Car Revs Hunting Up and Down?
Find Out What’s Wrong

The idle control valve (also called the idle air control valve or IAC) regulates the amount of air bypassing the throttle plate at idle to maintain a stable idle speed. When the valve becomes dirty or fails the idle speed becomes erratic — hunting up and down. Cleaning the valve with throttle body cleaner often resolves the issue without replacement.

A split or disconnected vacuum hose allows unmetered air into the intake. The ECU cannot compensate for this additional air — causing the idle to hunt as the system tries to maintain the correct air-fuel mixture. Vacuum leaks are identified using a smoke machine or by carefully spraying carburettor cleaner around intake joints while watching for idle speed changes.

The MAF (mass airflow) sensor or MAP (manifold absolute pressure) sensor provide the ECU with the data needed to calculate fuel delivery. A faulty reading from either sensor causes the ECU to continuously adjust fuelling — resulting in hunting revs. Fault codes will typically point to the specific sensor.

Carbon deposits on the throttle body plate prevent smooth airflow at low throttle openings. The ECU continuously adjusts to compensate — causing hunting. This is particularly common on direct injection engines which lack the cleaning effect of fuel washing past the throttle plate. Professional cleaning resolves most cases.

The coolant temperature sensor tells the ECU when the engine is cold so it can apply a richer fuel mixture for cold starting. A faulty sensor that reads incorrectly causes the ECU to apply cold-start enrichment even when the engine is warm — causing hunting idle and poor fuel economy.