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Thread: Throttle Sticking - any ideas for fixing?

  1. #1
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    Question Throttle Sticking - any ideas for fixing?

    Here's a couple links to video's showing my sticking throttle. And a couple things that enable it to close properly.
    https://youtu.be/kgT8Mum5fhQ
    https://youtu.be/h1SSb0Gd38E

    Quick symptoms list:
    * Slow application of the throttle, causes it to stick partially open.
    * Turning off the engine, allows it to close properly.
    * Unplugging the MAP vacuum line, allows it to close properly.
    * The throttle returns properly every time when engine is not running.

    More Info:
    This is not the first time I've had this problem. I've replaced MAP and TPS (twice) trying to get this to be fixed. None of those replacements fixed it. It's intermittent when driving...sometimes it'll have error codes. I suspect that it's frequently sticking, however under a load, and with the doghouse installed, I think I'm just not noticing it.

    Last time it was fixed (summer 2015), my mechanic adjusted the TPS position to a lower voltage and that fixed the problem. I have NOT touched the TPS mounting screws since then.

    Recently the engine was rebuilt and reinstalled. The problem reoccurred at that point. I've got ~800 miles on it, and the service engine light came on three times. Codes include:
    42 Ignition Error
    33 MAP sensor high
    22 TPS sensor low


    I asked for help with this before on a different forum.
    http://tunerpro.net/forum/viewtopic.php?p=14808#14808

    I can log info from tuner pro, I also plan to to check vacuum and TPS voltage when the vehicle is NOT running, and will follow up with a reply or edit this post with the findings.

    General info:
    1995 Chevy G30 HD van
    CA Smogged - passes visual and sniffer (set base timing back to 0, from the Harris recommended 10deg for test)
    350 TBI
    Summit AL CARB legal heads (design similar to vortec...with improved geometry/flow around valve stems in chambers)
    Stock exhaust manifolds
    L31R short block w/ stock cam from a 1996
    ECM: 16197427 BNKM
    Modified bin (from Brian Harris)
    Upgrades from Harris' recommendations...regulator spring, ultimate mods TBI, upgraded fuel pump.

    ~15k miles on upgraded TBI w/ larger bores/butterflies, modified gasket, and intake. The shaft and bushings are good, there is NO sticking, and the torsion spring is strong.



  2. #2
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    TPS info
    Tunerpro RT plugged into vehicle, looking for connection.
    Turn key to ON.
    TPS voltage displays 0.5, frequently switching between 0.4 and 0.5
    TPS throttle position % shows 0% initially, but then starts creeping up, gage moves up/down by a few tenths, or points...after a minute it's fluctuating around 3-4%, wait a bit, now it's steady at 5.1%

    Then V drops to 0.4, and TPS% fluctuates between 3.9 and 4.3%

    pedal to floor, shows 4.6V, 100%
    release pedal, shows 0.4-0.5V, 3.9-4.3%

    Tapping throttle, not exceeding 10% throttle position, and V varies/jumps between 0.7 and 0.2V, let it sit.
    V stabilizes between 0.2-0.3V, TP at 0.4-1.6%

    Pulled doghouse, and unplugged TPS from harness. Connected tunerpro, and switched key to ON, engine still not running.
    TP5 reads 0.0-0.1V, at 0%TP
    TPS sensor is rigidly attached to TBI as I expected, no movement there.
    Plug in sensor to harness, V jumps to fluctuate between 0.3 and 0.5V, and TP% jumps right up to 5%, and fluctuates.
    I find that I can tap on the sensor itself, and the reading fluctuates. Is this typical with the sensors? I've got two from my mod'd TBI provider.

    Also, I am getting continuous TUnerPro fluctuating between DA: Connected, and DA Data Error!

    Any ideas on why that would happen as well? I've tried various USB ports, and it behaves the same.
    I also tried turning OFF the connection within TP5, then turned off key, flipped TP5 connection on (status DA:Connecting...), and then turning key ON. No connection.
    Toggle the connection on/off, no change. Had to reboot TP5 twice to get a connection.
    Last edited by tony-baroni; 11-19-2017 at 01:01 PM.

  3. #3
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    Bore for the throttle shaft worn out so the shaft has play in the hole? You can find bushing kits fairly cheaply if you search Ebay.

  4. #4
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    Throttle shaft is a-ok...the TBI is as new - from SPR performance. Super smooth, no friction or binding.

  5. #5
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    OK, didn't see it mentioned in your posts. It has to be a mechanical binding of some sort. The vacuum might be pulling the blades in the direction to cause a bind.

    The TPS should not cause it if it's properly installed. The MAP sensor can't cause it.

  6. #6
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    It is puzzling...I like where you're going with the vacuum pulling blades in a particular direction. I can't feel ANY sticking when moving the blades or the cable attaching bracket. I have taken a bunch more video's of diagnosing this...and will update with new links when I get them up on youtube.

    FYI, I also had the throttle cable inspected by a mechanic back when it first occurred. I also checked it when I first had this problem with the new rebuild. There is no stiction in the cable.

  7. #7
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    My continued chasing a root cause has uncovered what appears to be three issues.

    1. Throttle blades intermittently sticking OPEN when engine is running, clearly noticeable w/o load, haven't noticed while under load.
    2. Intermittent issue with voltage output from TPS, being disturbed by tapping, moving wires on harness, and sometimes the voltage dropping to 0.2-0.3V. other times it works great.
    3. Intermittent issue with throttle position percentage, rising above 0, to as high as 8%, and staying there, when the throttle has NOT opened, and the TPS voltage is performing properly (~0.5V).

    Regarding throttle blades, the video's linked earlier in the thread show this in action. Still looking for a solution.

    Regarding intermittent voltage output from TPS, my first thought is a bad TPS, however the symptom is intermittent, and the symptoms re-occurred with a new TPS sensor, I think it's something else.

    Regarding the throttle position percentage issue, I think this may be a fault ECM issue.

    Here are some video's showing this odd behavior.
    https://youtu.be/qaS7B4cauSM

    The manual diagnostics requires a scan tool, which I don't have. I'm looking for some guidance in troubleshooting the root cause of these issues, and fix them, before I start tuning the engine/chip.

    Here is what I did to start troubleshooting.
    Test sensor values going INTO the ECM at the connector (sensor is working properly in this one)
    https://youtu.be/Hf6qk8nx4gI

    Confirm the ECM input pins, match the output from the ALDL cable. (sensor also working, but TP5 connection errors)
    https://youtu.be/cypYZXFOZkU

    https://youtu.be/f7A9RTLiXJU (same, another test, working this time)

  8. #8
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    As posted earlier I think you need to address the sticking throttle. It's got to be worn throttle shaft bushings, misinstalled throttle plates or throttle cable interference. I know you said they were good but??
    6395, BHDF, 7.4 BBC lightly modded now 6395 BMHM back to BHDF

  9. #9
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    Was able to spend time on this on Thanksgiving! :-)

    The TBI unit is a SPR Performance, ultimate mods remanufactured unit. It has less than 15,000 miles on it. Option #5 in their services website:
    http://www.sprperformance.com/services-1

    I removed the air cleaner, throttle cable and the TPS; then ran the engine and turned throttle by hand. The TBI still stuck open.

    When it's stuck just a small amount of force on the throttle cable attachment bracket causes the throttle to close. An easy fix would be to give the torsion spring one more turn. And testing throttle shaft clearance would be easier with the spring and butterfly valves removed.

    So I tried to remove the throttle shaft. I got to the point of removing the throttle blades, one came out easily, and then I stripped the torx screw holding the second one. Since I need to drive this Monday, I'm not going deeper into disassembly.

    I tested the shaft fit by hand, and there is a very small amount of play. Nothing is binding.

    During re-assembly of the butterfly plate, it is difficult to position the blade perfectly in the bore. The final tightening down of the torx screw puts pressure on the plate causing it to move slightly.

    I forgot to remove the cruise control cable for the test. I will try that today after I get the unit back together.

  10. #10
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    Here's a video of my diagnosis. During the video, in some spots you can hear me say there is no movement, but I was having difficulty seeing it while in the vehicle. When I viewed the video the movement was visible.

    I'm unsure if this amount of play is excessive.
    https://youtu.be/VcWU_TPqZMQ

  11. #11
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    Update from 11-24

    Reinstalled TBI, Fixed three vacuum leaks I discovered, two at connections of PCV system to TBI, and one at base of EGR (wasn't tightened).

    Tested w/ TPS installed, no throttle cable, no cruise cable connected. Still sticks open under vacuum pressure.
    Test drove w/ doghouse off, with both cables installed. Sticks under load as well, though it's not noticeable during driving. Probably not helping my mileage though, and I'd like this fixed before starting tuning.

    Going to try some other tweaks soon.

  12. #12
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    I'm tossing a dart...what's your IAC count? I read somewhere in this forum that more air across the throttle blades helps fuel atomization. Maybe open up your throttle so your IAC is down to single digits and see how it behaves from there...??? Don't forget to readjust TPS volts after.

  13. #13
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    I just had same issue with tps voltage and percentage fluctuations k.o.e.o. on tunerpro and dvm.I took the rubber seal off the tps connector and gave each terminal pin hole a little squeeze to tighten the contact in case that was the prob.it did make it more stable but still not perfect.at scrap yard recently I unplugged a tps on 95 gm v8.I see it's different than the one I have but I would check for it being loose with out the rubber seal.and on your tbi the idle screw should be backed out while tightening throttle blades as the body then centers them.then the idle and tps will need to be readjusted.also try driving with out the cruise cable hooked.sorry if I'm off but I could not watch the videos.
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

  14. #14
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    Thinking about the tbi it sounds more like a weak spring and or slight bind.if u never get it to stick with engine off then try a light return spring to see if any different. I would try the same if it sticks while running with no cables hooked up.check vacuum just to verify it's where it should be .the spring on the tbi is all that pulls the cables back.
    Jeeper
    87 sjimmy 4.5L /nv1500/ 8.8 3.55 rear/disc brakes.

  15. #15
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    Yeah...maybe over-thinking something as simple as adding a return spring!!! I forgot to mention...after watching the last video of wiggling the throttle shaft, I went straight to my bro's 240k mile tbi and tried to see if there was any play in the throttle shaft. Hard to tell comparing w/ video but it seems to me this 240k mile tbi has less slop then the one in your video.

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