Fluctuating RPM's at constant throttle (throttle hunting, surging, gear hunting)

Discussion in 'LR3' started by Efecta, Aug 13, 2015.

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  1. Efecta

    Efecta Member

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    2005 LR3 HSE 4.4 with 156k miles.

    RPM's rapidly fluctuate up and down even when on a flat road under constant throttle, speed and load. For example, RPM's quickly move from 1,250 to 1,500 at 45 MPH.

    This problem has been ongoing for two years (and driving me a bit mad).

    The condition is most pronounced when traveling between 15 and 45 MPH.

    I've never once noticed the condition at highway speeds (above 55 MPH.

    It feels like the vehicle is searching for the optimal gear as it seems to be quickly shifting up and down (e.g., between 2nd and 3rd gear).

    The sound of quick shifting back and forth can be heard near rear differential when seated in the rear of the vehicle.

    Condition does NOT exist when in manual shift mode.

    Condition seems to be most pronounced when the vehicle is cold and only slight when warmed up.

    Condition is worsened when climbing even a slight hill or grade.

    Condition exists even when cruise control is activated.

    Throttle body was carefully cleaned (very little gunk was found) and problem persists.

    Mass air flow filter (MAF) was carefully cleaned and problem persists.

    Air filter has been replaced.

    No other problems with transmission have ever been experienced with the exception of occasional "lunging" just before coming to a complete stop.

    Others in forums have suggested this may be a problem with the torque converter or coil pack capacitor.

    Please share your thoughts as I'm at a complete loss and this has been an unresolved issue on this any other forums for several years. Thank you in advance.
     
  2. kwlr3

    kwlr3 Full Access Member

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    My 2005 has the same issue. Mine has 131,000 miles and I have owned it from new. The condition only happens during the last minutes/miles of warm up. In other words, it doesn't do it with a cold engine and it doesn't happen with a fully warmed up engine, only in the last mile or so just before the temperature needle stabilizes. I have brought it up with the dealer and they have no solution. While the RPM does indeed fluctuate, the vehicle speed does not, so I have learned to live with it.

    I do have a theory as to why this happens based on some anecdotal experience. Here goes. The issue was there from new and seemed to go away when I hit 35,000 to 40,000 miles. I then replaced the thermostat and it started to happen again. Seemed to go away for the last 30,000 miles. Dealer replaced thermostat last week and now it's happening again. Here is my theory; the RPM fluctuation is the engine cycling between cold and warm. A cold engine has a closed thermostat and a warm engine has a fully open thermostat. Both are stable conditions. As the cold engine warms up to almost operating temperature, the thermostat cycles open and closed. The temp sensor could be positioned somewhere where it gets exposed to this alternating flow of warm and hot cooling water.

    OK doubters, you could be saying that if your thermostat is bad, you will be throwing codes because the engine does't reach operating temperature. Well, I live in South Florida, I could completely remove the thermostat and it would still reach operating temperature in less than a mile of driving. The first thermostat that I replaced had a dislodged rubber seal that allowed a small but constant amount of coolant to flow. These bad thermostats were an issue written about elsewhere on this site and if you live up north or someplace where you really do "warm up" before driving, they were throwing codes. My bad thermostat never caused a check engine light.

    If I really wanted to prove my theory, I should connect an engine analyzer and see if the rpm fluctuation coincides with a fluctuating temperature reading. None of this solves the problem but I 'll offer a couple of suggestions:
    1. Make sure the coolant system is fully bled.
    2. Make sure you have no coolant leaks, check the plastic bleeder valve under the engine cover. They are plastic and do go bad.
    3. Make sure you have no intake leaks and have good crankcase breather valve. This engine will tolerate some pretty loose connections on the intake side around the MAS and it will still run. The breather can clog, just look at how much oil accumulates in the bottom of the intake manifold.

    I would love to hear of other theories or solutions to this issue. Let us know what you do.
     
  3. Efecta

    Efecta Member

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    Thanks for sharing your experience and thoughts.

    After reading, "while the RPM does indeed fluctuate, the speed does not" in your post, it's clear we're experiencing the same condition. I tried to upload a video of the tachometer, but it doesn't appear possible to do that on this forum.

    I'll pass your advice alone to the new shop I'm going to work with and see what they can come up with. Thanks again.
     
  4. Jerichobrady

    Jerichobrady New Member

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    Same problem on my 05 Lr3. It's like the rpms are surging up and down but only between 30mph and 50mph. Then it levels off as I gain speed
     
  5. cmb6s

    cmb6s Full Access Member

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    The RPMs surging under a constant speed in that 30-50mph band certainly sound like a torque converter lockup problem. It sounds like the torque converter is locking and unlocking again and again. I don't know if there is much info about this issue on this particular board, but if you look on rangerovers[dot]net, you can probably find a lot of info on this particular subject.
     
  6. tlt

    tlt Full Access Member

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    Do to the fact the transmission operation is somewhat temperature depending, it makes sense that there are internal transmission problems or Torque converter issues, though torque converter issues are fairly rare on the Gas engines. On the TDV6 it is common, although they use the same transmission they use a different torque converter.

    It sounds like a transmission issue.
     
  7. vlonjati

    vlonjati Well-Known Member

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    Hi guys, new here so thanks for having me.
    Not to revive an old thread but I was wondering if you all had a resolution with this issue? I have the exact same behavior.

    Thanks.
     
  8. Efecta

    Efecta Member

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    Hi all. I'm curious if anyone has diagnosed a solution to this problem? Also, is it possible to repair or replace the torque converter or does that require a complete transmission rebuilt or replacement?
     
  9. Houm_WA

    Houm_WA Full Access Member

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    Efecta, you posted this issue about 1.5 years ago. Have you NOT gotten it fixed???
     
  10. Efecta

    Efecta Member

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    No. Even though the RPM fluctuation drives me a bit mad, it only occurs at low speeds in automatic. Manually shifting and chanting "serenity now" (Seinfeld) helps. Mechanically, the transmission is operating fine and I can't justify spending $5k - $6k to have a new transmission installed. The problem is caused by the hydraulics that control the torque converter and spending a few thousand on the valve body (if one could be found) is pointless when you have all the other wear on the gears and clutches inside the transmission itself.
     

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