ASE A1 Engine RepairFuel, Emissions, and Ignition Systems (General)Medium
A technician is diagnosing a vehicle that intermittently stalls at idle and experiences poor fuel economy. A scan tool shows that the engine control unit (ECU) is frequently adjusting the commanded idle speed. The technician suspects an issue with the idle air control (IAC) system. Which of the following is the most appropriate next diagnostic step?
- APerform a compression test on all cylinders.
- BTest the resistance of the oxygen sensor.
- CInspect the throttle body for carbon buildup around the throttle plate.
- DCheck the fuel pressure at the fuel rail.
Show answer & explanationAnswer & explanation
Correct answer: C. Inspect the throttle body for carbon buildup around the throttle plate.
If the IAC system is struggling to maintain idle speed, it often indicates insufficient airflow at idle. Carbon buildup around the throttle plate can restrict the bypass air controlled by the IAC, forcing the ECU to constantly adjust and potentially leading to stalls and poor idle control.
Why the other options are wrong
- A. A compression test is for internal engine issues and while it could cause poor idle, it's not the first step when the IAC system is suspected of struggling with idle speed control.
- B. Oxygen sensor resistance would be checked for air-fuel mixture issues, not directly for IAC system malfunction or idle speed control.
- D. Fuel pressure affects overall fuel delivery, not primarily the IAC's ability to control idle speed directly.
Idle Air Control (IAC) System Diagnosis
Diagnosis of IAC system issues often involves checking for physical obstructions, vacuum leaks, and proper sensor/actuator operation that affect idle air flow.
- IAC controls idle speed by regulating bypass air.
- Stalling/poor idle are common symptoms.
- Carbon buildup is a frequent cause of IAC malfunction.
Memory trick: IAC 'Stalls' when throttle 'Sticks' with 'Carbon'.