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Thread: Sporadic Engine Revs

  1. #1
    Join Date
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    Default Sporadic Engine Revs

    OK, guys. i need some ideas here. I have an issue with my Jeep the started yesterday. It is a 2006 TJ with the 4.0 inline 6 and a six speed manual transmission.

    Here are the symptoms.

    When sitting at an idle you can hear the RPM's rev up maybe just a couple hundred RPM's. It will sit there for about 5 seconds then come back down. It does this at what seems to be regular intervals.
    When driving, say in 4th gear at 1800 RPM's you can actually feel the Jeep pick up speed before dropping back down, again it seems to last for about 5 seconds at regular intervals.
    Finally, when coming to a stop and disengaging the clutch to take it out of gear it will rev up to about 2000 rpm or so before coming back down to idle and again it seems to be tied into those intervals because it doesn't do it every time.

    There is no check engine light on and the mechanic i usually use says they can't do anything without a check engine light. I really can't afford to bring it to a dealer to look at it so i am hoping someone here has some idea.
    -You should spend 10 minutes in a hammock everyday , unless you are busy, then you should spend an hour.

    2006 Jeep Wrangler X (TJ) - Stock.... except the 31" tires... just stock...
    ...oh no, wait, I bought a new top for it and also put up a sticky led light on the interior roof as a dome light and i put my first sticker on the body.

  2. #2
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    Default

    It seems like that might be a vacuum issue. I've seen the opposite issue (revs drop, about the same room, about the same duration) come about due to a small clog in the vacuum line coming to the throttle body. Popping off the lines and giving them a good clean can't really hurt anything, as long as the lines don't rip.

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    1997 Jeep Wrangler TJ "Barbara Ann": 4.0, 5spd, 4.10s, soft top, mostly stock. 31X10.5R15. Back to being a DD. What could go wrong?

    2013 Lincoln MKS Ecoboost "Lola": All stock, DD. Spun main bearing. R.I.P.

  3. #3

    Default

    That sounds pretty consistent, I suspect it's one of your sensors gone bad.

    I'd go to RockAuto order all of them and then replace them until you figure out which one it is, return the rest.

    It's probably a throttle position sensor, mass air flow sensor, or something else that controls the fuel or the air.

    Sounds to me like it's consistently adding too much of one which is going to rev up the motor.

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  4. #4
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    Rubicon is offline Ex-Coordinator & Ex-Mod Certified Trail Leader
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    Exclamation

    It does read like a vacuum leak or a TPS, though not steady enough. So start with the cheapest thing that it will need anyways, by cleaning the throttle body itself(the butterfly valve area) which allows the IAC to work properly again(probably the culprit). Actual throttle body cleaner works the best.

    Oh and it's not a MAF, as TJ's do not have them.

    Don't just "throw" money at it by buying sensors! At least test them first and they should trigger a code anyways. Maybe have the OBD II read anyways for possible saved pending codes ;)
    Traction, articulation, and gearing make a good off-road rig. Just a big lift, wide mud tires, and a winch simply do not.
    ~Shawn(the correct spelling ;)

  5. #5
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    Default

    Thanks, so I did clean the throttle body (I bought throttle body cleaner but it was in the car which was not ad home at the time so i used PB blaster which seemed to do a fantastic job). It was pretty clean on the top and underneath was kind of brownish, not the black like I saw in the videos I watched. I have a code reader but maybe i will bring it to O'Reilly's and ask them to check. They probably have a better one than I do.

    One other suggestion I had seen online was to unplug the battery for 8 hours to see if that would reset anything in the system. Is there anything to that or is that just crazy talk?

    I also changed the IAC since I had everything apart and it seemed like a a fair chance that was the culprit.

    That seemed to fix it.... for two days, now it is happening again. I would like to check for vacuum leaks but I have no idea where all the different vacuum lines are or really how to effectively test them at all.

    Maybe I should change the TPS next?

    Quote Originally Posted by Rubicon View Post
    It does read like a vacuum leak or a TPS, though not steady enough. So start with the cheapest thing that it will need anyways, by cleaning the throttle body itself(the butterfly valve area) which allows the IAC to work properly again(probably the culprit). Actual throttle body cleaner works the best.

    Oh and it's not a MAF, as TJ's do not have them.

    Don't just "throw" money at it by buying sensors! At least test them first and they should trigger a code anyways. Maybe have the OBD II read anyways for possible saved pending codes ;)
    Last edited by Dgagne; 03-11-2020 at 10:19 AM.
    -You should spend 10 minutes in a hammock everyday , unless you are busy, then you should spend an hour.

    2006 Jeep Wrangler X (TJ) - Stock.... except the 31" tires... just stock...
    ...oh no, wait, I bought a new top for it and also put up a sticky led light on the interior roof as a dome light and i put my first sticker on the body.

  6. #6
    Rubicon's Avatar
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    Quote Originally Posted by Dgagne View Post
    Thanks, so I did clean the throttle body (I bought throttle body cleaner but it was in the car which was not ad home at the time so i used PB blaster which seemed to do a fantastic job). It was pretty clean on the top and underneath was kind of brownish, not the black like I saw in the videos I watched. I have a code reader but maybe i will bring it to O'Reilly's and ask them to check. They probably have a better one than I do.

    One other suggestion I had seen online was to unplug the battery for 8 hours to see if that would reset anything in the system. Is there anything to that or is that just crazy talk?

    I also changed the IAC since I had everything apart and it seemed like a a fair chance that was the culprit.

    That seemed to fix it.... for two days, now it is happening again. I would like to check for vacuum leaks but I have no idea where all the different vacuum lines are or really how to effectively test them at all.

    Maybe I should change the TPS next?
    Defective IAC?

    It is more difficult to test the TPS, so if you can't do that, try to adjust it to see if that helps.

    Ether/starting fluid works great at finding vacuum leaks ;)

    The battery trick is old, but it is indeed a way to drain all the circuits and capacitors. However it will probably just end up re-learning again and put you back in the same place a couple of days later(kind of like what happened when you changed the IAC). So why not try a method that Bob posted a while ago: TJ-PCM-Adaptive-Memory-flash
    Traction, articulation, and gearing make a good off-road rig. Just a big lift, wide mud tires, and a winch simply do not.
    ~Shawn(the correct spelling ;)

  7. #7
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    Default

    2nd Shawn's suggestion of vacuum leak. You can spray around the lines carefully with a little starting fluid and see if it revs up higher.
    '99 TJ, 5.9L Magnum V8 swap, Atlas-4, Waggy 44s, Custom link susp F/R, 37" Pitbulls, etc etc etc, OMG what happened to all of my money?
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  8. #8

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    Did you diagnose and fix?

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  9. #9
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    Default

    Waiting for an answer too...my ford 2.9 is doing the same thing. I went around with starter fluid and couldn't find anything. The hoses are all 30 years old so I may just replace all the vacuum hoses anyhow.
    "I hope you get hit by a goose, right in the sway bar!" -anon

    2015 Tacoma Access 4-banger (aka 'Scratchy') -> 285/75r16 Grabber ATX, FJC wheels, 3" OME 887s/full dakar pack, CBI sliders and full skids, Pelfreybilt Hybrid bumper, ****tybilt 9.5 XRC winch, sway bar go by bye, ARB on-board air, many many zip-tie
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  10. #10
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    Default

    my 2 cents ,like was said but i would change all vacuum lines, tighten up all bolts around the throttle body and run major brand high test gas. my TJ always runs better with high test

    Good Luck ,let us know how you make out
    Scott / teamjeepcrawl / Xoverland

    1997 Jeep wrangler TJ ,4 inch rough county x- series lift. sitting on 33 inch BFG KM 2s ,tom woods drive shafts with SYE ,chrome molly axles ,smittybilt bumpers,rock-rails ,winch,skid plates ,ARB snorkel,CB radio , full roll cage built by Stinkys off road , along with a few outer moderate up grades and add ons. with a bunch of gear and tools just in case ..M101 Trailer with a roof top tent for those overland adventures.

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  11. #11
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    Default Wiggle this

    This is a symptom often seen when the TPS goes bad. However, it will not set a code nor show up in a datastream, unless you are testing for it. So, with the engine running, grasp the TPS and gently (GENTLY!!) twist it so that any abnormal play will allow it to flex. Don't twist the thing off the engine, be nice to it. If the engine changes RPM's, then you found it. If the engine remains stable, then do the same with the connector. Then, if the engine remains stable, gently twist and wiggle all connections of the harness under the hood and under the dash. make sure your A/C is off, as that will cause an increase in RPM's when it kicks on. This wiggle test should also be done with the vacume lines, both hard and soft. Don't forget to access the canister as there is a vacume line going there also. Please repost with results.
    1996 land Rover Discovery, 32" tires, 2" lift, running boards/Rock sliders, Blue LED dome lights and a custom cup holder. (project in progress)
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  12. #12
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    crank sensor
    Scott / teamjeepcrawl / Xoverland

    1997 Jeep wrangler TJ ,4 inch rough county x- series lift. sitting on 33 inch BFG KM 2s ,tom woods drive shafts with SYE ,chrome molly axles ,smittybilt bumpers,rock-rails ,winch,skid plates ,ARB snorkel,CB radio , full roll cage built by Stinkys off road , along with a few outer moderate up grades and add ons. with a bunch of gear and tools just in case ..M101 Trailer with a roof top tent for those overland adventures.

    Member of : Tread Lightly

    Please Leave no trace and try to carry out more then you carry in

  13. #13
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    Quote Originally Posted by jeep crawl View Post
    crank sensor
    My experience with those, is that they just quit, usually without any warning signs.
    Traction, articulation, and gearing make a good off-road rig. Just a big lift, wide mud tires, and a winch simply do not.
    ~Shawn(the correct spelling ;)

  14. #14
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    thats what i thought ,could not figure out my sons TJ same issues as listed ....was the crank sensor
    Scott / teamjeepcrawl / Xoverland

    1997 Jeep wrangler TJ ,4 inch rough county x- series lift. sitting on 33 inch BFG KM 2s ,tom woods drive shafts with SYE ,chrome molly axles ,smittybilt bumpers,rock-rails ,winch,skid plates ,ARB snorkel,CB radio , full roll cage built by Stinkys off road , along with a few outer moderate up grades and add ons. with a bunch of gear and tools just in case ..M101 Trailer with a roof top tent for those overland adventures.

    Member of : Tread Lightly

    Please Leave no trace and try to carry out more then you carry in

  15. #15
    Rubicon's Avatar
    Rubicon is offline Ex-Coordinator & Ex-Mod Certified Trail Leader
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    Quote Originally Posted by jeep crawl View Post
    thats what i thought ,could not figure out my sons TJ same issues as listed ....was the crank sensor
    It happens. Glad you found the issue Scott
    Traction, articulation, and gearing make a good off-road rig. Just a big lift, wide mud tires, and a winch simply do not.
    ~Shawn(the correct spelling ;)

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