Idle Speed: Adjustments

Throttle Body Assembly:

Curb idle and fast idle are controlled by the EEC system by way of the idle speed bypass valve and cannot be adjusted. Changes in idle speed usually occur because of problems elsewhere. Before performing idle speed setting procedures described in this article, eliminate these possible causes of idle speed problems:

1. Contamination within the throttle bore.
2. Contamination within the idle speed bypass valve.
3. Contaminated or defective oxygen sensor.
4. Throttle sticking or binding.
5. Engine not reaching operating temperature.
6. Vacuum leaks (air intake manifold, vacuum hoses, vacuum reservoirs, power brake booster, etc.).

PREPARATION

1. Prepare vehicle as follows:
a. Transmission in "P" or "N" (A/T) or neutral (M/T).
b. Parking brake applied (automatic brake release disconnected where applicable).
c. Wheels blocked.
d. Cooling system filled.
e. Engine at operating temperature.
f. Heater and accessories "OFF."
g. Throttle lever resting on throttle plate stop screw.
h. Ignition timing system operating properly.

2. Perform ON-BOARD DIAGNOSTICS (EEC QUICK TEST) and resolve any malfunctions indicated by service codes (bypass this step if you were sent to this section by an EEC Pinpoint Test).
3. With engine "OFF," disconnect negative battery terminal for a minimum of 5 minutes and then reconnect cable.
4. Start engine and allow to stabilize for 2 minutes. Goose throttle and let it return to idle. Lightly depress and release the accelerator and let engine idle. If engine idles properly, check is complete.
5. If idle is not correct, shut engine "OFF" and go to setting procedures.

SETTING PROCEDURES

1. Disconnect idle air bypass valve electrical connector.
2. With transmission in "P" (A/T) or neutral (M/T), run engine at 2500 rpm for 30 seconds and then let idle for 2 minutes.
3. Check idle rpm. If necessary, turn throttle plate stop screw to obtain 675 rpm.
4. Shut engine "OFF" and repeat steps 2 and 3.
5. Shut engine "OFF" and disconnect battery for a minimum of 5 minutes.
6. With engine "OFF," reconnect idle air bypass valve electrical connector. Verify that throttle is not stuck or binding and that linkage is not preventing throttle from closing.
7. Start engine and allow to stabilize for 2 minutes. Goose throttle and let it return to idle. Lightly depress and release the accelerator and let engine idle. If problem still exists, refer to DIAGNOSIS BY SYMPTOM.

NOTE: For step 3, if you must turn the throttle stop screw in, shut engine "OFF" and make an estimated adjustment. Start engine and repeat steps 2 and 3 before continuing.

For step 7, a condition may occur where engine rpm will oscillate. This can be caused by the throttle plate being open enough to allow purge flow. To verify this condition, disconnect carbon canister purge line and plug it. If purge is present, the throttle plate must be kept closed until purge flow induced idle oscillations stop.