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Another High Idle Thread

This is a discussion on Another High Idle Thread within the General Help forums, part of the LSx Technical Help Section category; Ok so my car has over 130kmi on it now, I've owned it for 5 years now I think and ...

  1. #1
    Member TransAmX2's Avatar
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    Another High Idle Thread

    Ok so my car has over 130kmi on it now, I've owned it for 5 years now I think and it has always had a high idle 1,100 or so. Over the years I’ve gotten used to it and through all it's modifications and upgrades the idle has remained constantly high. I have gone through everything I can think of that affects the idle and still it's 1,100 hot or cold.

    EGR deleted, headers, no cats, flowmaster cat back, LS6 intake, MTI lid, Free air baffle mod, TB Bypass, PCV redone with an inline catch can (oil separator), Ported TB, new plugs, frost tune, cleaned MAF, no vacuum leaks, throttle stop screw is all the way down, the blade is closed and not sticking.

    So after bouncing the issue off my mechanic friend he suggested I start it and unplug the IAC while it's running, it didn't change anything, his line of thinking is that the IAC was closing off as much air as it could to try and drop the idle already so disconnecting it changed nothing, if it was running as it should the IAC should be partially open to control airflow and once disconnected (on a properly operating car) it will default to closed and the idle will drop and even possibly stall out the car. Is this a correct assessment?

    To add to this theory upon further investigation I noticed about a 1/2" polished ring on the back side of my throttle blade around the idle hole in the throttle blade, leading me to conclude that someone previously drilled out the hole and touched the drill chuck to the blade in the process (making the polished ring around the hole), the batteries were dead in my digital calipers so my closest measurement is that a 5/32” drill bit fit in the hole. My research turned up that the factory hole is supposed to be .150” and the 5/32” hole would make it .15625”, so I’m thinking this is my issue and I’ll likely need a different TB. Anyone else able to confirm or contradict any of these conclusions? Thanks for any insight you can provide.

    I was thinking of putting a piece of tape over the hole and punching a smaller hole in it to see if it did indeed fix the idle (only for testing purposes, not going to drive it with tape on there and possibly suck it into the intake), just trying to pin down the issue before I spend $ on something I don’t need. I also considered JB Welding the hole and re-drilling it (if that is indeed the issue) but I’m not sure how confident I would be with the weld holding forever and not dropping a piece of weld down into the valves or combustion chamber, I think for peace of mind the $20-30 for a used stock TB would be worth it.
    Last edited by TransAmX2; 08-03-2011 at 07:43 AM.

  2. #2
    Spaz is My Mentor SMWS6TA's Avatar
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    Op, we both have very similar circumstances.

    When I got mine it was 98% stock (It had an aftermarket muffler) and I also have 145k on it today.

    In the last 2 1/2 yrs I've done all in my signature.

    My idle has always been at 1000 rpm, even with a Frost tune. I have also tried switching the TB blade to the stock blade on my P&P TB, have also replaced the IAC and still nothing has changed. I've even adjusted the throttle screw and only made the idle worse (higher).

    It may be that a batch of the TB blades have been drilled incorrectly. I say this because I matched up both blades and they both line up as well as the holes and both are the same size. I do not know what yr the P&P TB is from, just know it is a LS1 TB.

    In regards to the IAC, if you disconnect it it will not move. At least mine did not. I did pull the piece out to get as close to the hole to see if it would affect the idle. The answer is yes; it caused a couple of stalls. Then after a couple of restarts it position itself back to where it was and still idles at 1000 rpm.

    I too have thought of filling in the hole with a bolt & nut that was loctite and do another hole that is the correct size, but I also wanted the options of selling this TB if someone wanted a stock TB for porting.


    Subscribed for responses.....
    Last edited by SMWS6TA; 08-03-2011 at 12:46 PM.

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    Moderator Cutlass's Avatar
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    SMWS6TA is correct, the iac won't move when unplugged. It'll stay exactly where its at, so that theory is out the window. If you can get a hold of a scan tool that lists IAC counts, that can tell you something about whats going on. Typically if the idle is higher then what the computer wants, the IAC counts will be zero...meaning the IAC is completely closed. And that of course means that there is air getting in somewhere else (like a vacuum leak or an enlarged TB blade hole.

    I like the idea of putting a piece of tape over the hole just for a quick test. Just don't drive it. Even if the tape some how does get sucked into the engine it won't do any damage, I'd think.

    And FWIW, I did find out that the older Cadillac Northstar V8 throttle bodies are the same size as our F-body and have the same screw spacing. They also have no hole drilled in the blade. So if you try a blade with no hole, that's a path you could try. I used to have one of those laying around. I wonder if I can find it.

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    Member TransAmX2's Avatar
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    ok so my IAC self test didn't prove anything yet, good to know.

    I'll have to try a few other tests tonight if I have time, I'll let everyone know if I come up with anything interesting.

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    Spaz is My Mentor SMWS6TA's Avatar
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    Cutlass, in my research of this problem I've come across that some 98's were given the C5 cam due to shortage of cams in 98'. Could this different cam be the issue?

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    Moderator Cutlass's Avatar
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    If not properly tuned, sure it could.

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    Member 28Marcos's Avatar
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    1000rpm idle is high for a factory cam car, unless thats what the mail order tune does, im not sure. my car idles at 500 rpm but when it idles near 1000 i always have a vacuum leak.
    u said u already did everything you could think of and let me help u out with a checklist of what I did:
    air vaccuum line has a leak, line runs from valve cover way up in the dash lol then around to driver valve cover, u'll be suprised if the line is rotted in half, mine was.
    throttle position sensor? 50 dollar part but just return it if its not the problem.
    pcv valve needs to be replaced
    no codes are being thrown?
    maybe a gasket for the intake was pinched during the installation or just a bad gasket? throttle body gasket?
    try the old fashion way and spray brake cleaner with the engine running and pay attention if the engine revs up, thats how you can see if any lines have a hole.
    good luck

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    Veteran 0rion's Avatar
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    did you guys have the high idle prior to your P&P throttle body? If it's ported too much that will cause your high idle. Like cutlass said....you really need hptuners or a good scan tool so you can see iac counts and idle trims and things like that. It would definitely make trouble shooting this easier.

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    Spaz is My Mentor SMWS6TA's Avatar
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    98 T/A w/a little mods...

    Will you see these on a Dynojet? I'm fixing to do 3 pulls on one around the end of the month.

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    Moderator Cutlass's Avatar
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    I doubt it. Unless they are tuning at the same time...then maybe. Otherwise they just hook up a pickup for the rpm and run the thing WOT to see how much HP you make.

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    Member TransAmX2's Avatar
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    Quote Originally Posted by 28Marcos View Post
    1000rpm idle is high for a factory cam car, unless thats what the mail order tune does, im not sure. my car idles at 500 rpm but when it idles near 1000 i always have a vacuum leak.
    u said u already did everything you could think of and let me help u out with a checklist of what I did:
    air vaccuum line has a leak, line runs from valve cover way up in the dash lol then around to driver valve cover, u'll be suprised if the line is rotted in half, mine was.
    throttle position sensor? 50 dollar part but just return it if its not the problem.
    pcv valve needs to be replaced
    no codes are being thrown?
    maybe a gasket for the intake was pinched during the installation or just a bad gasket? throttle body gasket?
    try the old fashion way and spray brake cleaner with the engine running and pay attention if the engine revs up, thats how you can see if any lines have a hole.
    good luck
    Had high idle prior to P&P TB (I did not port the IAC Bypass port)
    I have not done a carb/choke cleaner spray check (didn't have any on hand) but I will just to cover that (I won't use brake parts cleaner)
    I have physically checked all lines and hoses (I believe)
    No EGR and entire PCV system is new and connected correctly
    Haven't tried the TPS, but i was going to look up what the readings should be at idle and just run my multi meter off it to see what signal it's sending.
    no codes thrown
    all gaskets were and are good (idled high before intake swap, idles high after)

    on a side note i did put a piece of duct tape over the throttle blade hole and it didn't change anything, i started it up and and the idle came down for 1/2 a second then went right back up, seems like the IAC must have opened up more to allow more air in. When I have the time I'm going to meter out all the sensors on the TB and just get a measurement of what signals they are all sending at idle and compare them to what it should be at stock and see if I have any red flags.
    haven't had time to work on it much lately, it's been really hot and humid out when i get out of work and my weekends have been really busy.

  12. #12
    Spaz is My Mentor SMWS6TA's Avatar
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    Quote Originally Posted by TransAmX2 View Post
    Had high idle prior to P&P TB (I did not port the IAC Bypass port)
    1. I have not done a carb/choke cleaner spray check (didn't have any on hand) but I will just to cover that (I won't use brake parts cleaner)
    2. I have physically checked all lines and hoses (I believe)
    3. No EGR and entire PCV system is new and connected correctly
    4. Haven't tried the TPS, but i was going to look up what the readings should be at idle and just run my multi meter off it to see what signal it's sending.
    no codes thrown
    5. all gaskets were and are good (idled high before intake swap, idles high after)

    6. on a side note i did put a piece of duct tape over the throttle blade hole and it didn't change anything, i started it up and and the idle came down for 1/2 a second then went right back up, seems like the IAC must have opened up more to allow more air in.

    1. Did it no changes in idle.
    2. Done this several times
    3. Egr/Air deleted during headers instal, have LS6 VCT & catch can - no changes in idle
    4. Same here, but have changed the IAC. I have done the TSP reset and still no change.
    5. Gaskets good, used new ones when installing LS6 Intake.
    6. Did the same this weekend and had the same reaction.

    I also too it to a shop and see if they could look for a leak. They didn't find any.

  13. #13
    Moderator Cutlass's Avatar
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    I have had a high idle on different car that was caused by a TPS reading a little high. I tried a new sensor and had the same results. So I ended up oblonging the screw holes on the TP sensor and repositioning the TPS on the throttle body so it would report a lower voltage to the PCM. worked great!

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    Member 28Marcos's Avatar
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    brake parts cleaner lol brain fart! haha
    just throwing another idea out there but i wonder if it has anything to do with something in the transmission..
    but i have a buddy who was having an issue with his 01 ws6 where it had an irregular stumble to the idle. originally he had two breathers one on each valve cover but once he got rid of those and reinstalled oe equipment it went away. maybe it has something to do with that??

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