Poor acceleration/stuttering around 3500 RPM - JeepForum.com
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post #1 of 18 Old 06-26-2019, 09:27 PM Thread Starter
EnterNerdyName
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Poor acceleration/stuttering around 3500 RPM

Hey everyone,

My 1999 2.5L auto TJ is exhibiting poor acceleration at approximately 40mph/3500 RPM. Seems like it is stuttering when I floor it and has little blips at idle, I can't think of how to explain the sound of either. I can't get past 40 mph. It was occasionally stalling at idle (mostly red lights) until I changed the MAP/IAC/TPS/PCV. Idle is a lot smoother since changing those, but not 100%.
I am at 127,00 miles and had the following rebuilt: transmission at about 115,000, engine and alternator just after 120,000, and transfer case around 125,000 miles. Oh, I have half a tank of good 93 in her.
The only code I'm throwing is P0138 downstream O2 sensor, high voltage. No codes pending.

Since this issue arose, I've replaced:
  • Catalytic converter (Flowmaster 222 series, exhaust smells clean)
  • Upstream O2 sensor (NTK 23506) (Plan to do downstream once she runs right)
  • Fuel pump
  • TPS
  • MAP
  • IAC
  • PCV & grommet
  • Spark plugs (Champion Copper+, wires tested good using a spark checker)
  • Intake air filter
  • Battery
  • Unrelated, but steering gear and thermostat.
Have read countless posts and done many searches thus far. I'm leaning towards injectors, but saw a post from Jerry (sometime back) stating he hadn't seen a TJ issue solved by replacing injectors.

Any ideas on where to search next? Thanks!


Last edited by EnterNerdyName; 06-26-2019 at 09:29 PM. Reason: Remove unintended white space
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post #2 of 18 Old 06-27-2019, 10:48 AM Thread Starter
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Ordered a fuel pressure regulator. Will update once its installed.
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post #3 of 18 Old 06-27-2019, 11:11 AM
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Quote:
Originally Posted by EnterNerdyName View Post
Ordered a fuel pressure regulator. Will update once its installed.
Did you test your fuel pressure with a gauge and determine it was low?

2000 TJ SE
2005 LJ Rubicon
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post #4 of 18 Old 06-27-2019, 11:22 AM
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Quote:
Originally Posted by EnterNerdyName View Post
Hey everyone,

My 1999 2.5L auto TJ is exhibiting poor acceleration at approximately 40mph/3500 RPM. Seems like it is stuttering when I floor it and has little blips at idle, I can't think of how to explain the sound of either. I can't get past 40 mph. It was occasionally stalling at idle (mostly red lights) until I changed the MAP/IAC/TPS/PCV. Idle is a lot smoother since changing those, but not 100%.
I am at 127,00 miles and had the following rebuilt: transmission at about 115,000, engine and alternator just after 120,000, and transfer case around 125,000 miles. Oh, I have half a tank of good 93 in her.
The only code I'm throwing is P0138 downstream O2 sensor, high voltage. No codes pending.

Since this issue arose, I've replaced:
  • Catalytic converter (Flowmaster 222 series, exhaust smells clean)
  • Upstream O2 sensor (NTK 23506) (Plan to do downstream once she runs right)
  • Fuel pump
  • TPS
  • MAP
  • IAC
  • PCV & grommet
  • Spark plugs (Champion Copper+, wires tested good using a spark checker)
  • Intake air filter
  • Battery
  • Unrelated, but steering gear and thermostat.
Have read countless posts and done many searches thus far. I'm leaning towards injectors, but saw a post from Jerry (sometime back) stating he hadn't seen a TJ issue solved by replacing injectors.

Any ideas on where to search next? Thanks!
Why was the engine rebuilt at 120k miles? What work was done during the rebuild?
Has the lack of power and blips at idle been a problem ever since the rebuild?
Have you performed a compression check on all four cylinders?
Have you checked the condition of your ignition cap and rotor?
Have you done a resistance check of your coil and verified you're getting full battery voltage to it?
What are your spark plugs gapped to and do they appear like you're getting the correct mixture?
Have you connected a vacuum gauge to check for abnormal readings or evidence of a restricted exhaust?

Please fill out your vehicle profile so we can know what transmission it has, axle gearing, tire size, etc.

2000 TJ SE
2005 LJ Rubicon
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post #5 of 18 Old 06-28-2019, 12:41 AM Thread Starter
EnterNerdyName
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Thanks for the questions, mukluk.


Quote:
Originally Posted by mukluk View Post
Did you test your fuel pressure with a gauge and determine it was low?
Having difficult attaching my gauge to the rail without it gushing fuel everywhere. Need to clean the threads up a little.



Quote:
Originally Posted by mukluk View Post
Why was the engine rebuilt at 120k miles? It was knocking pretty hard then wouldn't turn over, rebuilt in 2011. (Daily driver 2011-2014, 2014-2017 weekends, 2018 daily.)

What work was done during the rebuild? The specifics I'm not sure of. Think they were originally boring out a cylinder, but they sent a rod through the block which was then replaced with a new block.
Has the lack of power and blips at idle been a problem ever since the rebuild? No, they started last summer.
Have you performed a compression check on all four cylinders? Not yet. Was dreading this one.
Have you checked the condition of your ignition cap and rotor? Not yet, these were replaced around the time of the rebuild.
Have you done a resistance check of your coil and verified you're getting full battery voltage to it? Hadn't thought of this.
What are your spark plugs gapped to and do they appear like you're getting the correct mixture? 0.035 with normal wear on all 4.
Have you connected a vacuum gauge to check for abnormal readings or evidence of a restricted exhaust? Not yet. Will do this one tomorrow.

Please fill out your vehicle profile so we can know what transmission it has, axle gearing, tire size, etc.
Will do, here it is for now: 30rh, axle ratio 3.73, running 30" tires
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post #6 of 18 Old 06-29-2019, 11:57 AM Thread Starter
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Fuel pressure at the rail is 50 psi. Holds at around 10 psi when I turn it off.
Vacuum pressure is around 43 at idle, drops to 5 at load. I'll obstruct it tomorrow to see if it changes anything.
Havn't checked the coil resistance yet, will do tomorrow.
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post #7 of 18 Old 06-29-2019, 01:09 PM
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Fuel pressure shouldn't drop below 30psi in less than five minutes after shutdown -- is yours dropping to 10psi in this time frame?

I do hope your vacuum readings are in mmHg and not inches. Does the vacuum reading come back up if you hold the revs steady at 3000-3500 or does it decrease down near zero as revs increase and stay there?

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post #8 of 18 Old 06-29-2019, 04:22 PM Thread Starter
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Quote:
Originally Posted by mukluk View Post
Fuel pressure shouldn't drop below 30psi in less than five minutes after shutdown -- is yours dropping to 10psi in this time frame?
Almost immediately.

Quote:
Originally Posted by mukluk View Post
I do hope your vacuum readings are in mmHg and not inches. Does the vacuum reading come back up if you hold the revs steady at 3000-3500 or does it decrease down near zero as revs increase and stay there?
Sorry for the lack of units there, I gave cmHg. Using this HF gauge.
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post #9 of 18 Old 07-04-2019, 03:25 PM Thread Starter
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So just got finished swapping out the fuel regulator. It was clear there was no membrane left. She now hold 60 psi at idle and keeps 50 psi when I shut it down.


Went for a drive and now the Elmer Fudd stuttering starts at 2500rpm-25/30mph instead of 3500rpm/40mph.. Listened to the injectors; don't have a trained ear but they each sound a tad different with one being way off. Thinking injectors are due for a swap to 4 hole Boschs.



I'll be checking the cap voltage here soon.
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post #10 of 18 Old 07-11-2019, 12:24 PM Thread Starter
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Swapped the injectors, cap & rotor, spark plugs and wires, and now its running much rougher at idle. I can tell the injector swap was worth it as driving is a little more zippy. Ordering a compression gauge now. I made a video of the sounds shes making at idle, but couldn't upload the video directly so here is a link to it:
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post #11 of 18 Old 07-11-2019, 03:36 PM
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Where was the camera sitting? Have you checked for exhaust leaks?

Did you ever get around to performing this check:
Quote:
Does the vacuum reading come back up if you hold the revs steady at 3000-3500 or does it decrease down near zero as revs increase and stay there?

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2005 LJ Rubicon
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post #12 of 18 Old 07-11-2019, 07:35 PM Thread Starter
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Camera was just behind front passenger tire, under the body about 6 inches.

Found an exhaust leak just above the muffler inlet, 3 maybe 4 small streams. The rest of the pipe seems fine.

So before this recent round of work (injectors, cap, rotor, plugs, wires) vacuum pressure would stay around zero under load then bounce back at idle; probably just forgot to respond with the findings. As of 5 minutes ago, vacuum pressure doesn't go below 25cmHg and approaches 60cmHg around 3000rpms.

Last edited by EnterNerdyName; 07-11-2019 at 07:36 PM. Reason: remove unintended white space
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post #13 of 18 Old 07-12-2019, 10:06 AM Thread Starter
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Chopped off the muffler then rechecked vacuum in American units this time. At idle (800rpm) getting about 17inHg, increasing to 2k rpm goes up to 20inHg. It was difficult to keep it around the 3k range but it dropped to 6inHg and bounced back when I let off the pedal.


Took it for a short drive and sure enough Porky Pig is still in the engine bay. I can at least go 45mph now after passing a rough spot of stuttering at 2800rpm/30mph. I didn't take it on any roads where I could go faster.


Thanks for all the help so far mukluk.
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post #14 of 18 Old 07-12-2019, 11:20 AM
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Your vacuum readings are good, and you've already replaced a majority of the sensors and parts that may explain the issue.

Do you have your spark leads arranged on the cap matching the diagram below?
Did you ever get around to checking the coil resistance and verifying you're getting full battery voltage to it?
Try cleaning and tightening both ends of both battery cables, and verify you're getting full battery voltage to the PDC and PCM.
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post #15 of 18 Old 07-12-2019, 12:33 PM Thread Starter
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Quote:
Originally Posted by mukluk View Post
Do you have your spark leads arranged on the cap matching the diagram below?
Did you ever get around to checking the coil resistance and verifying you're getting full battery voltage to it?
Try cleaning and tightening both ends of both battery cables, and verify you're getting full battery voltage to the PDC and PCM.
Spark plug leads match the diagram.
I didn't get to check the coil resistance, couldn't find a resource that explained how to do it well. I'll keep looking.
I'll check the battery connections & PDC, PMC voltage tomorrow when I do the compression test.
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acceleration stutter , confused , help needed , rough idle , stalling at stop

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