Your engine is kind of like a little planet, or an organism -- a biosphere that must maintain numerous delicate balances just to survive. Your engine uses air and fuel just like your body does, oxidizing the fuel to produce usable energy. But air is a difficult thing to control; it changes in density by atmospheric pressure and heat, and it speeds up or slows down according to the shape of the channels it flows through. The idle air control valve is one mechanism that your engine uses to modulate this difficult substance in order to maintain that tenuous balance of oxygen and fuel.
Fuel Injection Basics
It takes around three parts pure oxygen to burn one part fuel. Since air is about 21 percent oxygen, your engine requires about 14.7 parts air to one part fuel to maintain stable combustion. Your foot, for the most part, controls airflow into the engine by opening or closing the main air valve -- the throttle plate. The computer uses a sensor in the intake to monitor the amount of air going through, and another couple of sensors to monitor temperature an pressure so it knows how much oxygen the air contains per cubic foot. The computer then changes the amount of fuel injected to keep the air-fuel ratio at a steady 14 to 1. An oxygen sensor in the exhaust double-checks the output to make sure that all went as planned in the cylinder.
The IAC Channel
Your main throttle plate controls airflow into the engine, and shutting it completely would stall the engine by starving it of air. Keeping the plate slightly open with a throttle stop is one way of providing adequate air at idle, and drilling or notching the edge of the throttle plate will allow air to flow through even with the plate completely shut. But most modern cars eschew the hole in the throttle plate in favor of a hole in the throttle body, the housing that contains the throttle plate.
Hypothetically, a car would run with nothing more than a hole in the throttle body housing. That would allow air to enter the engine at idle, just like the air bleed on a carburetor does. But the problem with this approach is that, as stated, air density does change with temperature and barometric pressure (as a function of either altitude or weather conditions). For instance, air density at 100 degrees Fahrenheit is about 9 percent lower than at 50 degrees, and it's about 18 percent lower at a mile above sea level than at sea level. So, in order to maintain the same airflow at idle, a car idling in the dead of summer in Denver would need a bleed hole 25-percent larger than one running in Florida in the fall.
Given the above factors, the obvious solution here is to use a bleed hole that can change in size to suit the conditions. The idle air control valve is a servo that moves a rod in the IAC channel. On the tip of that rod lay a cone-shaped plug that mates to a similarly shaped depression in the air channel. To reduce airflow to compensate for low altitude and high pressure, the computer signals the servo to push the cone plug closer to the orifice. Moving the plug farther away from the hole increases flow.
While IAC servos can and do fail, channel blockage is, by far, the IAC valve's greatest enemy. The problem here is that aerosolized oil droplets from the crankcase ventilation system end up sticking to the valve and channel walls while the car idles. These particles are a magnet for the dust and dirt that makes it through the air filter. Dust sticking to the oil absorbs more oil, and you eventually wind up with a nasty buildup of sticky, black junk in the IAC channel. This blockage gradually reduces airflow and will eventually choke the car into a stall. A proper IAC channel-and-valve cleaning usually involves removing the throttle body, soaking it in a solvent and then cleaning it with a proper throttle body cleaner and pipe brushes. Badly clogged IAC channels often indicate a bad PCV valve, which keeps oil out of the engine, and an air filter in need of replacement.
- Auto Fundamentals; Martin Stockel
- Engine Airflow: A Practical Guide to Airflow Theory, Parts Testing, Flow Bench Testing and Analyzing Data; Harold Bettes
- Photo Credit Karl Weatherly/Photodisc/Getty Images
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