Throttle Body: Adjustments
NOTE: The minimum idle air rate is set at the factory with a stop screw and is not normally readjusted unless repairs or replacement of the throttle body assembly has occurred. Engine idle speed is computer controlled through the IAC valve and is not adjustable.If there is a complaint of high idle speed, vacuum leaks should be considered the most likely cause. The Electronic Control Module (ECM) controls the Idle Air Control (IAC) valve, and it is less likely that a stalling complaint would be due to incorrect minimum idle speed. If either occurs see DIAGNOSIS BY SYMPTOM.
If the Minimum Air Rate is suspected, be sure the Idle Air Control (IAC) valve is not "lost" (not actually at the location indicated by current IAC "counts"). The IAC valve could be "lost" if ECM power has been interrupted with the ignition "ON," or the IAC valve has been disconnected with the engine running since the last reset.
CAUTION: To prevent engines from running at high speeds, be sure that the ignition is OFF and the vehicle is in PARK or NEUTRAL before connecting Idle Air Control (IAC) assembly, or installing/removing idle air passage plugs. Failure to do this may result in vehicle movement, which can result in personal and/or property damage.
MINIMUM IDLE AIR RATE CHECK
1. START engine and bring it to normal operating temperatures 185-212 degrees Fahrenheit (85-100 degrees Celsius). Turn engine OFF.
2. Remove air cleaner assembly. Remove hoses and plug any possible air sources on tube manifold assembly.
3. Disconnect throttle cable.
4. Ground diagnostic "test" terminal in ALDL connector. (jumper A to B).
5. Turn ON ignition, DO NOT START engine. Wait at least 10 seconds (this will allow the IAC pintle to extend and seat in the throttle body).
6. With ignition ON, engine stopped, test terminal still grounded, disconnect IAC valve electrical connector. (This disables the IAC valve in the seated position). Remove the ground from the diagnostic terminal.
7. Connect a scan tool to the ALDL connector and place in open mode. If a tool is not available connect a tachometer to the engine.
8. START engine. With transmission in NEUTRAL, allow engine rpm to stabilize. It may be necessary to depress the accelerator in order to start the engine.
9. Idle speed should be 550 to 650 RPM. Adjust idle stop screw if necessary. To remove the soft plug, puncture with an awl and pry out.
10. If the minimum idle air rate is within specifications, no further check is required.
Stop Screw Plug Removal:
11. Turn ignition OFF and reconnect IAC valve electrical connector.
12. Disconnect the scan tool or tachometer.
13. The IAC valve will be reset by the computer when the vehicle speed exceeds 43 MPH while the engine speed is above 2000 RPM, or when the diagnostic test connector is grounded when the engine RPM is above 2000 RPM. SEE IDLE AIR CONTROL RESET PROCEDURES.
14. Install air cleaner, adapter, gasket and vacuum hoses.
NOTE: The minimum idle air rate is set at the factory with a stop screw, which is covered with a tamper plug. This setting allows enough air flow by the throttle valves to cause the IAC valve pintle to be positioned a calibrated number of steps (counts), from the seat, during "controlled" idle operation. If the IAC valve is disconnected and reconnected with the engine running, the idle speed may be wrong. In this case, the IAC valve can be reset by doing the following: The IAC valve will be reset by the computer when the vehicle speed exceeds 43 MPH while the engine speed is above 2000 RPM or when the diagnostic test connector is grounded when the engine RPM is above 2000 RPM.
Engine idle speed is computer controlled through the IAC valve, by driving the IAC pintle in and out. Engine idle speed is a funtion of total air flow into the engine based on: IAC valve position, throttle valve opening (idle stop screw), calibrated leakage through the accessories, [1][2]PCV flow. "Controlled" idle speed is programed into the ECM, which determines the correct IAC valve pintle position to maintain the desired idle speed.