Think you’ve got a bad IAC? Don’t let testing myths fool you – get the real facts, right here.
The Idle air control (IAC) valve diverts air around the throttle blade to control idle speed. Seems simple enough, right? But how exactly do the internals of the valve operate? And how can you tell if one is bad?
There are misleading IAC testing myths floating around online. That’s why we’re here to dismiss such falsehoods to save you time and money.
Rotary-style IAC operation
Many European vehicles have a rotary-style IAC valve. As the name implies, this design uses a rotary valve to control airflow between the component’s inlet port and outlet port. A stepper motor or solenoid, which is controlled by the engine’s electronic control unit (ECU), opens and closes the valve. By adjusting the amount of airflow through the component, the ECU can control engine idle speed.
An infamous IAC valve testing myth
A lot of incorrect IAC testing information is in circulation. One of the most prominent – and most misleading – falsehoods is that you can identify a bad IAC by blowing through the inlet port or outlet port. The problem with this theory is that, when disconnected, rotary IAC valves do not typically default to the closed position.
Instead, when the valve is removed from its circuit, a spring inside forces it into a fixed position that allows a set flow rate. With the valve in this state, the engine is allowed to idle somewhere between 1,000 and 1,200 RPM. Therefore, in most cases, being able to blow through the valve is normal and does not indicate a faulty component.
How to properly test an IAC valve
Instead of using the unorthodox method of “blowing through the valve” you should always test an IAC in accordance with the manufacturer’s repair information. Most automakers recommend using a digital multimeter (DMM) to check the resistance between the IAC’s electrical terminals. Doing so will help determine the integrity of the component’s internal windings.
Some manufacturers also recommend applying power and ground to the IAC and watching for valve movement. Again, all vehicles are different, so be sure to consult the factory repair information to determine the correct testing and safety procedures for your application.
Also, keep in mind, incorrect engine idle speed or idle-related codes don’t necessarily indicate a bad IAC. Other issues – such as circuit faults, vacuum leaks or a bad sensor – can cause the same symptoms.
Very few replacement IACs are bad right out of the box. So, if you replace your IAC and the issue persists, chances are, the problem lies elsewhere in the system.
Sources:
https://www.lovehorsepower.com/ToyotaPDFs/60.PDF
https://www.autozone.com/repairguides/Volvo-240-740-760-780-940-960-1990-1998/ELECTRONIC-ENGINE-CONTROLS/Idle-Air-Control-Valve/_/P-0900c1528008c684
https://mbworld.org/forums/e-class-w124/503103-idle-air-control-valve-airflow-versus-control-voltage.html
https://www.autozone.com/repairinfo/repairguide/repairGuideContent.jsp?pageId=0900c1528004a161#hd1-1-7
