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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; Mine has similar symptoms. It doesnt do it until ive been driving for quite some time though, usually city driving ...

  1. #41
    Just a souped up sunfire. Ryans99LS1's Avatar
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    Mine has similar symptoms. It doesnt do it until ive been driving for quite some time though, usually city driving or long distances. Ill be coming up to a light, put it in neutral, and then idle will fall to about 1400 and stay there until i get to about 1-2mph or completely stop then it just drops to 600(where my idle is)

  2. #42
    Moderator Cutlass's Avatar
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    Quote Originally Posted by Too Fast View Post
    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.!
    Hehehe believe me I know exactly what you are talking about there. I even considered maybe I was doing that...but I have an UPDATE.

    My IAC count was at Zero. The desired idle was 800 rpm and I was sitting at 900 rpm with the IAC at zero (Throttle position was also 0% at like .55(edit) volts). So I started hunting for a vacuum leak. I sprayed the usual carb cleaner all over the usual suspects (fuel injector o-rings, TB gasket, IAC o-ring, and as much of the intake seals and vacuum hoses I could find). I came up with nothing.
    So I pulled the TB off to inspect and adjust the idle screw. I held the TB up to the lights and I can see daylight around the top half of the throttle plate. Bottom half seals up great to the housing. So I'm thinking "damn I got a bad throttle body from Shaner." I adjusted the idle screw down a bit. I couldn't go too far because the throttle blade starts sticking and binding against the housing. But I got it down a little bit.
    Reinstalled the TB and rechecked my IAC. Its idles down to the desired 800rpm and the IAC count is like 8 or 9. So at least it can control the idle but the count is definitely not at the desired 40 to 60 range or whatever. Driving down the interstate at 70mph, I pushed in the clutch and it idled right down to a little less than 1000rpms. Definitely way better then the 1200 to 1400 I had before. I almost think I can live with that.
    But now I'm considering contacting Shaner and see what he can do for me. I'm also considering plugging the throttle blade hole to help the situation. What would make a good plug thats not gonna fall off and go into my engine??
    Last edited by Cutlass; 04-10-2009 at 04:34 PM. Reason: I misread my TP Voltage on the scan tool. I changed it in the 2nd paragraph

  3. #43
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by dklowrider View Post
    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]
    Ya mine did. That is until I purchased HPtuner and rid the car of that stupid feature.
    One of the tables in there, either throttle cracker or throttle follower,,,I completely zero the table to get rid of what I called cruise control.
    I got tired of riding the brakes every time I was idling through 35 mph zones.
    It never had much engine braking either while cruising 65 mph...I couldn't simply let off the gas coming up behind someone, I had to hit the brakes. Completely irritating to drive.
    I've driven carbed cars all my life and still do,,,yes even the barbaric holleys as you put it
    It's the feel I'm used to and just wanted our fuel injected cars to work in the same fasion.

  4. #44
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Cutlass View Post
    considering plugging the throttle blade hole to help the situation. What would make a good plug thats not gonna fall off and go into my engine??
    I use silver solder on carburators just for this. If I have a hole that is too big I solder it and then it can be redrilled if necessary to the desired size.

  5. #45
    TunedbyFrost.com Tuner Frost's Avatar
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    Quote Originally Posted by Cutlass View Post
    Hehehe believe me I know exactly what you are talking about there. I even considered maybe I was doing that...but I have an UPDATE.

    My IAC count was at Zero. The desired idle was 800 rpm and I was sitting at 900 rpm with the IAC at zero (Throttle position was also 0% at like .8 volts). So I started hunting for a vacuum leak. I sprayed the usual carb cleaner all over the usual suspects (fuel injector o-rings, TB gasket, IAC o-ring, and as much of the intake seals and vacuum hoses I could find). I came up with nothing.
    So I pulled the TB off to inspect and adjust the idle screw. I held the TB up to the lights and I can see daylight around the top half of the throttle plate. Bottom half seals up great to the housing. So I'm thinking "damn I got a bad throttle body from Shaner." I adjusted the idle screw down a bit. I couldn't go too far because the throttle blade starts sticking and binding against the housing. But I got it down a little bit.
    Reinstalled the TB and rechecked my IAC. Its idles down to the desired 800rpm and the IAC count is like 8 or 9. So at least it can control the idle but the count is definitely not at the desired 40 to 60 range or whatever. Driving down the interstate at 70mph, I pushed in the clutch and it idled right down to a little less than 1000rpms. Definitely way better then the 1200 to 1400 I had before. I almost think I can live with that.
    But now I'm considering contacting Shaner and see what he can do for me. I'm also considering plugging the throttle blade hole to help the situation. What would make a good plug thats not gonna fall off and go into my engine??


    Man sounds like another TB ruined by 'porting'

    With IAC counts that low, your problem will return as soon as the weather gets about 10 degrees warmer or so than it was on the day that you noted the single digit counts.

    Quote Originally Posted by Firebirdjones View Post
    I use silver solder on carburators just for this. If I have a hole that is too big I solder it and then it can be redrilled if necessary to the desired size.

    This will definitely help and is certainly an accepted method.

  6. #46
    Just a farmboy V8MUSCLE's Avatar
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    i think it has something to do with the computer. mine does it and it is completley stock. i took of my air cleaner down to the throttle body and mounted a camera in there. it looks as if the "butterfly" doesnt close all the way till about 15mph.

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    Quote Originally Posted by Firebirdjones View Post
    Ya mine did. That is until I purchased HPtuner and rid the car of that stupid feature.
    One of the tables in there, either throttle cracker or throttle follower,,,I completely zero the table to get rid of what I called cruise control.
    I got tired of riding the brakes every time I was idling through 35 mph zones.
    It never had much engine braking either while cruising 65 mph...I couldn't simply let off the gas coming up behind someone, I had to hit the brakes. Completely irritating to drive.
    I've driven carbed cars all my life and still do,,,yes even the barbaric holleys as you put it
    It's the feel I'm used to and just wanted our fuel injected cars to work in the same fasion.
    glad im not having that problem cuz id be looking for a vacuum leak! cross my fingers. and if i have to start putting surveillance cameras in my intake im buying my 81 back.
    Last edited by dklowrider; 04-10-2009 at 12:49 PM.

  8. #48
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    Quote Originally Posted by dklowrider View Post
    glad im not having that problem cuz id be looking for a vacuum leak! cross my fingers. and if i have to start putting surveillance cameras in my intake im buying my 81 back.
    Ya I know what ya mean. Mine would do just as V8Muscle described,,,once at a slow enough speed the throttle would finally return. But while cruising along it was like a mild case of cruise control with very little (if any) engine braking. I couldn't stand it.

    No vacuum leaks at all,,,it was just a simple adjustment in the tune. Now it engine brakes so well I don't have to ride the brakes going through town anymore, and coasting down slight grades is now also very nice, no more riding the brakes all the time. The car actually slows itself.

    There was also an issue of the computer raising the engine idle if you were coasting in neutral. Sitting still it would hold 750 (where I currently have it) but if the car started rolling in neutral it would gradually raise the idle up to 1500 rpm or so.
    There was an idle table that worked off of MPH (speed sensor) and once that was corrected this issue went away also.

    Goofy computers,,,thats for sure.

  9. #49
    Senior Member Too Fast's Avatar
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    ^^^It has to be some sort of emissions thing. Why the hell else would that stuff be programmed in the PCM? I'm talking about the throttle cracker, or the high idle in neutral when rolling.

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    Moderator Cutlass's Avatar
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    I FIXED IT!!! Remember how I was saying I couldn't adjust the idle stop screw all the way out because the blade would bind against the housing. Well I found out it was because the throttle blade wasn't properly centered.
    Here's how I corrected it. I back the idle stop screw all the way out so that the throttle cam rested against the aluminum housing. Then I loosened the 2 torx screws holding the blade to the shaft. I opened and closed the throttle several times and then retightened the torx screws. I held it up to the light and BAM it was evenly sealed all the way around the blade (only an extremely small amount of light shining through all the way around the blade). I then screw the idle stop back in so that it just bumped the throttle cam...reinstalled everything...and now it idles great at 40 to 50 IAC counts. And it doesn't sail on when I wanna coast on the highway. I'll have to experiment a little more to see how it engine brakes and idles coasting in neutral.

  11. #51
    Moderator Cutlass's Avatar
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    Oh and while I was working a Cadillac Deville today I discovered their TB is also 75mm and the blade doesn't have a hole in it. I dug around and found 2 Caddy Northstar TBs and took the blades out. I was gonna use one in my LS1 TB if I couldn't get the idle down where I wanted it.
    So if you need a TB blade w/out a hole, I have a few. One from a 99ish looks like the same screw hole spacing as our LS1s, the other is a little newer and the screws are spaced out more but could easily be redrilled.

  12. #52
    Senior Member Too Fast's Avatar
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    Quote Originally Posted by Cutlass View Post
    I FIXED IT!!! Remember how I was saying I couldn't adjust the idle stop screw all the way out because the blade would bind against the housing. Well I found out it was because the throttle blade wasn't properly centered.
    Here's how I corrected it. I back the idle stop screw all the way out so that the throttle cam rested against the aluminum housing. Then I loosened the 2 torx screws holding the blade to the shaft. I opened and closed the throttle several times and then retightened the torx screws. I held it up to the light and BAM it was evenly sealed all the way around the blade (only an extremely small amount of light shining through all the way around the blade). I then screw the idle stop back in so that it just bumped the throttle cam...reinstalled everything...and now it idles great at 40 to 50 IAC counts. And it doesn't sail on when I wanna coast on the highway. I'll have to experiment a little more to see how it engine brakes and idles coasting in neutral.

    Nice. Now we'll see how the cracker holds things up.

  13. #53
    Moderator Firebirdjones's Avatar
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    Quote Originally Posted by Too Fast View Post
    ^^^It has to be some sort of emissions thing. Why the hell else would that stuff be programmed in the PCM? I'm talking about the throttle cracker, or the high idle in neutral when rolling.
    Probably. Starting in about 1971 GM had the same thing on the V8 cars. They used a throttle solenoid that was triggered by the transmission. Everytime the transmission shifted to 3rd (auto) or 4th (4-speed) it would turn on the switch which kicked up the idle slightly.

    This was mainly for cruising mode (hence the gear selection) so when you let off the gas the throttle didn't move as far,,,,so that when you got back on the gas there was less movement as well,,,less pump shot from the carb meant better gas mileage.

    Another thing this carb switch also triggered was the vacuum advance for the distributor. The hose was running to it,,,but only supplied vacuum (and subsiquently more timing) to the distributor when the switch was tripped in high gear, again for better mileage. It was common back then (and now) to hook the distributor directly to a constant vacuum source to increase gas mileage for around town driving as well.

    You are right, emissions has something to do with it as well,,,California started implimenting this setup as early as 1970 on Cali built cars, my 70 Formula built in Van Nuys has it. And there is an emissions sticker on the car telling the owner "not to remove the vacuum advance from the switched source" When I had it on the sniffer in California it had to be hooked up properly,,,otherwise the extra timing at idle would increase the hydro carbons to an unsafe level,,,,,or some crap like that,,,,haha.

    Sorry to get off subject,,,but this damn high idle while moving crap on these computer cars just reminds me of that stuff.
    Last edited by Firebirdjones; 04-10-2009 at 05:03 PM.

  14. #54
    Moderator Cutlass's Avatar
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    Quote Originally Posted by V8MUSCLE View Post
    i think it has something to do with the computer. mine does it and it is completley stock. i took of my air cleaner down to the throttle body and mounted a camera in there. it looks as if the "butterfly" doesnt close all the way till about 15mph.
    WHAT?? Hows that possible? The computer doesn't control the throttle, its cable operated. If your foot's off the pedal, the TB butterfly is closed. Plus, driving around without a MAF sensor, probably didn't run too well, huh?

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    Senior Member Too Fast's Avatar
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    Quote Originally Posted by Cutlass View Post
    WHAT?? Hows that possible? The computer doesn't control the throttle, its cable operated. If your foot's off the pedal, the TB butterfly is closed. Plus, driving around without a MAF sensor, probably didn't run too well, huh?
    I was wondering that too but was too scared to ask.

  16. #56
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    Quote Originally Posted by Cutlass View Post
    WHAT?? Hows that possible? The computer doesn't control the throttle, its cable operated. If your foot's off the pedal, the TB butterfly is closed. Plus, driving around without a MAF sensor, probably didn't run too well, huh?

    the computer does have the throttle cracker which can adjust the idle.

  17. #57
    Moderator Cutlass's Avatar
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    YEAH...but it controls the Idle Air Control valve, NOT the throttle body butterfly.

  18. #58
    Moderator Cutlass's Avatar
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    Ok so here's my update on engine braking and idle rpm coasting in neutral. I'd stay it now engine brakes about as normal as any other car I've driven. Of course I don't get much engine braking in 6th gear below say 55mph, mostly because the rpms are so low.
    I have also coasted with the clutch in or with the trans in neutral and the rpms are around 800 to 900 rpm, exactly where it should be I'd say. So I don't know if my '99 has throttle cracker or not but it doesn't seem to affect much.
    I'd be curious if the same fix I did, would help you other guys out.

  19. #59
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    i always thought it did this for when you're going over say 30 mph so when u shift into 5th or 6th it lines up... i also noticed when u downshift from 6th to 5th it kind of automatically revs up to where the 5th gear rpms would be (but by the nature of reducing the load on the engine it may attribute to this)

    or possibly its programmed to do this again if you're over some speed (say 30 mph again) and if ur a new driver to stick and put it in too low of a gear without rev matching it compensates slightly to reduce wear on the driveline compared to if it was at 900rpms


    i honestly have no actual facts but thats just what i have always thought logically... some kind of "safety" mechanism built into the PCM or whatever to lessen wear on the driveline in general... (basically the principle of driving faster = rpms need to be higher no matter what gear...especially if downshifting.)

    idk if i explained it correctly but thats kind of what i thought because i noticed it only happens when going atleast over 30... i see it mostly happen over 45 where 6th can be engaged)
    Last edited by mammoth713; 10-23-2009 at 08:29 PM.

  20. #60
    What Would Earl Do? Vexzer's Avatar
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    Quote Originally Posted by mammoth713 View Post
    i always thought it did this for when you're going over say 30 mph so when u shift into 5th or 6th it lines up...
    Nope, not it. Not a new Nissan LOL!

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