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This "unintended acceleration" business...

Discussion in '2nd Gen. Tacomas (2005-2015)' started by Caddmannq, Sep 7, 2008.

  1. Sep 7, 2008 at 9:07 AM
    #1
    Caddmannq

    Caddmannq [OP] MotoNerd

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    No, I haven't experienced it, and my complaint is minor, but this is what I do feel:

    First my 09 Tacoma is a V6, automatic 5-speed, and it's not broken in yet. I have maybe 200 miles on it of which 175 is stop & go traffic on the main boulevards here. that means traffic gets from 0~45 MPH and back to zero every couple blocks.

    When I'm slowing to a stop, I can feel the engine accelerate a little as the truck slows down, and suddenly the brake pressure I'm applying isn't slowing the truck quite as much as it was. Instead of tapering off the pedal I'm having to push harder at the "middle" of a stop, because the engine's pulling me just a little when it should be braking me more and more (as manifold vacuum rises.)

    Because of this I've taken to shifting the trans down manually, which is less smooth, but I get a more steady and controlled feel under braking.

    I'm suspecting that the smog controls are bypassing some air to the intake to prevent a richening of the mix as the trans shifts down, but I'm not sure I could tell that even with a manifold gage.

    I've had other auto trans cars do this (though much worse) when they developed a minor vacuum leak or a balky idle air bypass motor/solenoid.

    As I said, it's minor, but certainly annoying. If it doesn't clear up a bit as the truck starts to get broken in, I'll probably take it in and complain to the dealer.
     
  2. Sep 7, 2008 at 9:16 AM
    #2
    WilsonTheDog

    WilsonTheDog Kylie's dad

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  3. Sep 7, 2008 at 10:11 AM
    #3
    RealSouthernPride

    RealSouthernPride Well-Known Member

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  4. Sep 7, 2008 at 10:29 AM
    #4
    Caddmannq

    Caddmannq [OP] MotoNerd

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    Yeah, I live in the desert. It's been on almost 100% of the time since I bought the truck.

    So are you telling me this happens because the A/C clutch kicks out as I'm slowing down? Every time I come to a stop? Seems kinda strange. As you slow down the A/C has to work harder because of less airflow and less compressor RPM. I'd think the A/C would tend to kick in as I slowed down.

    OK, it's gonna be 102 today. I'll go try it without the A/C and see what happens.
     
  5. Sep 7, 2008 at 10:30 AM
    #5
    RealSouthernPride

    RealSouthernPride Well-Known Member

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    No AC! your an animal.......Wait till the evening to try it.
     
  6. Sep 7, 2008 at 11:03 AM
    #6
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    FlimFlubberJAM
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    Sounds to me like the feeling you are haveing is the tranny downshifting. :confused:
     
  7. Sep 7, 2008 at 11:09 AM
    #7
    RealSouthernPride

    RealSouthernPride Well-Known Member

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    don't think it's the tranny.... try the AC first to rule it out! then if this is a consistent problem take it to the dealership and let them know you feel that the vehicle is unsafe and could pose a danger to you and the public. if they can not resolve the issue call toyota
     
  8. Sep 7, 2008 at 12:17 PM
    #8
    WilsonTheDog

    WilsonTheDog Kylie's dad

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    That is the general consensus, yes.
     
  9. Sep 7, 2008 at 12:21 PM
    #9
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    Im still thinking its the tranny. I have tried this with the AC off, and it stil does it. As the tranny down shifts, more tq. is applied to the wheels via an increase in gearing makeing it need more brake peddle aplication to maintain the desiered stopping distance. :confused:
     
  10. Sep 7, 2008 at 12:22 PM
    #10
    WilsonTheDog

    WilsonTheDog Kylie's dad

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    ^I used to think that but I, and many others, simply don't have it with the A/C off. Not that I'm doubting you but are you sure it's off completely?
     
  11. Sep 7, 2008 at 12:28 PM
    #11
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    Yep. I also monitored the RPMs with a scan tool. Seems that the tranny is holding 2nd gear until after the vehicle is stopped, then it shifts to first and applies more tq. lockup to keep the rpms. down around 500 or so. ( I have verified this with some Toyota techs.) That seems to be when I feel a "lunge". NOw, that being said, I cant accurately verify that the AC compressor is "off". The switch is off, and I ran a video camera under the hood and aimed it at the compressor so I could see if it was indeed "off", but there was not enough light under the hood to get an image. The more I dig into this issue, it seems that some people DO have an issue related to the AC compressor, while otheres are simply experiencing the tranny shifting down to first gear. Like I mentioed before, though, in my case, I have turned on the AC when stopping, stopped, etc., and found that I can rarely feel it engauge. :confused:
     
  12. Sep 7, 2008 at 12:31 PM
    #12
    WilsonTheDog

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    ^See, that's the other theory, that it's an a/t pressure issue.
     
  13. Sep 7, 2008 at 2:26 PM
    #13
    Caddmannq

    Caddmannq [OP] MotoNerd

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    Ummm...why not just tap a voltmeter into the compressor clutch circuit and see when it kicks up and down?
     
  14. Sep 7, 2008 at 2:27 PM
    #14
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    OK, now thats much to easy to do. :( I should have thought about that. :eek:
     
  15. Sep 7, 2008 at 2:28 PM
    #15
    Caddmannq

    Caddmannq [OP] MotoNerd

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    I could definately see that happening, but with this fly-by-wire business I have no idea what's actually happening to the throttles. Again I think the ECU could be bypassing air to help burn off emissions.
     
  16. Sep 7, 2008 at 2:30 PM
    #16
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    That very well could be. I havent thought about it doing that.
     
  17. Sep 7, 2008 at 2:32 PM
    #17
    Caddmannq

    Caddmannq [OP] MotoNerd

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    I will try the A/C first. In fact I'm going out to do so directly.

    So far I do not feel the vehicle is unsafe. I can certainly stop it. I just don't seem to be able to stop it as smoothly as I'd expect a Toyota to stop, due to this issue.
     
  18. Sep 7, 2008 at 3:50 PM
    #18
    Caddmannq

    Caddmannq [OP] MotoNerd

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    OK I'm back from my test ride, and what I think is actually happening is this:

    First, it is only noticible under very light braking, when coming slowly to a smooth stop. I haven't made a panic stop yet, but under even middling moderate braking pressures the effect is undetectable.

    As I'm slowing I'm getting pretty normal (for an automatic) engine braking in 3rd gear, but then as it shifts down to 2nd there's a long pause where it feels like the truck is coasting with no engine braking at all, until it slows almost to a stop. It's at the point where the truck starts to coast that I have to apply the brakes harder.

    Also, I do notice much less of this effect when I switch the A/C off. It still happens but is far less pronounced. In fact if I wasn't actually looking for it to happen I probably wouldn't have noticed it at all.
     
  19. Sep 7, 2008 at 4:01 PM
    #19
    chris4x4

    chris4x4 With sufficient thrust, pigs fly just fine. Moderator

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    In my research, I found this to be when the torque converter is locking up to a degree to lower the engine RPM, and shifting into first. This information was given to me from an editor at Popular Mechanics, who said that he talked with "Toyota" about the issue. :confused: One of the reasons given WAS for emissions, but I was told that by keeping the rpm's down, less fuel is burned, thus lower emisions.
     
  20. Sep 7, 2008 at 4:05 PM
    #20
    Taco's for All

    Taco's for All Well-Known Member

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    I felt the same thing when I first got my 09. I notice it most under light braking and I think it just may be how the transmission is downshifting at higher RPM's. I had a 94 Mustang with a shift kit in the AOD and it used to downshift much harder than the Tacoma so I am relatively used to it. I'm by no means a mechanic but I think that it's just the transmission downshifting. My way of thinking about it is that if it weren't supposed to do it, then my truck wouldn't have made it out of the factory.
     
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