Function Of Too Much Air Or Too Little Air, Iab Valve And Idling Of 94-95 Gt's

Discussion in '1994 - 1995 Specific Tech' started by from6to8, Jul 9, 2013.


  1. from6to8

    from6to8 There's suction so I used that end O_O

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    'Anyone know the function of having too much air i guess as to the more than 2 turns and having too little air as to the less than 1/2 turn on the IAB screw? I reset my idle and have about 1 turn, maybe slightly more than a turn on the IAB screw and the car's idle hangs when come to a stop and sometimes die but will crank back up. It idles fine going out of a driveway now, no bucking or jerking. I do notice though it seems to maybe be slightly high, though its idling at about 1000 but it almost , well i can hear the idle seeming to be revved up some on it's own just a little bit. I will just turn the screw close all the way and try it and see how it does then and make adjustments counterclockwise in small increments from there depending on how it's doing after closing the screw. I just wanted to hear you guys take on the amount of air being too little or too much and it's effect on idle
  2. toyman

    toyman Active Member

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    The screw on the TB is air bleed and is used just to tweak the idle. These cars are controlled by the computer (ECU) so you need to understand some basics before generally messing with things. First, the idle is a command parameter in the ECU. The computer needs to know how much air is required to match the fuel at various loads from idle through WOT. The MAF provides the signal to measure the air mass and the Intake Air Temperature (IAT) sensor signal allows the (ECU) adjust the air mass calculation based on the air temperature. The feedback to the ECU comes from the O2 sensors which tells the ECU how rich or lean the fuel mixture is by measuring the amount of oxygen remaining in the exhaust gasses. The ECU then can adjust the fuel to the air mass by shortening or lengthening the injector pulse width. Now to the Idle Air Control (IAC) valve (not IAB). The IAC is pulsed by the ECU to open and close (there is no in between position) to allow the correct amount of air to meet the targeted AFR at the commanded idle of about 650 rpm or 700 rpm for an auto. This AFR target value is 14.7 and is calculated based on the O2 feedback. The ECU knows when the engine is idling from the TPS signal. Then there is the Engine Coolant Temperature (ECT) sensor. The signal from this sensor tells the ECU if the engine is cold or warm and adjusts the fuel based on this signal as well at idle.

    All these sensors, to function properly, require good circuits. The circuit that is usually overlooked is the ground side. That's why the battery to engine and engine to chassis grounds are critical. Because of the interaction between the signals from all these sensors is why the ECU stores Data Trouble Codes (DTCs) if a part is not giving a signal within it's specified parameters. So, one needs to check for codes to see if the ECU has logged an event that may explain the issue. The point to all this is to show that guessing at possible fixes is not the best way to diagnose an issue. You'll waste time and end up often replacing good parts based on the guess.
  3. from6to8

    from6to8 There's suction so I used that end O_O

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    cool. Yeah i will turn that IAB screw closed all the way and adjust increments and see how it does. The throttle stop screw is adjusted correctly

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