Idle control maintains stable engine speed when the vehicle is stationary. The ECU regulates airflow (via throttle or idle air control valve), ignition timing, and fuel delivery to achieve target idle RPM under varying loads.

Overview

Target idle speed varies with coolant temperature, A/C load, electrical load, and transmission state. The ECU uses closed-loop control to maintain RPM within a tight window.

Controlled Signals

• Throttle position / IAC valve
• Ignition timing
• Fuel injection
• A/C compressor clutch
• Alternator load

Maps Involved

• Target Idle RPM Maps vs coolant temp
• Idle Air Correction Maps
• Idle Ignition Timing Maps
• A/C Load Compensation Maps
• Drive Engagement Maps (automatic trans)

Logic Sequence

Target Idle RPM Lookup
         ↓
Load Compensation (A/C, electrical)
         ↓
Airflow Control → Throttle / IAC
         ↓
Timing & Fuel Adjustment
         ↓
RPM Feedback → Closed-Loop Correction

Calibration Objectives

• Smooth, stable idle under all conditions
• Fast load compensation
• Minimize idle emissions

Calibration Strategy

• Lower idle RPM for better fuel economy (within stability limits)
• Increase cold idle for faster warm-up
• Verify stability with A/C cycling

Diagnostics

• Idle speed control faults
• Hunting or surging idle
• Stalling under load

Best Practices

• Clean throttle body and IAC valve periodically
• Address vacuum leaks before adjusting idle calibration