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What Is An Idle Control Valve?

One of the worst things you can experience as a car owner is a car that just seems to stop randomly at the lights. Or even a car that just seems to shake and drop the RPMs randomly before just carrying on as normal. So why is this such a common problem (especially on cars from the early 2000's-2010's)?

In a lot of cases, poor idle problems and stalling at the lights can be related to a faulty idle control valve (IAC/ISC/ICV).

Let's get into it a bit deeper though. While a lot of car owners have a passing understanding of what a misfire is, it's not very common to find a vehicle owner who knows what an idle control valve is or what it does.

Like almost every one of our Kashy blogs, we have to start with a bit of history. Back in the olden days, cars were simpler. In fact, for most cars from the 1880's to the 1980's the most advanced way to meter the amount of fuel going into the engine was called the carburettor.

If you haven't seen a carburettor before, it's a shockingly simple (and beautiful) piece of equipment. By using the vacuum created by the engine to draw air past a metering needle, they essentially suck fuel into the combustion chamber to keep the car running smoothly.

Technical drawing and cross section of an SU style slide carburettor
A 'simple' carburettor

Carburettors of the day were actually able to fairly accurately meter the amount of fuel needed depending on the amount of throttle you were using. For example, if the throttle was closed, very little air would come past (meaning little fuel) and as the throttle was opened, the volume of air (and fuel) increased to match.

Now, this all changed in the 70's and 80's when electronic fuel injection became the go-to.

Electronic fuel injection sort of does exactly what it says on the tin. It takes readings from various little sensors around the engine (including the mass air flow, throttle position and oxygen sensors), sends it to a little computer and that determines exactly how much fuel to add based on some computer maths. It then uses a set of little electronic injectors to send the precise amount of fuel to the engine.

You might be thinking, "Gee, that seems needlessly complicated", and you'd kind of be right. However, while carburettors are fairly good at their job, they require a lot of maintenance, are easily susceptible to changes in weather and altitude and (worse of all) are bad for the emissions (a major problem in the 70's).

However, because computers have historically been a little... slow, and because the throttle was now default closed, manufacturers had to come up with a way to change the amount of air being let into the engine to make up for discrepancies when idling.

This is where the idle control valve (IAC/ISC/ICV) comes in. By using a tiny solenoid valve to allow a throttle bypass passage to open and close based on the engine RPM's and the readings of the oxygen (O2) sensors, manufacturers were able to control the idle running once again.

So why don't we have issues with idle control valves (IAC/ISC/ICV) on modern cars?

Well, it's all pretty simple. Throughout the mid-2000's and early 2010's manufacturers started changing one key part, the throttle body. By changing from a throttle controlled by a cable to one that was able to change its position electronically, the need for an idle control valve suddenly ceased to exist.

In these newer cars, the throttle is not connected to the pedal at all and instead, the car uses the engine computer to tell an electronic motor to change the amount of air coming into the engine (i.e. the throttle angle).

So how can you tell that you've got a faulty idle control valve (IAC/ISC/ICV)?

The best way is to get familiar with a multimeter. By finding the specification online for the resistance of the solenoid and voltage at the connector, you can very easily pinpoint a common problem.

Otherwise, the symptoms of a faulty idle control valve are usually very easy to spot. High or low idle, rough idle, not idling at all. Realistically, anything to do with poor idle could be attributed to a failing idle control valve.


If you have any questions about this blog post, would like a second opinion from a mechanic or would like to find an honest mechanic in Brisbane, check out Kashy here.

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