Chevrolet Full Size Cars 1979-1989

Throttle Body Injection (TBI)

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MINIMUM IDLE SPEED ADJUSTMENT





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Fig. Fig. 1 For access to the idle stop screw, pierce the cap with an awl and carefully pry the cap from the side of the throttle body

Beginning in 1985, some engines were available with a throttle body fuel injection system. All throttle body injected vehicles are controlled by a computer which regulates idle speeds and supplies the correct amount of fuel during all engine operating conditions. No periodic adjustments are necessary. However, if throttle body is replaced AND a proper idle speed cannot be obtained, there is an adjustment which may be made. An idle stop screw is set at the factory and then covered to discourage tampering. If all other components of the fuel system are working properly and there is still a problem with the idle speed, the screw may be adjusted.

Incorrectly adjusting the minimum idle speed stop screw will result in the IAC valve pintle to constantly bottom on its seat leading to an early valve failure.

  1. Set the parking brake and block the drive wheels.
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  3. Start and run the engine until it reaches normal operating temperature. Make sure all accessories are turned OFF.
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  5. Stop the engine, then disconnect and plug any vacuum lines, as required.
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  7. Using an awl, pierce the idle stop screw cap and carefully pry the cap from the throttle body. The cap and screw can be found on the opposite side of the throttle body from the Idle Air Control (IAC) valve and Throttle Position (TP) sensor.
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  9. Connect a tachometer to the engine.
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  11. Ground the diagnostic terminal of the Assembly Line Data Link (ALDL) connector by using a jumper wire to connect the terminal to the Electronic Control Module (ECM) system ground terminal. The diagnostic and ground terminals are the 2 top right terminals on the ALDL connector which is found under the dash, near the steering column.
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  13. Turn the ignition
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ON , but DO NOT start the engine. Wait at least 45 seconds for the IAC valve pintle to extend and seat in the throttle body.

  1. With the ignition
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ON and the ALDL test terminal still grounded, unplug the IAC valve connector. This will keep the pintle extended throughout the procedure and prevent the ECM from adjusting idle speed using the valve.

  1. Remove the ground from the ALDL terminal, then disengage the distributor set timing connector in order to prevent the possibility of engine speed changing during the procedure due to timing changes.
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  3. Place the transmission in Neutral, then start the engine and allow it to idle.
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If equipped, double check to make sure that the cruise control cables do not hold the throttle open.

  1. Wait until the idle stabilizes, then check and adjust the idle speed as necessary:
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    1. For 1985 vehicles, set the idle screw to obtain 500-600 rpm.
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    3. For 1986 vehicles, have an assistant place the transmission in Drive and apply the brake pedal, then set the idle speed to 400-450 rpm.
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    5. For 1987-88 vehicles, set the idle screw to obtain 400-450 rpm.
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    7. For 1989 vehicles, set the idle screw to obtain 400-450 rpm for the 4.3L (VIN Z) engine or to 450-500 rpm for the 5.0L (VIN E) and 5.7L (VIN 7) engines.
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  3. Turn the ignition
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OFF , then reconnect the IAC valve and distributor timing wiring.

  1. For 1985 vehicles, turn the ignition
  2.  

ON , but do not start the engine, then adjust the TP sensor using a digital voltmeter to backprobe terminals A and B of the TP sensor connector. If necessary loosen the sensor retainers and pivot the switch in order to achieve an output of 0.525-0.075 volts, then tighten the retainers to secure the switch. Make sure the switch does not move when tightening the retainers or adjustment will be lost.

  1. Use silicone sealant to cover the idle stop screw, then reconnect and vacuum lines which were plugged for the procedure.
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  3. Start the engine and check for proper idle operation.
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