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Car Care Advice

5 Crucial Symptoms of a Bad Idle Air Control Valve (+ How to Fix)

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Seeing a Check Engine Light (CEL) on your dash along with rough idling and stalling?

A bad idle air control (IAC) valve may be the culprit. If you don’t sort it out quickly, it can lead to costly engine issues.

But fear not!
We’ll help you identify the most common symptoms of a bad idle air control valve, how to diagnose and fix a faulty IAC valve, and answer some frequently asked questions.    

This Article Contains:

Let’s begin. 

5 Common Symptoms of a Bad Idle Air Control Valve 

As you release the gas pedal, the throttle position sensor closes the throttle plate, and the IAC valve opens to maintain the engine’s idle speed. But if the idle air control valve is bad, it causes abnormal idle RPMs and several other issues.  

1. Illuminated Check Engine Light

Idle air control valves impact your car’s engine performance by regulating the air intake. When the Engine Control Unit (ECU) registers an IAC valve or air intake problem, it activates the Check Engine Light.  

However, the Check Engine Light doesn’t provide specific information about the issue, such as the engine’s idle speed. To accurately diagnose the Check Engine Light, you’ll need to read the trouble code registered by your car’s ECU.

2. Irregular Idle Speed 

The idle control valve allows air to bypass the throttle plate to regulate the idle speed. If you have a malfunctioning IAC valve, incorrect amounts of air can enter the engine. When that happens, you’ll experience abnormal engine idle speed, called a “rough idle.”

How do you know if your idle RPMs are abnormal?
Idle speed usually ranges between 500-1000 RPMs on standard vehicles. So, if your engine speed surges outside this range, you likely have a rough idle. 

Note: A vacuum leak also results in abnormal idle speed. So, while a vacuum leak doesn’t cause a malfunctioning IAC valve, it causes similar symptoms (like an engine stall, abnormal engine idle, and an active Check Engine Light). 

3. Engine Stalling

If the idle air control valve doesn’t allow enough air to bypass the throttle plate, It leads to combustion failure and engine stalling. This often happens without warning. However, in some cases, engine stalling is caused when you use the car’s electronic systems (like AC), which affect the engine load. 

Note: Avoid using these systems to reduce the engine load while driving with an active CEL, and take your car to a mechanic as soon as possible. An engine stall can be dangerous in bad traffic conditions.

4. Backfiring

A backfire is when fuel ignites outside the combustion chamber. If the IAC valve causes it, you’ll notice the backfire when you disengage the gas pedal to slow down. 

How does a bad idle control valve cause it?
A bad idle control valve leads to poor air intake, causing a car’s engine to run rich. This results in excess unburnt fuel leaking out into the exhaust and igniting. 

5. The Engine Starts, Then Dies 

A faulty IAC valve can starve your car’s engine of oxygen. This makes combustion difficult and, in some cases, impossible. And if combustion can’t occur, the engine dies

Now that you know the symptoms, let’s check out how to confirm if your IAC valve has gone bad and what you can do to fix it.  

How to Diagnose and Fix a Bad Idle Air Control Valve? 

Accessing the IAC valve can be tricky, so rather have a mechanic diagnose and fix the issue to save you time.
Here’s how they would tackle a bad IAC valve:

A. Diagnose a Bad Idle Air Control Valve

After checking the idle speed, there are two primary ways a mechanic will diagnose a bad idle control valve:

  1. Identify the diagnostic code: Connect the OBD2 scanner to the vehicle’s computer system and read the error code. A P0505 error code indicates a bad idle control valve.  
  1. Measure the electrical current: Attach the multimeter to each valve terminal and note the reading. Any reading outside 0.00-0.05 indicates a bad idle control valve. 

If the valve is bad, the mechanic will clean or replace it. 

B. Clean the Idle Air Control Valve

To clean idle air control valves:

  1. Locate the IAC valve and remove the intake hose from the throttle body. 
  1. Detach the vehicle’s battery cable from the negative terminal.
  1. Loosen the screws connecting the idle control valve with the throttle body.
  1. Disconnect the electrical plug and wires. 
  1. Remove the hoses and clamps connected to the IAC valve.  
  1. Take off the gasket around the valve.
  1. Use a carb cleaner to remove debris from the idle control valve, intake manifold, and throttle body
  1. Replace the gasket.
  1. Reconnect the IAC valve to the throttle body, followed by the electric plug and hoses.
  1. Reconnect the battery. 
  1. Turn on the vehicle and leave it running to determine if there’s still an idle issue. The idle control valve is likely working if the engine idle is consistent.  

If the engine idle fluctuates, it means the idle control valve still isn’t working. So they’ll replace it. 

C. Replace the Idle Air Control Valve

Here’s how to replace the idle air control valve:

  1. Locate the idle air control valve and detach any parts in the way. You’ll typically find it within the throttle body. 
  1. Unplug the electrical connector.
  1. Remove the gasket and old idle air control valve.
  1. Attach the new idle control valve to the throttle body.
  1. Attach the electrical connector and any other previously removed components. 

Still have some questions about a faulty idle control valve? 
Let’s settle your curiosity.  

4 Bad Idle Air Control Valve FAQs

Here are the answers to some common questions regarding a bad idle control valve:

1. What Causes a Bad Idle Air Control Valve? 

There are three primary reasons an IAC valve goes bad:

2. Can You Continue Driving with a Bad Idle Air Control Valve?

While you may be able to drive your car with a faulty idle control valve, it’s best to avoid it. Generally, you should minimize driving as soon as your Check Engine Light comes on. 

Besides the illuminated Check Engine Light, a bad idle control valve can lead to stalling, which can be dangerous if you are on a high-traffic, fast-moving route.   

3. How Do You Prevent Idle Air Control Valve Issues?

Routine maintenance ensures the longevity of your idle control valve and reduces the likelihood of any issues. 

Moreover, most problems stem from debris buildup. So, regularly cleaning the idle control valve with a carb cleaner helps prevent an idle issue. But, if you notice irregular engine idle speed or other symptoms of a bad idle air control valve, get your vehicle checked immediately to stop the problem from getting worse. 

4. How Much Does a Bad Idle Air Control Valve Replacement Cost? 

Typically, you’ll spend $70 to $400 on an idle control valve. Replacing a bad IAC also incurs additional labor costs, typically between $50 to $100.  

Final Thoughts

There are several tell-tale signs of a bad IAC valve, including an activated Check Engine Light, rough idling or abnormal idle speed, and stalling. These bad IAC symptoms can harm your vehicle and put you in danger.

So why not contact a mobile mechanic service like AutoNation Mobile Service to resolve the bad IAC in your driveway?
We offer easy online booking and are available seven days a week. We also provide a 12-month | 12,000-mile warranty on every auto repair.

Reach out to us today for all your auto repair needs.