AEM 30-6905 Universal Programmable EMS-4 - Manual (Page 150)

Page 150 of 279 EMS-4 Install and Tuning Guide_Rev 1.6
The Start Extra vs Temp and Start Extra Decay tables work together. Both are vs coolant
temperature. An enrichment is applied from the Start Extra vs Temp and decays away
according to the time defined in the Start Extra Decay table.
Idle Air Control
Idle air control (IAC) systems are used to stabilize idle speed during cold engine and after
warm-up operations. The IAC system regulates the volume of air bypassed around a closed
throttle butterfly valve. The EMS controls the IAC system by applying various input signals from
the user-programmable software file. There are two major types of IAC systems. The stepper
idle air control motor is, typically used by Toyota, Dodge, Mitsubishi, etc. The pulse width idle air
control valve is commonly used by Honda, Subaru, Ford, Nissan, Volkswagen, etc. EMS-4
systems are only compatible with pulse width (PWM) style control valves.
The pulse width duty cycle valve regulates air bypass volume by utilizing a duty cycle controlled
solenoid. This electric solenoid controls an air valve which blocks passage of air from the intake
pipe to the intake manifold. Since this solenoid is incapable of flowing high air volume, a
separate mechanical air valve is typically used to perform cold fast idle. With this type system,
the EMS varies bypass airflow by changing the duty ratio of the command signal to the IAC
valve. By increasing the duty ratio, the EMS holds the air bypass valve open longer, causing an
increase in idle speed.