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Miss at idle

PavementPrincess

NAXJA Forum User
Location
Oregon
I’ve got a 2001 cherokee 4.0 I’m having idle issues. I have searched and searched but haven’t 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. Doesn’t run hot, right at 210 regardless of temp. Idle doesn’t change based on temp either. Idles the same when it’s cold or hot. Plugs done less than 1k miles ago. Was gonna pull em and read em. Like I said it’s fine all the time only at idle. I’ve got a scanner I can read live data but to be honest I don’t know how to interpret all of the readings from the 02 sensors. Where would you guys start if this was your rig? I’m gonna test each emission sensor according to online procedures and hope I get lucky. I’ve read about fuel pressure issues but I can’t figure out why it would only have issues at idle. I’ll 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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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.
 
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.
 
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.
 
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.
 
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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Update on this and still need help. I’ve 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. I’ve 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. I’m running out of ideas.


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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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Still need to run some injector cleaner, the more mileage the higher the likelihood they don't perform equally!
 
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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It’s been a babied xj. One owner old lady who drove it daily and parked in a garage. They took good care of it that’s why I’m not too concerned the miss is something serious. I’ll keep on inspecting :(


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Still have no clue what’s going on. When it’s a cold start it really seems to miss bad until about 20 seconds then it smooths out a bit. That’s the only change I’ve noted. I’m going to change injectors just because they are 20 years old. After that I need to quit just throwing parts at it. I’m stumped at this point. Any jeep gods in the Pacific Northwest(Portland), wanna give me a few pointers? I’ll gladly supply beer,whiskey, and food!!! Or he’ll at this point anyone have a shop near Portland they trust. I don’t have the tools to diagnose anymore than I have already.


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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/showthread.php?t=1012701&highlight=oxygen+sensor+test

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.

Still have no clue what’s going on. When it’s a cold start it really seems to miss bad until about 20 seconds then it smooths out a bit. That’s the only change I’ve noted. I’m going to change injectors just because they are 20 years old. After that I need to quit just throwing parts at it. I’m stumped at this point. Any jeep gods in the Pacific Northwest(Portland), wanna give me a few pointers? I’ll gladly supply beer,whiskey, and food!!! Or he’ll at this point anyone have a shop near Portland they trust. I don’t have the tools to diagnose anymore than I have already.


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Are you sure it is a miss or could it be a bad motor mount? Vibes are the worst at very low rpms.
 
Update on this: I still drive daily and still have the same issues. I will be swapping out injectors this week and I’ll check back in. I’ve checked for vacuum and it’s in spec. Still haven’t checked other sensors. Also plan on doing motor mounts as I consider maintenance items. The idle clears up with just a little increase in rpms. Like still under 1k. Still kinda stumped, if injectors do nothing idk what else to check


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Instead of swapping injectors, run a can of BG44K through the fuel first.
 
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