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  #1  
Old October 14th, 2017, 15:29
PavementPrincess PavementPrincess is offline
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Miss at idle

Iíve got a 2001 cherokee 4.0 Iím having idle issues. I have searched and searched but havenít found anything specific enough to help. Motor has 115k miles auto trans. Has new head and head gasket. Only problem is a miss at idle only. Starts fine hot or cold. No check engine light or codes stored. Get consistent 17.5 mpg however I drive it. Doesnít run hot, right at 210 regardless of temp. Idle doesnít change based on temp either. Idles the same when itís cold or hot. Plugs done less than 1k miles ago. Was gonna pull em and read em. Like I said itís fine all the time only at idle. Iíve got a scanner I can read live data but to be honest I donít know how to interpret all of the readings from the 02 sensors. Where would you guys start if this was your rig? Iím gonna test each emission sensor according to online procedures and hope I get lucky. Iíve read about fuel pressure issues but I canít figure out why it would only have issues at idle. Iíll test the pressure and report back. If I do a live data reading on my scanner and post it maybe I can get some feedback. Another thing when u do a live reading do you want a cold reading and operating temp? I feel lost trouble shooting when I have no codes or obvious failures.


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  #2  
Old October 14th, 2017, 19:22
anthrax323 anthrax323 is offline
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Re: Miss at idle

Roughly what is your idle RPM when this is happening? Does applying even the slightest bit of throttle make it go away?

If you're experiencing a low idle, the engine can feel like it's stumbling or misfiring. A sticky or failing idle air controller would be the likely culprit in this case. When mine failed, it flat-out wouldn't idle unless I gave it throttle which made it an easy diagnosis.
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Old October 14th, 2017, 20:48
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Runnin'OnEmpty Runnin'OnEmpty is offline
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Re: Miss at idle

If one of the coils is marginal, that will show up as a possible miss only at idle. Don't ask me why, but it might be because of the longer
coil dwell time when idleing.

You could test them until you found a weak one. Also make sure all the coil connectors are plugged in.
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  #4  
Old October 15th, 2017, 08:37
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Tim_MN Tim_MN is offline
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Re: Miss at idle

Make sure all the tune-up parts are fresh. For a 2000-01 I prefer Autolite APP985 Double Platinum spark plugs, all other years should use basic Champion or NGK plugs. Test the fuel pressure. Test the fuel injectors. Inspect the fuel injector wire plugs for corrosion and the wire harnesses for damage. The coil rail can be tested, but it is not a very likely suspect. Add some fuel injector cleaner to the next 2 or three gas tank fill ups. Clean the Idle Air Controller. Make sure the O2 sensors are functioning correctly. O2 sensor are routine maintenance parts and their service life is about 100-150,000 miles.
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  #5  
Old October 15th, 2017, 18:51
trippled trippled is offline
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Re: Miss at idle

Check engine vacuum for good steady reading and compression test? If this has been since head replacement I'd suspect a valve with poor sealing. If nothing else the above test are cheap and quick to rule out a lot of mechanical problems.
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  #6  
Old October 16th, 2017, 20:25
PavementPrincess PavementPrincess is offline
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Miss at idle

Plan on tune up since vehicle is new to me. Coil pack and plugs are on the list. Do I need a mopar coil pack or can I safely use others. Heard there picky about some


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  #7  
Old November 14th, 2017, 17:36
PavementPrincess PavementPrincess is offline
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Miss at idle

Update on this and still need help. Iíve checked and replaced the plugs and they looked fine. Replaced with champions recommended. Replaced coil rail with new mopar unit. Compression tested near perfect specs according to fsm. I cleaned the throttle body and replaced the iac. Iíve checked for vacuum leaks everywhere. Not sure how to test a o2 sensor or a vacuum test. Gonna check fuel pressure at the rail tonight. Ohmed out injectors and all about 13.6 ohms. Iím running out of ideas.


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  #8  
Old November 14th, 2017, 19:04
PavementPrincess PavementPrincess is offline
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Miss at idle

Checked fuel pressure at the rail and got 48psi and it stayed that way for 5 minutes with car off. Here are my compression test numbers. Cylinder #1-152psi, #2-154psi, #3-160psi, #4-163psi,#5-159psi, #6-164psi


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  #9  
Old November 15th, 2017, 05:42
RCP Phx RCP Phx is offline
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Re: Miss at idle

Still need to run some injector cleaner, the more mileage the higher the likelihood they don't perform equally!
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  #10  
Old November 15th, 2017, 13:13
PavementPrincess PavementPrincess is offline
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Miss at idle

I be run multiple injector cleaners. I may just buy some and have em cleaned or buy new ones. They are 20 years old


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  #11  
Old November 15th, 2017, 18:36
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Green Mesa XJ Green Mesa XJ is offline
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Miss at idle

Good compression on that engine!

It might be worth it to get a new set on injectors if you think it is a problem. On newer vehicles the upstream Oxygen sensor can be the cause because the computer is constantly adjusting timing and fuel trim, not sure how complex the 2001 XJ were.

I had a engine miss at idle then eventual cruising speed on the interstate with a f150, never got a code and it would do it randomly.
Replaced TPS, throttle body, upstream oxygen sensors, spark plugs and coils. Turned out to be one of the injectors, replaced them all since I was in there doing the job. Found out later ford had a defective batch of injectors and it was a common problem for my model F150.

Just a thought, it might not carry over to the XJ.




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  #12  
Old November 15th, 2017, 18:58
PavementPrincess PavementPrincess is offline
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Miss at idle

Itís been a babied xj. One owner old lady who drove it daily and parked in a garage. They took good care of it thatís why Iím not too concerned the miss is something serious. Iíll keep on inspecting


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  #13  
Old November 25th, 2017, 12:31
PavementPrincess PavementPrincess is offline
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Miss at idle

Still have no clue whatís going on. When itís a cold start it really seems to miss bad until about 20 seconds then it smooths out a bit. Thatís the only change Iíve noted. Iím going to change injectors just because they are 20 years old. After that I need to quit just throwing parts at it. Iím stumped at this point. Any jeep gods in the Pacific Northwest(Portland), wanna give me a few pointers? Iíll gladly supply beer,whiskey, and food!!! Or heíll at this point anyone have a shop near Portland they trust. I donít have the tools to diagnose anymore than I have already.


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  #14  
Old November 25th, 2017, 16:33
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Ecomike Ecomike is offline
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Re: Miss at idle

That sounds like the heater inside the O2 sensor is bad, on the pre-Cat forward O2 sensor(s). Yours may have 4 O2 sensors, so it may be a loss of relay power to the heater wire in the O2 sensors, or a bad heater wire in one sensor.

http://www.naxja.org/forum/showthrea...+sensor+t est

Is the bible here on O2 sensor tests.

Yours has 0-1 volt output. 2 ground wires, and the fourth is 12-14 volts (IIRC on that year) power to the heater in the sensor. If the heater is not working the O2 sensor is too cold on start up to work properly and causes a miss...while the exhaust gasses warm it up.

Quote:
Originally Posted by PavementPrincess View Post
Still have no clue whatís going on. When itís a cold start it really seems to miss bad until about 20 seconds then it smooths out a bit. Thatís the only change Iíve noted. Iím going to change injectors just because they are 20 years old. After that I need to quit just throwing parts at it. Iím stumped at this point. Any jeep gods in the Pacific Northwest(Portland), wanna give me a few pointers? Iíll gladly supply beer,whiskey, and food!!! Or heíll at this point anyone have a shop near Portland they trust. I donít have the tools to diagnose anymore than I have already.


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  #15  
Old November 27th, 2017, 00:50
PavementPrincess PavementPrincess is offline
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Miss at idle

Iíll take a look into this, appreciate the response.


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