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high idle only when moving

This is a discussion on high idle only when moving within the Computer & Tuning forums, part of the LSx Technical Help Section category; I considered that and the IAC at the very start. I'm thinking with it doing it at neutral idle, I'M ...

  1. #21
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    I considered that and the IAC at the very start. I'm thinking with it doing it at neutral idle, I'M doubtful, even if it is registering a false throttle level w/ the butterfly shut from 0 mechanical actuation the car shut sputter. Further, it isn't too often you see a sensor give a bad "spot" problem without showing some signs through the rest of the range. But float type sensors (mechanical potentiometers and some that are capacitive) are more likely than other sensors and though I am not sure, the TPS is likely a simple potentiometer!

    Now a speed sensor maybe could as well...honestly, I don't know if our cars have those. But I have had to troubleshoot things more than a thousand times. If something happens after you change something...I 998/1000 it is related to the change! So I'm going over everything TB and Intake mounted first.

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    Junior Member blackonblackws6's Avatar
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    Quote Originally Posted by TransAm11973 View Post
    also could be your throttle positioning sensor
    I've seen the TPS cause this symptom in the past. Use a scan tool to varify the TPS is at 0% when closed. I've seen a couple a/m TB's have problems with this. adjusting the TB stop only bandaids the problem for one key cycle then the PCM rereads the TPS position at restart. It does sound like a vacuum leak if its a issue that's just popped up and is progressing. Seems like the PVC hose, the rubber connection cracking at the intake have been a common failure. check all vacuum sources before you go turning screws and fooling with tuning. good luck

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    I was out looking at this again a few days ago and I didn't like how the PVC hose looked. It looks likes it is binding where the valve goes (pulling at an angle and causing some gaping at the connection. I should get to take it out and drive tomorrow.

    I want to do one thing at a time so I can be sure what was causing the problem. But I had a vibration coming around 35 mph last time I was driving, felt like it. I am new to this area so I want to take it out to a clean road (I've thought I had vibrations before...and it was just the road.) I'm also going to check my lug torques before I move it.
    Last edited by BowTied; 01-15-2009 at 04:29 AM. Reason: typo

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    Junior Member blackonblackws6's Avatar
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    Quote Originally Posted by BowTied View Post
    I was out looking at this again a few days ago and I didn't like how the PVC hose looked. It looks likes it is binding where the valve goes (pulling at an angle and causing some gaping at the connection. I should get to take it out and drive tomorrow.

    I want to do one thing at a time so I can be sure what was causing the problem. But I had a vibration coming around 35 mph last time I was driving, felt like it. I am new to this area so I want to take it out to a clean road (I've thought I had vibrations before...and it was just the road.) I'm also going to check my lug torques before I move it.
    The tires on my TA are very soft, if I don't drive it for a week or so they get flat spots that take a few miles to smooth out (but they do) and I get the same vibration you speak of. It took me a while to figure this one out, I thought oh damn, here's another issue to deal with. I notice it more when the temps are low. I always put it on stands when its stored. Not sure if that applies to you but just a thought.

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    Quote Originally Posted by blackonblackws6 View Post
    The tires on my TA are very soft, if I don't drive it for a week or so they get flat spots that take a few miles to smooth out (but they do) and I get the same vibration you speak of. It took me a while to figure this one out, I thought oh damn, here's another issue to deal with. I notice it more when the temps are low. I always put it on stands when its stored. Not sure if that applies to you but just a thought.

    I am so damn glad you said that!

    I thought about that, stopped and looked at things, and thought it again, and worried I was wishful thinking. I hadn't drove it for 7 weeks (out of town) and my tires are very soft. Comp tires only rated for 20k miles.

  6. #26
    Senior Member Too Fast's Avatar
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    Quote Originally Posted by BowTied View Post
    I was out looking at this again a few days ago and I didn't like how the PVC hose looked. It looks likes it is binding where the valve goes (pulling at an angle and causing some gaping at the connection. I should get to take it out and drive tomorrow.

    I want to do one thing at a time so I can be sure what was causing the problem. But I had a vibration coming around 35 mph last time I was driving, felt like it. I am new to this area so I want to take it out to a clean road (I've thought I had vibrations before...and it was just the road.) I'm also going to check my lug torques before I move it.
    Sounds like you found the vacuum leak, causing your high idle issues. Let us know!

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    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by blackonblackws6 View Post
    The tires on my TA are very soft, if I don't drive it for a week or so they get flat spots that take a few miles to smooth out (but they do) and I get the same vibration you speak of. It took me a while to figure this one out, I thought oh damn, here's another issue to deal with. I notice it more when the temps are low. I always put it on stands when its stored. Not sure if that applies to you but just a thought.
    Nothing a little spin balance won't cure I mean a nice little burnout.

    I have a ventura that sat for 20 years, talk about flat spots. No amount of driving got rid of it. One nice burnout did Then I rotated the tires and did it again,,,,no more flat spots

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    TJEA Retirement Home Dr.Crash's Avatar
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    where do you go to adjust the "throttle cracker"?

  9. #29
    Senior Member Too Fast's Avatar
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    ^^^ Racist!!!!!! LOL, refer to my post # 10. But, I don't know where to adjust it.

  10. #30
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    What you dont like Black cars?

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    Senior Member Too Fast's Avatar
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    LOL, throttle "cracker". You do know what that slang means, eh?

  12. #32
    TJEA Retirement Home Dr.Crash's Avatar
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    Quote Originally Posted by Too Fast View Post
    LOL, throttle "cracker". You do know what that slang means, eh?
    ya i was J/K

  13. #33
    Junior Member edcmat-l1's Avatar
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    Most of the time, cruise control effect is caused by too much base idle airflow, or the throttle blade cracked too much.

    First step in getting it correct is getting the base idle airflow right, and the throttle blade opening right, so that the IAC counts are 40-60 warmed up. After that, the throttle cracker/follower can be adjusted to prevent stalling.

    There are several idle pids that need to be logged to figure out what's up. STIT, LTIT (long term idle trims should really be turned off), expected airflow, throttle cracker airflow, throttle follower airflow. Using these pids, you can dial the idle and return to idle in nicely.

  14. #34
    Moderator Cutlass's Avatar
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    My car seems to be doing this also AFTER I installed a port and polished Shaner S3 throttle body. I could be wrong...maybe the engine was racing like that before, but I think I would have noticed. Anyways there are some damn good suggestions in this post and I'll have to try them out soon. I think I'll be taking the scan tool home from work a few times this week to check out some TPS and IAC data!! I'll update with my results!!

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    Quote Originally Posted by Cutlass View Post
    My car seems to be doing this also AFTER I installed a port and polished Shaner S3 throttle body. I could be wrong...maybe the engine was racing like that before, but I think I would have noticed. Anyways there are some damn good suggestions in this post and I'll have to try them out soon. I think I'll be taking the scan tool home from work a few times this week to check out some TPS and IAC data!! I'll update with my results!!
    did you use your stock throttle body gasket? If so, I would look into replacing it with a new one, they last pretty long, but i've heard of these causing a high idle before when either defective due to heat cycles, or installed incorrectly when it's pinched or something behind the throttle body.

  16. #36
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    my car has a few places it kind of gets into a "groove" and i can let off the gas and it holds the speed for awhile, I think its normal

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    Quote Originally Posted by Spikito View Post
    my car has a few places it kind of gets into a "groove" and i can let off the gas and it holds the speed for awhile, I think its normal
    theres nothing normal about an engine not returning to idle when the throttle is closed. or at least deccelerating if not disconnected from the drivetrain

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    Senior Member Too Fast's Avatar
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    Quote Originally Posted by dklowrider View Post
    theres nothing normal about an engine not returning to idle when the throttle is closed. or at least deccelerating if not disconnected from the drivetrain
    Well, if there is a speed sensor input to the PCM, sometimes the PCM logic calls for a little higher than normal idle, hence the throttle cracker table.

    Now if you're talking old school carburetors, then you are correct.

  19. #39
    Senior Member Too Fast's Avatar
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    Quote Originally Posted by Cutlass View Post
    My car seems to be doing this also AFTER I installed a port and polished Shaner S3 throttle body. I could be wrong...maybe the engine was racing like that before, but I think I would have noticed. Anyways there are some damn good suggestions in this post and I'll have to try them out soon. I think I'll be taking the scan tool home from work a few times this week to check out some TPS and IAC data!! I'll update with my results!!
    Since you, as well as me, are an ASE Master tech, I could be that you are paying more attention to it now, since you made a change. You know how customers are, when you fix one thing on their car they suddenly get hyper sensitive to any noises/vibration/leaks/steering wheel crooked/idle issues/ etc. etc.!

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    Quote Originally Posted by Too Fast View Post
    Well, if there is a speed sensor input to the PCM, sometimes the PCM logic calls for a little higher than normal idle, hence the throttle cracker table.

    Now if you're talking old school carburetors, then you are correct.
    but would a speed sensor input call for a high enough idle that the car will not change speed when you let off the throttle? i know there are times when the engine will not immediately return to base idle but some of the conditions described here sound pretty extreme. didnt mean to say popping off of the throttle would cause an immediate snap back to idle at all times. [a holley 4 barrel is so barbaric, but fun to tune or mess with anytime you feel like it]
    Last edited by dklowrider; 04-09-2009 at 04:51 PM.

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