Idle Speed: Adjustments

CURB IDLE SPEED ADJUSTMENT
Explanation
Curb idle speed (RPM) is controlled by the Electronic Engine Controls (EEC IV) processor and the idle speed control device. If the control system is operating properly, these speeds are self-compensating and cannot be changed by traditional adjustment techniques.

Adjustments are sometimes required to establish the correct operating limits within which the Idle Speed Control (ISC) system can properly function. Misadjustment of the operating limits could restrict the operating range of the ISC system, so adjustments to the system should never be made unless a specific causal factor has been identified.





Procedure
- Set parking brake and block wheels. Place the transmission in Neutral/Park.
- Bring the engine to normal operating temperature and turn off.
- Turn the A/C-Heat selector to the Off position.
- Verify that the throttle linkage is free, that travel is unobstructed and that the cruise control cable (if so equipped) is not holding the throttle open.
- Check for vacuum leaks.
- Place transmission in Neutral and operate engine at 1800 rpm for at least 30 seconds.
- Place transmission in Drive (A/T) or leave in Neutral for (M/T). Allow engine to stabilize and check idle speed within 30 seconds. If engine is idling longer than 30 seconds, repeat two previous steps.

NOTE: After 3 seconds the idle speed may increase because of strategy parameters.

- If curb idle speed is equal to or less than specification, do not adjust. Specifications
- If idle speed exceeds specification turn engine Off and disconnect the negative (-) terminal of the battery for 5 minutes and then re-connect. Repeat three previous steps.
- If idle speed still exceeds specification, perform EEC IV Quick Test and resolve any indicated defects. Testing and Inspection
- If idle speed still exceeds specification:
- Back out idle air set screw until the idle speed reaches 600 +/- 20 rpm (A/T) or 700 +/- 20 rpm (M/T).
- Then back out the curb idle screw one-half additional turn to bring the throttle plate linkage into the nominal operating range of the ISC system.