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Unread 07-16-2013, 11:56 AM   #1
hardbodyjeep
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1998 jeep grand cherokee laredo 4.0 idle problem

Hi I have a 1998 jeep grand Cherokee laredo 4.0 automatic 2wd that I purchased from a little buy here pay here with 124k miles a week after I got it home I noticed it had a idle problem while sitting the rpms fluctuate up and down but not a lot and almost feels like its missing like spuddering runs fine driving just little rough at idle and is getting annoying I've already replace a few parts and checked for vacuum leaks also ran fuel injector cleaner through it please somebody lead me in the right direction I'm tired of guessing


Here are the parts that I've already replaced and didn't help

Fuel pump/sending unit
Iac sensor
Map sensor
Ignition coil
Distributer cap and rotor
Spark plugs/wires
Radiator


And here are the things I'm thinking about doing next that mite help but who knows also let me know what order I should do them in or if there's anything else that would help thank you

Crank position sensor
O2 sensor
Tps sensor
Fuel injectors

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Unread 07-16-2013, 12:52 PM   #2
dellis
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I'm not exactly sure what you mean by "rough" ... at your Jeep's age, here's some comments on what you've already done:

(1) A general tune up (plugs, wire, distributor cap, distributor rotor, and coil) helps a lot with smoothing idle ... which you've already done. The plugs should be just a plain old copper type, nothing fancy.
(2) Cleaning the throttle body and the IAC. You replaced the IAC, but did you clean out it's housing and the throttle body?
(3) Yes, some form of fuel injector cleaner (Techron or Seafoam) will help to clean up injectors and knock the carbon off valves ... might want to do more than one treatment.

Concerning what you are thinking about doing.

(1) The Crank Position Sensor will not affect idle. The downstream O2 Sensor will not affect idle.
(2) The Upstream O2 Sensor or the TPS may affect idle, but I wouldn't expect them to correct something small ... usually, it seems that these sensors cause big problems when they are going bad.

Some other ideas:

(1) Replacing the two motor mounts and the transmission mount will help to smooth out the feel of the Jeep at idle. A lot.
(2) Clean the carbon and other goop off your Intake Air Temperature sensor.
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Unread 07-16-2013, 02:22 PM   #3
hardbodyjeep
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Quote:
Originally Posted by dellis View Post
I'm not exactly sure what you mean by "rough" ... at your Jeep's age, here's some comments on what you've already done:

(1) A general tune up (plugs, wire, distributor cap, distributor rotor, and coil) helps a lot with smoothing idle ... which you've already done. The plugs should be just a plain old copper type, nothing fancy.
(2) Cleaning the throttle body and the IAC. You replaced the IAC, but did you clean out it's housing and the throttle body?
(3) Yes, some form of fuel injector cleaner (Techron or Seafoam) will help to clean up injectors and knock the carbon off valves ... might want to do more than one treatment.

Concerning what you are thinking about doing.

(1) The Crank Position Sensor will not affect idle. The downstream O2 Sensor will not affect idle.
(2) The Upstream O2 Sensor or the TPS may affect idle, but I wouldn't expect them to correct something small ... usually, it seems that these sensors cause big problems when they are going bad.

Some other ideas:

(1) Replacing the two motor mounts and the transmission mount will help to smooth out the feel of the Jeep at idle. A lot.
(2) Clean the carbon and other goop off your Intake Air Temperature sensor.
Thank you for the reply but I mean rough as in it seems like it has a mild cam or a miss it drops to around 500 rpms then back up to 650 over and over and u can hear it in the exhaust kinda lopey and little bit of a shake in the seat while it does it just enough to annoy you

Last edited by hardbodyjeep; 07-16-2013 at 03:12 PM..
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Unread 07-16-2013, 03:15 PM   #4
Oldfrog
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dellis gave you good advice, I would only add this:
1. what spark plugs are you using? They should be the cheap Champion copper plus.
2. Are they gapped properly ? ( .035) This is important, especially on an older engine.
3. Clean out the inside of your distributor cap with electrical contact cleaner and wipe it down thoroughly in there.
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Unread 07-16-2013, 03:39 PM   #5
dellis
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Quote:
Originally Posted by hardbodyjeep View Post
Thank you for the reply but I mean rough as in it seems like it has a mild cam or a miss it drops to around 500 rpms then back up to 650 over and over and u can hear it in the exhaust kinda lopey and little bit of a shake in the seat while it does it just enough to annoy you
lol ... when you said your idle was fluctuating "not a lot", I was thinking 710-730 ... but, you are talking about quite a large fluctuation.

If this fluctuation is NOT happening in the first 30 seconds after your Jeep is started, but begins happening shortly after that (when the PCM goes into closed loop), then I would suspect that your upstream O2 sensor is causing your issues (the upstream 02 sensor is between the exhaust header and the catalytic converter).

If the observation above is happening, then to verify the O2 sensor as your cause, simply disconnect your upstream O2 sensor and see if that corrects the idle problem. The O2 sensor plug can be found mounted on the side of your engine, below the exhaust header, and close to the transmission cooling lines. (It's OK, you can run your engine a little with it disconnected).

So, if that is it, then let us know. If not, we'll lead you through some other possibilities and tests.
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Unread 07-16-2013, 10:27 PM   #6
hardbodyjeep
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I'm using copper ngk spark plugs which I gapped .035 and the distributer cap was brand new so I didnt bother wiping it down but ill deff unhook the o2 sensor in the morning see if that work ill try anything at this point ill let you now how it goes
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Unread 07-16-2013, 10:29 PM   #7
Oldfrog
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Jeeps seem to like champion plugs.....but I dont think that's your issue.
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Words of wisdom, Posted by Zeejay:
"..it's not air, it is the opposite of air... suckage so to speak."

Stalling? Work through Zeejay's list
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Unread 07-16-2013, 10:57 PM   #8
hardbodyjeep
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Ok soo on my way to work tonight once it was warm at the stop light idled at 500rpm then dropped to about 400 and continued up and down did not do it in the first 30 sec of starting the car when I first start acts normal goes up to about 1k then comes down like a.normal running vehicle then it starts fluctuating once the rpms come down
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Unread 07-17-2013, 12:35 PM   #9
Donnyd21
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Change the O2 sensor, its likely due anyway, stay with a OEM one, you can get them off amazon for a few bucks more than the Bosch units.
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Unread 07-17-2013, 01:37 PM   #10
Oldfrog
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Not trying to get you sidetracked....but check the tightness of your distributor cap. Humor me and eliminate that possibility. Also, check the bearing slack in the distributor itself by wiggling the post under the rotor from side to side. Other than that..right now I'd only suggest a throttle body cleaning, then then one by one, wiggling all the sensor wiring while it's idling. TPS, MAP, etc. Look for a change in RPMs.
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Words of wisdom, Posted by Zeejay:
"..it's not air, it is the opposite of air... suckage so to speak."

Stalling? Work through Zeejay's list
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Unread 07-17-2013, 02:04 PM   #11
dellis
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By the way, in talking about the O2 sensor, I am assuming that you have already cleaned out your throttle body and your IAC is working properly.

In my estimation, the oscillating that you are experiencing is the result of the engine overcompensating air/fuel adjustments based on the data it is receiving from the O2 sensor (which will look like an oscillating waveform). The idle will always oscillate a little, but a large oscillation is the result of the PCM thinking it is compensating properly but instead the system (PCM plus actuators) overcompensates ... this could be due to poor readings from the O2 sensor, or poor performance of something unexpected by the PCM in the air/fuel mix system (e.g. vacuum leak, but you say that you've eliminated this).
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Unread 07-17-2013, 05:13 PM   #12
hardbodyjeep
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Quote:
Originally Posted by dellis View Post
lol ... when you said your idle was fluctuating "not a lot", I was thinking 710-730 ... but, you are talking about quite a large fluctuation.

If this fluctuation is NOT happening in the first 30 seconds after your Jeep is started, but begins happening shortly after that (when the PCM goes into closed loop), then I would suspect that your upstream O2 sensor is causing your issues (the upstream 02 sensor is between the exhaust header and the catalytic converter).

If the observation above is happening, then to verify the O2 sensor as your cause, simply disconnect your upstream O2 sensor and see if that corrects the idle problem. The O2 sensor plug can be found mounted on the side of your engine, below the exhaust header, and close to the transmission cooling lines. (It's OK, you can run your engine a little with it disconnected).

So, if that is it, then let us know. If not, we'll lead you through some other possibilities and tests.
Ok update I took throttle body off and cleaned it up wasn't to bad but didn't fix anything also disconnected o2 sensors one at a time and ran jeep for a few minutes each one and unplugged felt the exact same and didn't throw any codes wouldnt that mean they are bad? While looking at the o2 sensors I noticed they were 2 different kinds does this matter? Also I checked to make sure cap and rotor were on good and no play and they were fine. What to do next?
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Unread 07-17-2013, 06:12 PM   #13
ZeeJay1997
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The PCM wont set a DTC for one O2 failure. It doesnt mean your O2 sensors are bad.

Check for a large vac leak.

Listen to your injectors with a stethoscope or a long screwdriver with the handle to your ear and the tip on the injector.
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Unread 07-17-2013, 09:04 PM   #14
dellis
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Quote:
Originally Posted by ZeeJay1997 View Post
The PCM wont set a DTC for one O2 failure. It doesnt mean your O2 sensors are bad.
I've disconnected just the upstream O2 sensor, and (if I remember correctly), a DTC was set on the second drive cycle ... nothing in the first drive cycle.
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Unread 07-18-2013, 03:40 AM   #15
hardbodyjeep
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I guess here in a few hours ill go back over the lines again and listen to the injectors and ill let you know what I find
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