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98 Grand Cherokee: 4.0--symptom: Low idle..exhaust..40 mph..diagnose…

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98 Grand Cherokee, 4.0--symptom: Low...
98 Grand Cherokee, 4.0--symptom: Low idle after start, almost as though there is a blockage in the exhaust. Exists (until warm) between idle and about 35-40 mph when in top gear also. Can't diagnose. Recent work I've done includes fresh cap, wires, plugs--O2 sensor, throttle position sensor, exhaust manifold replacement. It's making me crazier than normal. Please help me!!

Thanks,

Kevin,
Milwaukee

I forgot to add that if I trounce on it, it does push through the problem, but only if I head up to highway speeds.
Submitted: 8 years ago.Category: Jeep
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Answered in 4 minutes by:
6/24/2010
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Tim West
Tim West, ASE Certified Technician
Category: Jeep
Satisfied Customers: 4,638
Experience: 10 year jeep owner/ ASE certified Service Manager
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Thanks for visiting Just Answer,
This problem sounds like it could be caused by a throttle issue or idle air control motor problem.
First, make sure the battery has a good charge, weak batteries cause all sorts of low idle complaints on this vehicle . Check the battery state with the engine cold, connect a volt meter across the battery. Remove the fuel pump relay. Crank the engine for 15 seconds monitoring the voltage. It should stay above 9 volts.

Inspect the throttle body for any build up or sludge, clean with a rag as needed.
If the problem still remains, you could have a sticking idle speed motorgraphic
Tim West
Tim West, ASE Certified Technician
Category: Jeep
Satisfied Customers: 4,638
Experience: 10 year jeep owner/ ASE certified Service Manager
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Customer reply replied 8 years ago
There is no IAC on the 4 liter, only on the 4.7 liter. Do you mean the IAT?
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago

Sure it does, graphic
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Customer reply replied 8 years ago
Yeah, I found it. I changed it, slight improvement then back to the same. Do you think the MAP sensor may be a problem?
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Is the MIL (engine light) on?
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Customer reply replied 8 years ago
That's what got me--no check engine light. You know how that shaft in the motor is telescopic as it extends due to electricity being put to it? Well, I cleaned the old one and it ran much better then returned to its original problem so I put the new one in. Ran fine then the same problem with slightly different symptoms but similar enough that anyone but me might not notice. And, again, once it was warm and running for a while there were no problems. It happens soon after start and first drive of the day or after it has sat for a more than a half hour or so. Goofy. Thanks for your help so far. I do appreciate it. Didn't mean to say that there was no IAP motor, the Haynes just called it something else. Thanks!
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
It's no problem what did the haynes call it? the AIS? it's all good.
Have you checked the engine for vacuum leaks?
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Customer reply replied 8 years ago
I'll have someone do the vacuum check. I'm not entirely sure where to begin on that.
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Well, to check for a vacuum leak you can try blocking the throttle and verifying it stalls, or try spraying some carb cleaner around the engine and listen for the engine to react to the presense of external fuel.
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Customer reply replied 8 years ago
Thanks, XXXXX XXXXX that idle air motor will need to extend and retract a few times to get it settled in?
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
No, It's possible the IAC or AIS could be bad. I was just talking to another expert with the same problem. Replaced the IAC and the problem didn't change. So after a day of checking other areas, we ended up replacing the IAC again, and it worked.
So the possibility of getting a bad part is there. Just don't over look that as an option.
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Customer reply replied 8 years ago
Yeah, I thought that too. But before I replaced it, I removed the old one, cleaned it and put it back on. It ran much better for a few minutes--the choke progressed down nicely then it started its crap again so I just changed it and the progress was exactly the same. I think I'm on the right track with the fuel/air mixture route. Oh yeah, do you know whether or not I can buy replacement t-20 head screws for those sensors from anyone in particular? Thanks.
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Autozone sells t-20 have you considered loosening the exhaust?
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Customer reply replied 8 years ago
How does one loosen exhaust? I wouldn't know where to begin.
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Try to loosen the cat bolts

REMOVAL

WARNING: if torches are used when working on the exhaust system, do not allow the flame near the fuel lines.

  1. Raise and support the vehicle.
  2. Saturate the bolts and nuts with heat valve lubricant. Allow 5 minutes for penetration.



  1. Remove exhaust clamp from the catalytic converter and exhaust pipe connection.
  2. Remove exhaust clamp from the catalytic converter and muffler connection.
  3. Disconnect oxygen sensor wiring.
  4. Heat the exhaust pipe, catalytic converter and muffler connections with an torch until the metal becomes cherry red.
  5. While the metal is still cherry red, twist the catalytic converter back and forth to separate it from the exhaust pipe and the muffler.

INSTALLATION

  1. Position the exhaust clamp over the exhaust pipe/catalytic converter connection. Tighten the nuts to 61 Nm (45 ft. lbs.) torque.
  2. Install the muffler onto the catalytic converter until the alignment tab is inserted into the alignment slot.
  3. Install the exhaust clamp at the muffler and catalytic converter connection. Tighten the clamp nuts to 61 Nm (45 ft. lbs.) torque.
  4. Connect oxygen sensor wiring.
  5. Lower the vehicle.
  6. Start the engine and inspect for exhaust leaks and exhaust system contact with the body panels. Adjust the alignment, if needed.



 

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Customer reply replied 8 years ago
Cool--A little more detailed than I am able to accomplish with my limited tool box but thanks for the information. I'll have to take it in and get it done. Thanks very much for your help, I'm sure I'll be in touch with further Jeep anomalies. Have a great weekend.

Kevin
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Thanks Kevin you too.
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Customer reply replied 8 years ago
Hey Tim--Hope you are having a good holiday weekend. Can you tell me what the symptoms, if any, there are for a failing or malfunctioning ECU? I've got a friend who knows a guy who thinks my problems could be ECU related. If you get some time to answer, I would appreciate it. Thanks!
Jeep Mechanic: Tim West, ASE Certified Technician replied 8 years ago
Well a failing computer can react in many many ways. There are unlimited ways a computer can act when it fails so saying it acts in a certain way wouldn't be correct.
the computer communicates with every sensor.
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Customer reply replied 8 years ago
Gottcha--Ok, I've changed the throttle position sensor, the idle air sensor, the ignition system, O2 sensor and exhaust manifold. I'll start the truck, it turns over normally, idles normally for about a minute then the idle drops and it doesn't respond to light/normal throttle inputs as though it is misfiring or there is an obstruction in the exhaust. After the temperature gets to normal (between 180-200), the thing idles fine. This, however, does not preclude the idle from dropping off once the truck has come to a stop after having driven it for a while. It's an anomalous symptom at best but that is as accurate as I can be. Also, no "check engine" light. That's why I was leaning toward the ECU. I'm ready to shoot the thing and go back to Nissans, but I don't want to spend the cash. Bonus for you for sure if I can get this thing running right. Thanks, Tim.
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