What Happens When The Idle Air Control Valve Goes Bad?

Maybe you are standing in front of your car with the hood up, scratching your head and wondering why your car is fluctuating in speed without even moving. We can attribute this to the idle air control valve (IAC) and the need to replace it, but we don’t always have to go to the mechanic.

What Happens When The Idle Air Control Valve Goes Bad?

What Is an Idle Air Control Valve and How Does It Work?

The IAC is a small valve that ensures the engine receives the right amount of air and associated RPMs during idling. Located on the throttle body next to the intake manifold, the IAC takes information from the car computer, ensuring the RPMs change in sync with the engine’s requirements. However, when it goes bad this is no longer the case.

What Happens When The Idle Air Control Valve Goes Bad?

When we have our car idling we usually hear a consistent speed from the engine. When this consistency starts to turn into more of a polyrhythmic drum solo from the engine, we need to take a look under the hood. If your valve goes bad, you will notice massive fluctuations on the dashboard tachometer and vibrations through the car. Eventually, the car will become underivable if the IAC is left unattended, so look out for the warning signs!

Top 6 Symptoms of a Bad Idle Air Control Valve

​​​​1. Check engine light is illuminated

An obvious symptom but something that anyone can forget to take notice of if your check engine light is illuminated, don’t shrug it off. Cars are designed so that even the tiniest problems with an engine will set off the check engine light, making sure you don’t let a problem go beyond the point of no return. Of course, if you notice any of the other symptoms then you can be assured the IAC is the culprit for the problem. However, the car's central computer will notify you of any issue and thus you should consult a mechanic immediately if you are unable to determine the source of your car's check engine light being illuminated.

2. Engine is freezing

You might start accelerating and notice the car simply won’t move, this is a sign that your engine is stalling or freezing. The engine will remain running but you won’t be able to drive the car if stalling is happening. The whole valve system is to blame for this, malfunctioning just after starting ignition usually. When the engine is only beginning to stall, you should be able to make it to a nearby mechanic or safe place for towing as stalling will happen every couple of minutes in the beginning. However, once the engine stalls completely and freezes, if you are away from home call a tow truck immediately.

3. Rough idling

There is a long list of reasons that your car may be producing a rough idle, but it’s important to first understand what a rough idle really is. A rough idle is intense vibrations that start when your vehicle is stopped with the engine running. Less air is going into the engine during its idle state, therefore any issues which limit this will force the car to shake rapidly. Bad spark plugs, carburetor issues, dirty fuel injectors, and clogged air filters are just a few of the problems which can produce a bad idle alongside a bad IAC. Be on the lookout for further symptoms to be sure issues with your IAC are to blame.

4. An increased load is causing stalling

It can feel embarrassing and seemingly meaningless when the simple act of turning on our air conditioner makes our car start to seemly die right under our feet. But if your car is stalling from something as simple as this, it’s a clear sign that you need to have a look at your IAC. Turning something such as the heater on when you have a bad IAC will see your engine stall immediately. You may also notice your steering wheel feels like it’s being dragged to one side or another quite heavily as well. Thankfully, these are easy problems to solve until you are in a safe space to investigate your IAC issues. For a temporary fix, turn off the aforementioned heater or AC unit and this will reduce the load on the engine. It’s also recommended you let the engine cool for a couple of minutes. There can be deeper issues with your radiator or cooler which are triggered by turning on your AC or heater, however, these should be considered after investigating your IAC safely. 

5. The brake pedal is responding less

If you need to push further than usual on the brake pedal at a red light, this could be a sign to check under the hood. When you aren’t pressing on the accelerator at idle speed, it should be very subtle braking to avoid creeping forward. However, if you have a bad IAC this means the engine idles faster than usual, requiring additional effort to keep the car at a standstill. Of course, not everybody has the ability to determine when they are in need of pushing down further than usual on the brake pedal. Listening out for any abnormally fast or harsh sounds from an idle engine can help to clarify whether your brake foot is going further than normal.

6. Speeds are inconsistent

While the IAC controls the idle speed of the engine, this has many knock-on effects. With a damaged IAC, the power of the engine will be pushed up and down sporadically, resulting in inconsistent speeds. If you notice your speed starting to change without any difference in how you are driving, give your IAC a good look.

Step by Step Guide Fix The Problem

For many people with an IAC that isn’t completely destroyed, a quick spray with any carbon cleaner will get the valve working again. If you remove the valve and see some buildup, this will most likely be all you need to do. Give it a spray with your cleaner, a quick scrub to make sure it’s all gone, then screw it back in once it’s dry. However, if the valve looks relatively clean then you may have a defective valve or need to replace it for other reasons. Make sure to disconnect the battery for 15 to 30 minutes afterward before testing.

For those of us that need to replace the valve, consulting a mechanic will be our first thought. With the average price of a mechanics visit for an IAC replacement running you anywhere from $120 to $500, having the skills to complete this simple replacement will save you a bit of heartbreak. Not to mention new parts are relatively inexpensive with some only setting you back as little as $45.

If you do need to replace the IAC it is a very quick job, only taking about an hour at best. Here’s a step by step guide!

If you are worried that you might place cables in the wrong place after removal, it’s helpful to use masking tape to label where wires and hoses plug into.

Step 1: Get your tools ready

You are going to need a replacement valve (obviously), throttle body cleaner (there are plenty on the market, pick whatever one suits your budget but make sure it’s throttle body cleaner and not generic cleaner), pliers and a socket set with a ratchet.

Step 2: Disconnect the battery

Only the negative battery cable needs to be disconnected and set aside, but if you want additional peace of mind then completely disconnect both cables and set aside.

Step 3: Locate the problem

You are going to have to consult your vehicle’s manual as the location of the IAC varies based on make and model. However, the valve is nearly always located on the intake manifold.

Step 4: Disconnect the harness for the wiring

The wiring harness will be connected to the valve, release the electrical terminal from the valve. You will also notice a clip or tab to disconnect, this might be an easier route for removal with a pair of pliers.

Step 5: Remove the old valve

The valve will have a number of retaining bolts, remove these and pull the valve straight out of place amongst where the bolts and wires once were.

Step 6: Ensure a clean seat

As the seat for the valve is now exposed, your throttle body cleaner will be of use to you. Clean the area you will attach your new valve to, ensuring there is a clean seal between the seat and valve.

Step 7: Install your new valve

Before undertaking this important step it’s crucial that you understand over-tightening bolts when installing a new valve will be detrimental to functionality in the future. Overtightening can lead to leaks or improper function, limiting your new valve's lifespan and opening up the potential to damage other areas of the engine.

With this in mind, ensure all the attachment areas on your new valve match the old one. Once you are ready to proceed, install the new valve and the retaining bolts, tighten these bolts by hand firstly. Once snug you can use your socket and ratchet to ensure each is flush one by one.

Step 8: Reinstall the wiring harness

This is a very straightforward step, however, you need to be mindful of the terminal making a proper connection, the clip has to be completely engaged to ensure a secure connection.

Step 9: Reconnect your battery

You are now almost finished with your installation, reattach the cables you set aside back onto the battery. Tighten the bolts, this ensures any excess vibration from the engine won’t pose a risk to them rattling loose.

Step 10: Test your new valve

There are a lot of ways you can test your new valve, but the best and simplest method is to start the engine and take notice of the idle speed. All vehicles have different idle speeds, especially considering the ambient temperature of your region. Most vehicles should hold between 550 RPMs and 1000 RPMs steadily, with smaller RPMs for hotter temperatures and vice versa for higher RPMs.

Optional Step 11: Replace your coolant

Coolant plays an important role in providing the IAC with the correct conditions needed to run efficiently. If you want to take all precautions to make sure your new IAC is working at its best, a coolant replacement is a good idea. Not to mention, you should be replacing your coolant along with your oil amongst regular checks of your car. Most manufacturers recommend changing your coolant every 30,000 miles.

If you decide a coolant change would be a good step as part of your maintenance, it's a relatively straightforward process.

  • All you need is coolant, some wrenches and screwdrivers, jack stands (you should have one with your tyre change kit), a drainage pan (anything with a large opening to store liquids will do), and a hose removal tool.
  • Firstly, ensure your car engine is cool then have a look at your existing coolant. If your coolant has a rusty look or has oil and other materials floating on top, take it to a mechanic. Otherwise, you are ready to replace your coolant so jack up the vehicle with some jack stands.
  • Place a drainage pan roughly under the radiator, loosening the lower radiator hose clamp with your tools, removing the hose. If the hose is stuck, use your hose removal tool but only to loosen it.
  • Fully drain your water pump and radiator, reattaching the drain plugs afterwards. Now you can begin refilling with your new coolant.
  • Some suggest investing in a refilling tool for this step, to ensure there are no air pockets as you refill your coolant. However, this is not a complete necessity. When you are done refilling the coolant, you only need to screw the tank caps back on and remove the jacks.

Leave a Comment