location: san francisco
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It worked

XJokee - we replaced the o2 sensor and it worked.... i appreciate the help and thanks again... happy jeepin
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post #7 of 9 Old 01-09-2011, 11:07 PM
connor0770
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I have this same issue. Any chance those O2 sensors would throw a code?

97 TJ Dana 30 w/ARB Solid Cover, Currie 4" lift, Ford 8.8 w/ Riddler Cover, Currie Steering, Trail Gear Front Bumper, LOD Rear Tire Carrier, Smittybuilt XRC8, 35" KM2's, 5100 Bilsteins, Teraflex Oil Pan Skid, Poison Spider Sliders, Rock Lights etc.
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post #8 of 9 Old 01-10-2011, 01:52 AM Thread Starter
sfsutro
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Location: san francisco
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There were no codes thrown from this problem... We did disconnect the tps at one point and it threw a code... See if that's ur problem...
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post #9 of 9 Old 03-15-2014, 07:50 PM
Logsdon2000xj
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1997 XJ Cherokee 
 
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Location: Caneyville
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In some cases the upstream sensor when failing won't throw a cel code and the but sensor is bad and not reading right..so I'd say more than likely it's your upstream 02 sensor
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