ENGINE PERFORMANCE II

LESSON 8

IDLE SPEED CONTROL

The PCM controls idle speed to ensure good driveability. The PCM will only attempt to maintain the desired idle speed when it “knows” the throttle is closed. A throttle switch or the TPS input is used to determine when the throttle is in the idle position. Motors and solenoids are used by most PCM’s to control idle speed. Many PCM’s are programmed to attempt to control the idle speed by adjusting spark timing and then by controlling the idle speed actuator. Some computers of the early 80's did not control idle speed.

The PCM adjusts idle speed for a variety a reasons:

cold engine start-up, transmission engagement, engine overheating,

A/C engagement, low system voltage, high power steering pressure

The PCM uses the idle control actuator to allow the engine speed to drop slowly when the throttle closes rapidly to avoid engine stall.

(Dashpot effect)

IDLE SPEED CONTROL METHODS

1. Throttle opening control –

opens and closes throttle to control idle airflow

PM reversible motor - used on early TBI systems. The PCM moves the throttle with a motor plunger that extends and retracts. The motor plunger has a throttle switch or idle tracking switch which tells the PCM when the throttle is closed.

Throttle-by-Wire systems use the throttle actuator PM motor to control idle speed.

2. Throttle bypass air control –

opens and closes an air passage, which bypasses the throttle, controlling idle airflow

Stepper motor - moves a pintle which opens and closes the air passage. Stepper motor position is reset by the PCM during the drive cycle by running it all the way in and re-establishing the “zero” position. Used by GM & Chrysler

Scan tool PID in steps: 0 steps bypass air is closed,

15-35 steps is normal at idle

Bypass solenoid valve - a duty cycle controlled valve which controls airflow bypassing the throttle. The greater the duty cycle the longer the valve is open and the higher the idle speed command. Used by Ford and Chrysler.

Scan tool PID in duty%: 0% bypass air is closed,

15-35% is normal at idle

PM reversible motor - the PCM controls airflow using a pintle or rotary valve by reversing motor polarity. Not common.

3. Spark timing control –

the PCM advances timing to increase idle speed and retards timing

to decrease idle speed, before using the idle speed actuator to

control idle speed.

Idle quality and control can be affected by:

Vacuum leaks -

PCV system failures -

EGR valve leaks -

Carbon build-up in the throttle body -

Many PCM’s require an idle re-learn after its power or idle control actuator has been disconnected. (Check service manual for specific procedures)

Idle Speed Adjustments

Idle speed is not adjustable on most PCM controlled systems. The throttle stop and actuators must be adjusted however, to assure proper system performance.

Most systems require at least one of the following adjustments:

A. Minimum air rate or minimum authority - this is set to a specific engine RPM on a warm engine when the bypass air is completely blocked or the throttle control actuator is fully retracted. This adjustment is accomplished by turning the throttle stop screw, correctly positioning the closed throttle. This allows the PCM to accurately control idle speed.

B. Maximum Authority - this is set on a fully warm engine to a specific

RPM when the throttle control actuator is completely extended. It is accomplished by turning the plunger screw. This adjustment sets the maximum speed the PCM can command. Only applicable with throttle opening controlled systems.