Weird Low Idle Issue

[4x4 AG]

I am chasing something that has me baffled. So a long time lurker, and user of all the info on here, is signing up to beg for help.

Issue, 2004 Jeep Wrangler LJ 4.0 auto, starts fine. runs fine down the road, good off the line, etc. but when coming to a full resting stop, at a light or stop sign, the engine idle will drop to like 540 rpms. If the AC is on, it does it a little less, but still drops. This causes a rough low idle, shakes the car and mirrors. If I come to a rolling stop, california roll, the idle doesnt drop.

So far I have, checked for vacuum leaks visibly, replaces IAC and TPS with mopar, cleaned the throttle body, new plugs OE ones, pulled the injectors and flushed them with new o rings. I have checked live data on an OBDII tool, and nothing is off. has new cats and all for Mopar new O2 sensors.

HELP.....

 

[Chippy]

I think I have the same issue. Car drives fine otherwise for me. When you blip the throttle to raise the RPM slightly does the engine feel way smoother? My side mirrors are vibrating just sitting there idling but if I very slightly give it some throttle it’s the smoothest idle known to man

 

[Christian.mers]

Bunch of long shots here but how’s your transmission running? Any issues with your Torque Converter or Lockup? If you come to a complete stop and it’s shaking, put it into Neutral and see if your shakes stop

Idle in D and pressed the brake in and out and see if rpm changes at all

 

[4x4 AG]

Bunch of long shots here but how’s your transmission running? Any issues with your Torque Converter or Lockup? If you come to a complete stop and it’s shaking, put it into Neutral and see if your shakes stop

Idle in D and pressed the brake in and out and see if rpm changes at all

I will try both of these. but seems to shift fine.

I have watched the Idle numbers on the OBDII live data, and it mainly drops lower in Drive. But will also drop down in Park, in which, as a kid, park used to idle higher, and Drive was lower.

Used to remember like 800-900 rpm was good for park or nuetral, and like 680-750 for Drive or Reverse. But that is old knowledge from carb and no electronic days. when you used to adjust the idle.

 

[4x4 AG]

I think I have the same issue. Car drives fine otherwise for me. When you blip the throttle to raise the RPM slightly does the engine feel way smoother? My side mirrors are vibrating just sitting there idling but if I very slightly give it some throttle it’s the smoothest idle known to man

same for me, if i slightly set my foot on the accelerator, the idle comes up and it smooths out.

 

[macleanflood]

I'd definitely be checking for vacuum leaks and making sure your intake bolts are properly torqued. Smoke test might help too.

-Mac

 

[Christian.mers]

same for me, if i slightly set my foot on the accelerator, the idle comes up and it smooths out.

Same reason the AC helps. It raises the base idle rpm because there’s load on the engine. This is why I mentioned the Transmission. When there load the TCC activates, when there’s no load it deactivates and idles freely, if it doesn’t deactivate then you can have a low idle like that. This is because the engine is being forced to turn your drivetrain like you’re in gear and not idling putting excess load (excess in comparison to the load it should be seeing at a complete stop) on it and bogging it down

 

[Christian.mers]

I will try both of these. but seems to shift fine.

I have watched the Idle numbers on the OBDII live data, and it mainly drops lower in Drive. But will also drop down in Park, in which, as a kid, park used to idle higher, and Drive was lower.

Used to remember like 800-900 rpm was good for park or nuetral, and like 680-750 for Drive or Reverse. But that is old knowledge from carb and no electronic days. when you used to adjust the idle.

Definetly a thought

The first test should identify if it’s something Transmission related

The second test should help to identify something up with the brake booster, maybe a small internal leak only detectable at full pedal pressure?

 

[4x4 AG]

I'd definitely be checking for vacuum leaks and making sure your intake bolts are properly torqued. Smoke test might help too.

-Mac

I have a smoke machine on order. Will be doing that.

I am planning to smoke test the Evap system and the Intake manifold. Hopefully that helps.

 

[4x4 AG]

Same reason the AC helps. It raises the base idle rpm because there’s load on the engine. This is why I mentioned the Transmission. When there load the TCC activates, when there’s no load it deactivates and idles freely, if it doesn’t deactivate then you can have a low idle like that. This is because the engine is being forced to turn your drivetrain like you’re in gear and not idling putting excess load (excess in comparison to the load it should be seeing at a complete stop) on it and bogging it down

ok. so i better understand what i am testing here. I have to run to town in a bit, so it will get time to warm up and i can check it.

when i am at a stop sign, fully stopped and rested, and the idle drops, i want to slide it into neutral?

If in neutral, i want to see the RPM go up and smooth out? If it doesn't, i have an issue with the transmission?

Sorry, i am only slightly mechanically inclined. Youtube certified only mechanic.

 

[Christian.mers]

ok. so i better understand what i am testing here. I have to run to town in a bit, so it will get time to warm up and i can check it.

when i am at a stop sign, fully stopped and rested, and the idle drops, i want to slide it into neutral?

If in neutral, i want to see the RPM go up and smooth out? If it doesn't, i have an issue with the transmission?

Sorry, i am only slightly mechanically inclined. Youtube certified only mechanic.

Drive it long enough to be fully warmed up then come to a complete stop in Drive (foot on brake). When the idle drops and it starts to shake, shift to Neutral while still stopped and watch what the RPM does.

If it goes up, transmission
If it stays the same, idle control or vacuum

What’s mileage?

 

[4x4 AG]

Drive it long enough to be fully warmed up then come to a complete stop in Drive (foot on brake). When the idle drops and it starts to shake, shift to Neutral while still stopped and watch what the RPM does.

If it goes up, transmission
If it stays the same, idle control or vacuum

What’s mileage?

111,000 and change.

I will report back with findings later today.

 

[Christian.mers]

Also let me know what the long term fuel trims are once the vehicle is warmed up in Park at idle, in drive at idle, and finally in Park but above idle in the1200-1500 RPM range. That could help too.

Ultimately I have already filled out the solenoid just because the lock-up remaining applied could cause that kind of feeling coming to a stop yes, however on this particular transmission it could not be caused by a bad solenoid. The reason is that solenoid has dual function, if the solenoid was not fully close when it should, it would lock-up the wheels on the 1-2 shift.

Like you said, you’re not having driving issues so my first hypothesis was incorrect but still continue forward with the test and also the tests above. Maybe when you return home after your drive

 

[4x4 AG]

Also let me know what the long term fuel trims are once the vehicle is warmed up in Park at idle, in drive at idle, and finally in Park but above idle in the1200-1500 RPM range. That could help too.

Ultimately I have already filled out the solenoid just because the lock-up remaining applied could cause that kind of feeling coming to a stop yes, however on this particular transmission it could not be caused by a bad solenoid. The reason is that solenoid has dual function, if the solenoid was not fully close when it should, it would lock-up the wheels on the 1-2 shift.

Like you said, you’re not having driving issues so my first hypothesis was incorrect but still continue forward with the test and also the tests above. Maybe when you return home after your drive

havent made it to town yet, but i have been watching the Fuel Trims.

When i got home this AM after a good drive in it. both short terms were close to 0. fluctuated slightly negative to slightly over. never more than 1. something either way.

long terms were both in the 7s. i have seen them in the 4-5s, but i think that was cooler and shorter run time. This was close to an hour of driving and in the 7s.

 

[4x4 AG]

 

[4x4 AG]

No real change in the rpm’s when shifting from D to N while at a resting stop. Some change but felt more like pump engaging and disengaging.

Rpm’s fluctuate when I watch it on the OBD screen. Small fluctuations, but 10-15 up and down.

Will get screen shots of fuel trims at higher RPMs on the way home.

 

[4x4 AG]

Here are the Fuel Trims at 1500 or so RPMs.

 

[Christian.mers]

Sounds great, so we’ve got LTFT at 1,500 RPM in P.
What was your LTFT B1/2 in:
Park at idle
Drive at idle (foot on brake)

To determine if it’s a transmission issue I’d need to know all 3 at these exact conditions. For this testing STFT is relatively useless but it may help on the engine side of things (if we do rule out the transmission portion)

 

[4x4 AG]

I will have to get drive at idle another day after driving it some.

Idle in park is the first screen shots. So in the 7s.

Second screenshots shots are 1500ish rpm in park.

 

[GoldenGorilla]

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A vacuum leak would almost certainly result in a fast idle, not a slow one. I think we have to look somewhere else. I think an issue with the torque converter seems plausible. Alternatively and issue with something on the accessory belt. Power steering, alternator, water pump and AC plus the tensioners and pulleys. Pull the belt and see if something is producing more resistance than expected.

I have an 04 also, but with the manual. I've been tuning mine with HP Tuners since March. I have started to get a pretty good understanding of it. One thing I don't understand though is the STFT reported numbers. My STB1 is reported crazy high like your STFTB2. But when you look at the fuel pulse widths and the O2 sensor voltages, everything is totally normal as far as I can tell. No exhaust leak, no misfires, no apparent problems.

There is a reported idle desired rpm number available on the ODBII, but I wouldn't expect it to sit that low without opening the IAC position. I have set my idle at 824rpm because I have some MORE motor mounts and didn't like the vibration they transmitted through the body at the stock setting 672rpm. I do see it oscillate the IAC value around 29-34 when idling normally. I'm not sure if you can monitor that value on your ODBII.