2005 LJ throttle hesitation / stall when warm

longheep

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Hey everyone,

I have an interesting issue that I'd love some feedback on. I have a 2005 LJ, non-rubicon, 195,000 miles, with the 42RLE automatic transmission. I'm running into an interesting issue where the engine will have a very small moment of almost stalling when hitting the throttle at operating temperature. I've attached a video of what I mean - you can hear the revs drop right as I hit throttle and then climb back up from there. I also have intermittent issues with stalling when in reverse. In trying to solve this issue I have:

  • Replaced spark plugs (NGK as recommended by the forum)
  • Replaced timing belt
  • Replaced OPDA (original replaced with Crown unit but kept OEM CPS)
  • Full fluid service (oil, coolant, transmission, brakes)
I am not showing any codes, no CEL. I have checked for vacuum leaks and there do not appear to be any. I have also tried disconnecting the upstream O2 sensors while the engine is running to see if those could be the issue, to no avail.

Any help or pointers from the community would be greatly appreciated! Video below.

 
Bumping this for any potential insights out there. Though I've tried unplugging the upstream O2 sensors once hot to see if that helped, I'll be replacing both as well as the Crankshaft Position Sensor to see if the problem is fixed. IAC has been cleaned recently but if new O2 sensors and CkPS don't fix the issue I'll try it again. If none of that works I may try WranglerFix for a new PCM.
 
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Well, both upstream O2 sensors have been replaced as well as the crankshaft position sensor (replaced with MOPAR OEM), and unfortunately there has been no change with regards to this issue. The engine is running as good as it ever has while in park, but driving around town still gives me trouble. I'm kind of at a loss as to what the issue could be here, and many searches of both this forum and the internet at large have yielded no results. The only similar issue I have read about someone else having a similar issue also ended up unresolved (I've lost track of the specific thread). My guesses at this point are kind of shots in the dark:

1) PCM failure - I am having some intermittent transmission issues in addition to the stalling and surging issues, though the symptoms do not directly align with a bad PCM.

2) Issue related to OPDA replacement and timing - when I replaced my OPDA, I had a bit of a mental lapse and installed the new OPDA without the pin in it and tried to start the Jeep. I realized my mistake after the Jeep would not start and had to re-time the engine using the top-dead-center mark on the front of the crankshaft pulley. I also took out plug #1 to confirm that I had found top-dead-center on the compression stroke.

I have also checked all electrical grounding points and much of the wiring harness to make sure there are no loose connections or bare wires/ shorts.

Any input from the forum on what could be happening is appreciated, I've been chasing this for a while now and am pretty well at my wit's end.
 
Sorry to hear you're dealing with this too! Honestly the hesitation on throttle is annoying, but I'm able to drive around it for the most part. I'd say the most dangerous part is the stalling in reverse as well as the RPM bounce that can happen when trying to feather the throttle - giving a little bit of gas at slow speeds and low RPM can cause a dip in revs, then a quick bounce up in revs and a surge forward if you're not on the brakes.

Taking another jab at this issue, this morning I gave the PCM a reset following the procedure outlined here on the forum. With about 10 miles on the clock since, my transmission appears to be shifting well and I have not noticed the reverse gear stalling issue. I also grabbed an OBDII scanner to monitor for any trouble codes that could come up as the PCM re-learns. The above issue of an RPM bounce/ jump did occur even after the PCM reset.

One more guess I have is that this issue could be related to the 33in tires I have mounted and the general state of a Jeep with so many miles on the clock. Maybe the increased tire size on a 3.73 axle gear ratio is presenting a torque challenge or introducing more driveline slack?
 
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I'd say the most dangerous part is the stalling in reverse as well as the RPM bounce that can happen when trying to feather the throttle - giving a little bit of gas at slow speeds and low RPM can cause a dip in revs, then a quick bounce up in revs and a surge forward if you're not on the brakes.

I had some of this going on (stalling and surging), but most of it cleared up after I cleaned the IAC.
I'm on 33s, but with 4.10s.
 
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You may still have a sticking IAC or a bad IAC. Did you take it out to clean it?

Check the MAP sensor reading with the ignition on, but the engine off. Compare that to the absolute atmospheric pressure at your location to see if the MAP is reading correctly. You can't use the weather station reading without correcting for altitude. You can download a barometer app to your phone that will read pressue.

My 06 had a MAP that was not reading correctly and causing a lean condition. It would sort of hesitate and then surge under acceleration.
 
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I had some of this going on (stalling and surging), but most of it cleared up after I cleaned the IAC.
You may still have a sticking IAC or a bad IAC. Did you take it out to clean it?
Thank you both for your feedback on the IAC - I had cleaned it when this issue first came up about 18 months ago, both using the method Jerry describes and removing the IAC to clean it with carb cleaner and a rag. That said, it is possible I missed some nooks and crannies, or just as well the sensor itself could be bad. I'll give it a really good clean once more and see where I get!

Check the MAP sensor reading with the ignition on, but the engine off.
That's a great idea, thank you. Do you know if MAP sensor data is viewable on an OBDII scanner?
 
Gotcha, thanks! I'm going to get some more throttle body cleaner and give one more go at cleaning the IAC. Here's hoping I just did a bad job last time 🙃
 
Well, after putting the better part of half a can of throttle body cleaner through the IAC, nothing much has changed. However, I was able to play around a little bit more with bringing on the surging/bouncing idle and stall, and was able to record a video of the RPM surge leading to a stall. Video attached.



This bouncing idle is brought on by giving small, quick throttle inputs while in gear - it does not happen while in park or neutral. This condition is very unstable and the engine will stall after about 5 bounces unless I give a more sustained throttle input or shift out of gear. Of note as well is I have both the e-brake on as well as basically standing on the brake pedal as the last time this happened I only had the e-brake on and the RPM surge is enough to overcome the e-brake. I also had my scanner hooked up for a few of these events and was able to see that the TPS indicates no change (indicating ~13% throttle) while this occurs, MAP shows fluctuations between 30-75 kPa, and RPM bounces between 400-1700 before stalling. I've also checked and this same situation can be brought on with the AC blowing.

Back to square one for the nth time, starting to feel like I may just be hooped here. The forum's input is appreciated as always!
 
An update for anyone following along:

Mark and his team at WranglerFix are everything they're cracked up to be, thank you Mark for the communication! Though we weren't able to conclusively nail these symptoms down as being PCM-related, I ordered a new PCM and installed it yesterday. I've since put on about 20 miles running around town. The great news is the Jeep runs better than ever in most respects - smoother idle, smoother shifts, and better overall transmission performance than before. I also, to the extent that I've been trying to reproduce it, no longer encounter the noticeable hesitation on throttle at stoplights and stop signs.

Though I still consider this a major win and certainly worth the expense, the 4.0 still bounces revs as before when giving small throttle inputs. In attempting to verify this, I did notice that it is harder to bring about this stall condition and the Jeep appears to recover itself faster and smoother than before. Again, this is mostly good news, but I'm definitely left scratching my head as to what could be left to cause this issue. It is still quite noticeable driving around in parking lots and driveways. My thoughts are:

1) Hidden vacuum leak. I may need to acquire a smoke tester to double check the vacuum system and verify this is indeed not the issue. Could also be a leak related to the vacuum side of the evap system/ NVLD.
2) Something transmission-related? Considering this really only happens in gear I'm thinking it could be a possibility. I have a tranny rebuild on the long-term agenda.
3) Faulty IAC.
4) An issue with fuel delivery. While inspecting my fuel delivery line I did see a small kink in the hard line, though I'm not sure if this is an intentional bend. Maybe fuel delivery is impaired and causing the engine to run lean/ harder to keep up with demand?

As always, any input from the forum is appreciated.
 
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One other thing to check is the TPS, you can watch it's operation on a scanner or you can use an ohmmeter to watch the resistance. The readings should smoothly change, any jumping around and the TPS is shot.