When off the gas, not decelerating as normal

[Lasal]

Recently cleaned throttle body, MAF sensor, changed oil.

Now, when I let off the gas it will sometimes hang at a given RPM and maintain speed. Can happen in any gear, high or low range. 4SPD auto. 4RLE Just light brakes, then it decelerates. But it's definitely getting a little fuel when it hangs as it will maintain even on light grades. Steeper uphills and problem won't occur.

It idles perfectly at 600 rpm, no idle issues or surging at all, warms up normally no issues. Throttle cable fine. Cruise control fine. Throttle position sensor is fine, as I watch it on the OBD2 screen. Idle control valve works fine, *unless* it's only misbehaving when I let off the gas. Evap purge solenoid has no vacuum leaks. Vacuum line to throttle body has no leaks.

All readiness sensors read ready, no faults but when I do a vehicle test with TORQUE PRO there is one "NOT OK" MID: $3D TID:$84 EVAP monitor (0.020") Min 0.061% Max: 100.006% Current 0%

I think this might be related to the EVAP purge solenoid, but I can't find any info. I put a vacuum on the upper inlet that goes to the throttle body, and it holds fine, so it's not stuck open.

I've been maintaining my 06 Liberty since new, 115k now. (Getting ready for an off road trip around Crested Butte for the fourteenth year in a row with this Jeep, OME suspension and skid plates 275 AT Revo 3s)

Thanks for any help!

 

[Lasal]

What would you guys do to diagnose?

 

[LibertyTC]

Did you clean the IAC? IAC controls the idle, it and the port can get quite carboned up.
You can remove the IAC for cleaning and see if that helps, if not replace the IAC, with a mopar one.
Also Check to make sure that the throttle body butterfly is opening & closing smoothly without any binding.
I've had to lightly oil the throttle plate on sides, to get an even, completely free moving plate.
https://www.jeepkj.com/threads/idle-air-control-no-screws.44826/

 

[Lasal]

Yes, I did clean the IAC with MAF cleaner, instead of throttle body cleaner since it has plastic parts, and I cleaned the port it goes into on the throttle body. It warms up normally and drops to 600RPM, so I'm thinking the IAC is ok, but I'll test further. I'll re-check the throttle body plate too. Thanks!

(My original post says 275 tires, they are actually 245/75 15s. the largest size that fitsd with the OME suspension.)

 

[Lasal]

Update:

I tested the IAC by letting the engine warm up, shutting it down, unplugging the IAC and restarting and test driving. No hanging revs, and idle is about 530, not 600. So, I replaced the IAC with a new one, idle returned to 600, and the hanging revs on deceleration also returned! So, the ECU is calling for the idle air control valve to open somewhat.

Speculation: there is an EVAP issue that is not creating a fault, but the ECU is opening the IAC when the throttle closes for a longer period of time than is needed. Perhaps the MAF sensor is reading low after cleaning it? Anyway, the old Liberty will get us to Crested Butte the back way this weekend!

 

[Squamish_Steffen]

I'm having a very similar issue, but not initiated by cleaning anything. To be clear, was your Liberty not doing this rev-hang prior to cleaning your throttle body + IAC + MAF ?

Mine is showing crazy bad rev-hang. Warming up makes no difference. I'm not sure if its especially bad in 1st and 2nd gear, or just more noticeable. Probably just more noticeable. If I accelerate to 3000rpm in 1st gear, its a solid 3 seconds before I can get the gearbox into 2nd gear. Its annoying but not a real problem if driving slowly, but can almost be dangerous in some cases when cars behind me have to hit the brakes because they expect a more linear acceleration in traffic. I should add that I have a very slightly rough idle.

My next move was to clean the throttle body and parts just like you have. My suspicion is that either the butterfly valve is sticking, or that the cable is sticking on return, or that the return spring on the cable is tired.

Edit: Another thought: we know that ECU's purposefully slow the air-intake when you let off the gas for environmental reasons (causing the modern rev-hang). Is it possible that the IAC plays a similar role in mech throttle body vehicles?

 

[Lasal]

I'm having a very similar issue, but not initiated by cleaning anything. To be clear, was your Liberty not doing this rev-hang prior to cleaning your throttle body + IAC + MAF ?

Mine is showing crazy bad rev-hang. Warming up makes no difference. I'm not sure if its especially bad in 1st and 2nd gear, or just more noticeable. Probably just more noticeable. If I accelerate to 3000rpm in 1st gear, its a solid 3 seconds before I can get the gearbox into 2nd gear. Its annoying but not a real problem if driving slowly, but can almost be dangerous in some cases when cars behind me have to hit the brakes because they expect a more linear acceleration in traffic. I should add that I have a very slightly rough idle.

My next move was to clean the throttle body and parts just like you have. My suspicion is that either the butterfly valve is sticking, or that the cable is sticking on return, or that the return spring on the cable is tired.

Edit: Another thought: we know that ECU's purposefully slow the air-intake when you let off the gas for environmental reasons (causing the modern rev-hang). Is it possible that the IAC plays a similar role in mech throttle body vehicles?

No, my Liberty was not rev hanging before cleaning, but I actually didn't notice it until right after the last oil change, which was about a week after the TB cleaning. I don't use this Jeep much, and I'm not sure what precipitated the hanging. I just took it on a 500 mile trip with about 3-4 hours off road in low gears, and the problem has gotten slightly worse, although I don't think the off roading is related.

Yes, the IAC does a similar thing in mechanical TB's. To test, I unplugged the IAC valve and there is absolutely no rev hanging. In fact, the engine is so smooth it idles perfectly at 520 rpm with the IAC unplugged and therefore closed. So, the PCM is keeping the IAC open when decelerating to the point that it is like cruise control.

I just plugged in the OBD2 and all the readiness monitors are green, no fault codes present. However, I use Torque Pro and when I look at the test results, one EVAP Monitor says "not ok" (I'll post the screenshot.) Funny that it's not the EVAP Monitor that was "not ok" last time I checked. That one now is in the green, reading "ok". I also looked at the O2 sensors in real time, and they look like they are reading fine. I looked for a vacuum leak, and didn't find one, however I don't have a smoke tester. I might just replace the purge valve, and I'm going to check the canister by the fuel tank.

I'd like to fix this, the rev hanging makes mountain driving a pain--where you want some engine braking on a regular basis.

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