Manual idle adjustment

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  1. #1
    Member anyoldiron's Avatar
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    Manual idle adjustment

    l thought my idle was fine on my 85 until i cleaned the intake, EGR system and took it for its ltalian tune up, cam home and it was 1100rpm idle and 1600rpm cold start the next day. l looked around for instructions and could find any (no that hidden screw didn't change anything)
    l played around with the linkages and it changed nothing.......................then i found these 2 grub screws and captive nuts. Apologies for lack of accurate direction as it was on going (yellow arrows point).

    1. Car wants to be proper warm.

    2, loosen both lock nuts

    3, leaving the engine running fiddle with the screws, i cannot remember which way is up or down, but YOU HAVE TO MOVE BOTH SCREWS to get a change.

    4, When you have blipped the throttle a few times and are happy with the rpm, tighten the locknuts. When you do this it WILL alter the RPM, you have to 'load' the screw, a bit like tappets on old cars and bikes.

    l did this a while ago and now it starts cold at 1300rpm and idles at 800rpm
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  3. #2
    CelicaSupra.com Member williamb82's Avatar
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    Sounds like the passage on the TB from the valve cover is clogged. That hidden screw should have adjusted the idle.
    william
    Black 84 7mgte/R154, MS2, truetrac, bbk, etc...soon to have 2jzgte and t56
    Blue 85 Beater. TRD Header, HKS Exhaust, custom 3in intake pipe, cone filter
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  4. #3
    Member anyoldiron's Avatar
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    Quote Originally Posted by williamb82 View Post
    Sounds like the passage on the TB from the valve cover is clogged. That hidden screw should have adjusted the idle.
    lt isnt, that much i do know.

    ....................i do way more reading than posting and that screw subject is bordering on a oil subject, but since you opened the door.
    OK lts not a true idle adjustment screw, more a 'air screw'. Yes it can effect the idle, but not in correct way. This is the route ive gone down and are happy with the results.

    Here is some concise information from a source with more post count than me that may resonate better with you. This is according to 'Hoppyjim,' Jim Hopkins an authority on Toyotas ;


    On ALL Cressidas, Supras, etc., with an electronic idle speed control
    (ISC) motor, the air passage adjusting screw in the throttle body must
    be turned in fully clockwise, seating the screw and completely closing
    off the air passage around the throttle plate.

    This idle air passage and adjustment is used to set the idle speed on
    engines that do NOT have an electronic idle speed control motor. It
    serves no function on engines with an idle speed control, as it only
    parallels the air passage regulated by the idle speed control (ISC).
    The useless screw, and it's air passage around the throttle plate was
    eventually deleted from later throttle bodies (it is not found on
    7M-GE models), because it is redundant. If you back out the idle speed
    adjusting screw in the throttle body on your Cressida with a computer
    controlled idle speed, you will open the air passage, which will act
    just like a throttle plate that sticks a little further open and lets
    more air into the engine. If this screw is backed out, there will be a
    parallel air path created around the throttle plate and the idle speed
    controller.

    If you back the screw out a little the computer may be able to
    compensate for the additional air by closing the idle speed controller
    down a little. However, if you back the screw out too far, and open
    the air passage far enough the idle speed control (ISC) will be forced
    close down completely in an attempt to adjust the idle speed, and
    reduce it down to the target value. If you back out the screw even
    further, the engine will then idle faster than the computer's
    programmed idle speed, but the idle speed control will be unable to
    reduce the idle speed further. If you stay within the idle speed
    control's range of compensation, the system can usually overcome the
    idle air leak you have created. However, you will have only succeeded
    in forcing the computer controlled automatic idle speed control (ISC)
    to drop a couple of notches further closed to compensate for the
    additional air leak. Unfortunately, this compensation will only occur
    when the engine has warmed up a little, and as long as the idle speed
    control is functioning is in closed loop.

    However, at start up when the idle speed control reverts to it's full
    open (default) position, the car may be harder to start, as more air
    will enter the engine leaning out the cold start injector's initial
    squirt of fuel. At start up the engine will also initially idle much
    faster wasting fuel, making the initial fan noise at start up even
    more pronounced. The faster initial idle RPM at start up will also
    increase engine wear as there is already an inherent lack of
    lubrication on initial start up.

    As well, the higher idle RPM at start up will increase shift shock
    when "D" or "R" is engaged after initial start up. Even the "anti
    squat" feature of the automatic transmission, which momentarily
    engages second gear when the brake pedal is depressed, and the shifter
    is moved from "N" to "D" may not be able to cope with the additional
    cold engine RPM caused by the air leak in the throttle body. The
    computer and the idle speed control will do it's best to conceal an
    idle speed passage that has been caused to leak by inadvertently
    unscrewing it, but the system can only do so much to remedy the
    situation.

    I have heard countless tales of how mechanics discovered this amazing
    secret throttle body screw that the engineers apparently overlooked,
    and all the magic it can do. Forget it! Any benefit is hogwash. On
    cars with an ISC, turn the idle screw all the way in where it was
    originally set at the factory, and leave it alone. If there is an idle
    speed control screw on your Cressida's throttle body, be sure that it
    is fully seated, then ignore it, as it serves no purpose.

    With a warm engine, when the screw is initially backed out, the engine
    will idle slightly faster, until the computer closes the idle speed
    controller down to compensate for the increase in idle speed. When the
    screw is initially turned inward, the engine will idle a little slower
    until the computer responds and opens the idle speed controller to
    compensate for the decrease in idle speed. There is a second or so of
    computer response time delay to prevent the engine idle RPM from
    aimlessly cycling up and down, hunting for the exact target idle
    speed. Do not mistake this time delay, or lag in computer response
    time for any real RPM change that turning the idle screw may have made
    to hot idle speed.

    The engine computer will control the idle speed. These engines are
    very frustrating as they are so well engineered that there is no Holy
    Grail of that little screw that only needs part of a turn to make your
    car perform better. There is no mention in any repair manual of ever
    tampering in any way with this screw. Don't mess with a good design.
    Make it run as the engineers designed it to run, and it will run very
    well indeed.

    JIM HOPKINS
    Last edited by anyoldiron; 06-25-2019 at 06:42 PM.

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  6. #4
    CelicaSupra.com Member williamb82's Avatar
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    He confirmed it will adjust the idle in that long diatribe. It may not be the recommended method, but it does the exact same thing as what you did, as technically, after adjusting those screws, you now need to re-calibrate the TPS, which puts you right back at square one. So backing the screw out to mimic a throttle plate that is further open(which is stated in what you posted), or adjusting the throttle stop to actually hold the throttle plate further open as you did, does the exact same thing, and has the exact same end result. You just went about it a more difficult way.The ECU is still going to try to compensate for the raised idle by shutting the IASCV further to lower the idle speed, until you have opened it enough that even with the IASCV fully closed, the idle will stay above the RPM the ECU tries to meet.
    william
    Black 84 7mgte/R154, MS2, truetrac, bbk, etc...soon to have 2jzgte and t56
    Blue 85 Beater. TRD Header, HKS Exhaust, custom 3in intake pipe, cone filter
    Black 86 Triple Weber 6m Eventually
    Rusty 85 Frankenstein 6M/w58 with DIYPNP

  7. #5
    Member anyoldiron's Avatar
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    Quote Originally Posted by williamb82 View Post
    He confirmed it will adjust the idle in that long diatribe. It may not be the recommended method, but it does the exact same thing as what you did, as technically, after adjusting those screws, you now need to re-calibrate the TPS, which puts you right back at square one. So backing the screw out to mimic a throttle plate that is further open(which is stated in what you posted), or adjusting the throttle stop to actually hold the throttle plate further open as you did, does the exact same thing, and has the exact same end result. You just went about it a more difficult way.The ECU is still going to try to compensate for the raised idle by shutting the IASCV further to lower the idle speed, until you have opened it enough that even with the IASCV fully closed, the idle will stay above the RPM the ECU tries to meet.
    Bollocks, my way is telling the engine what to do, your way is trying to tell the engine how to do it. its really as simple as that. Otherwise the thing would not be running right would it. You are really trying to tell me that adjusting the air mix and adjusting the throttle is the same.........LMAO

    Anyway, have at it, im done........................the cars just about finished and trying to contribute to this forum is like pissing into the wind.

  8. #6
    CelicaSupra.com Member supraz's Avatar
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    So for someone who does not know much about the throttle body and linkage, what are the functions of these two adjustments?

    I had a quick look and from what I can see, the lower stop is for a dashpot that slows the throttle closing after closing the throttle.
    Is the upper adjustment there to act as a linkage stop to prevent twisting of the throttle shaft? Is there an actually throttle blade stop in the throttle body itself? I can't find any adjustment information in the TSRM so I would perhaps naively assume that you make sure the throttle blade stop and the linkage stop are adjusted so that they stop at the same point. Or is this the only throttle blade stop present?

    I'm agnostic here. I don't know enough about the throttle body to add anything to the discussion. I'm just trying to understand what is being discussed.

    Dale
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  9. #7
    CelicaSupra.com Member williamb82's Avatar
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    Adjusting the air mix? You do realize the throttle body is NOT a carburetor right? It doesn't change the air mix at all to adjust that screw, as it is still ingesting the same air that has been metered by the maf. So whether the throttle plate is cracked further open, or that bypass screw is backed out to allow air to circumvent the throttle plate, it does the SAME EXACT THING! I know you made this post trying to be helpful and sound smarter than everyone else, but you have failed miserably. You went around your ass to scratch your elbow!

    Quote Originally Posted by anyoldiron View Post
    Bollocks, my way is telling the engine what to do, your way is trying to tell the engine how to do it. its really as simple as that. Otherwise the thing would not be running right would it. You are really trying to tell me that adjusting the air mix and adjusting the throttle is the same.........LMAO

    Anyway, have at it, im done........................the cars just about finished and trying to contribute to this forum is like pissing into the wind.
    william
    Black 84 7mgte/R154, MS2, truetrac, bbk, etc...soon to have 2jzgte and t56
    Blue 85 Beater. TRD Header, HKS Exhaust, custom 3in intake pipe, cone filter
    Black 86 Triple Weber 6m Eventually
    Rusty 85 Frankenstein 6M/w58 with DIYPNP

  10. #8
    CelicaSupra.com Member
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    I retract!

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