How to Reset the Baseline Idle in a GM TBI

by Lee Sallings

The GM TBI (Throttle Body Injection) system uses a pair of low pressure fuel injectors mounted above the throttle body opening. Unlike a port fuel injection system, that sprays fuel into the intake manifold runner just above the intake valve, the injectors in this system spray fuel directly into the throttle body above the throttle plate. The air gap between the throttle body and the throttle plate is calibrated to allow enough air to pass by it for the engine to run at base idle speed. The IAC (Idle Air Control) valve utilizes the on-board computer to fine tune the idle speed according to engine conditions.

Step 1

Remove the wing nut on top of the air cleaner and lift the air cleaner off the throttle body to access the vacuum lines. These are attached to the bottom of the air cleaner. Twist and pull the vacuum off of the bottom of the air cleaner and lay the air cleaner aside. Plug the vacuum lines with golf tees or other suitable item.

Step 2

Remove plug over the idle screw located on the right front of the throttle body by using a hammer to drive a punch into the plug and then using the punch to pry the plug out of the throttle body. Discard the plug. Start the engine and let it run until it warms up.

Step 3

Turn off the engine and leave the ignition key in the "Off" position. Unfold a paper clip and bend it into a "U" shape. Push the ends of the paper clip into the "A" and "B" terminals of the ALDL (Assembly Line Diagnostic Link) connector, also called the diagnostic connector. In GM vehicles that use this TBI injection, the connector is a black plastic rectangular connector located under the driver side of the dash. The "A" and "B" terminals are located on the top right of the connector.

Step 4

Unplug the IAC valve located on the passenger side, rear of the throttle body. Start the engine. Insert a T-30 size Torx driver into the Idle adjustment screw that is located behind the plug removed earlier. Turn the screw to adjust the idle speed to 650 RPM +/- 50 RPM.

Step 5

Turn off the engine and disconnect the negative battery terminal from the battery using a wrench. Reconnect the IAC valve. Reinstall the air cleaner and remove the paper clip from the ALDL connector. Reconnect the battery terminal after 5 minutes and test drive the vehicle.

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