The idle air control valve, also known as the IAC valve, is an essential component in a car’s engine management system. Its primary function is to regulate and control the engine idle speed, allowing for a smooth and consistent idle. However, when the IAC valve fails or malfunctions, it can cause issues like a high idle speed, low idle, rough idle, or even stall out. In such cases, bypassing the idle air control or removing the IAC valve as a temporary fix may become necessary.
Before bypassing the valve (IACV), it’s important to diagnose the problem correctly and check for any air leaks, vacuum leaks, or other issues that could cause idle problems. Once confirmed, the IAC valve can be bypassed by simply unplugging the electrical connector and removing the valve from the throttle body. Blocking off the IAC passageway on the throttle body with a metal plate allows air to bypass the idle valve completely. This should set the engine to a default factory idle speed, although further tuning may be required.
In our guide, you can learn more about maintaining your Honda engine’s smooth operation and how to bypass the fast idle valve if required. By the end, you’ll better understand how the valve regulates air into the engine and how using the bypass procedure helps keep your car running smoothly. (Learn How To Clean Oxidized Aluminum Engine Parts)
What Does The Idle Air Control Valve Do?
The IAC valve should be located near the throttle body or intake manifold of your Honda Civic and sits on the side of the engine while connected to the air intake pipe. It is an electrically-controlled valve that opens and closes to control the amount of air that can bypass the closed throttle plate and enter the engine when it is idling to keep the engine working properly.
When the engine is cold, the valve allows more air to enter when you start the engine and let it idle faster to warm quicker. Once warm, the fast idle control valve closes to adjust the idle speed. The valve also helps compensate for loads on the engine from things like the air conditioner or power steering. If the IAC valve gets stuck open, it could cause a high idle. If it gets stuck closed, it could cause a low or rough idle or stall. So, the idle thermo valve becomes integral to smooth engine operation and idle control.
Symptoms My IAC Valve Is Bad
Some common symptoms that cause you to check the idle air control valve include:
- High idle speed
- Low idle or stalling
- Rough idle
- Surging idle that goes up and down
- Check Engine light is illuminated
- Engine dies soon after starting
- The vehicle struggles to start
- Poor fuel economy
If you are experiencing any of these valve is bad symptoms, it could point to the IAC valve is not working properly. But there are ways to test it yourself before replacing it.
How To Test The Idle Control Valve
Before replacing the IAC valve, you should troubleshoot it to confirm it is the cause of your symptoms. Here’s how to test the IAC should you see symptoms of a bad IAC.
- Listen for abnormal noises from the valve when the car is idle. A bad valve may make scraping or clicking noises.
- Check for vacuum leaks, damaged hoses, and proper valve connections around your vehicle’s engine. Air leaks can impact idle control.
- Use an OBDII scanner to check for any idle air control codes. Common codes include P0505, P1508, P050A.
- Monitor the idle speed in park and when revving the engine. Constant idle speed should be steady.
- Unplug the valve when idling and see if the idle changes. Unplugging increases idle speed, which often indicates a bad valve.
- Test valve resistance with a multimeter. Resistance should be within factory spec range.
If testing confirms the IAC is functioning poorly, it likely needs to be replaced, or you can clean the valve and possibly reset the valve first and see if it works. Also, there are some temporary methods when the control valve is not working to bypass it while you wait for a replacement part. (Read Vehicle Shaking When Idle)
How To Bypass Idle Air Control Valve
The idle air control valve, commonly referred to as the IAC valve, is an important component in many modern fuel-injected vehicles. Its purpose is it helps to regulate and control the engine’s idle speed by adjusting the amount of air that bypasses when the vehicle is idling, and the throttle is closed to the minimum.
A faulty IAC valve can cause various issues, such as causing the engine to idle rough, stalling, high idle, poor acceleration, and reduced fuel economy. In some cases, a malfunctioning IAC valve may need to be replaced. However, several methods exist to temporarily bypass the IAC valve to keep the vehicle running until the valve can be repaired or replaced.
If your IAC valve is confirmed to be malfunctioning, here are a few ways to bypass it temporarily to regulate idle speed when the engine is running. It is also important to note this fix to a problem with the IAC valve should be no more than a temporary solution. This valve regulates the airflow, and with an engine running too high or too low, it could cause more engine issues.
Unplug The IAC Electrical Connector
The simplest bypass method is to locate the IAC valve. The valve should be located on the throttle body so you can quickly unplug the electrical connector going to it. This will stop power from reaching the valve, essentially disabling it.
With the valve inactive, the engine computer will revert to its default programming for controlling idle speed. This allows air to bypass even if the throttle plate is closed and isn’t restricted by the valve.
Be aware that this may cause your car to idle too high at around 1500 rpm as the engine gets too much air. But, even with this high engine idle on your car, it can keep the vehicle running until the new idle valve is located and you get the valve back in working order.
Use A Paperclip Or Jumper Wire
You can also put a paperclip or jumper wire in the IAC valve electrical connector pins to keep the circuit complete while disconnecting the valve in place rather than removing it.
Some vehicles require a certain resistance value in the circuit for proper idle control. So, jumping the connector instead of fully unplugging it simulates that resistance and can give better idle speed control.
Disconnect The Air Line From The Valve
Open the hood and locate the plastic air intake pipe that runs to the IAC valve from the throttle body. This allows air to flow through the valve into the engine. Use pliers to loosen the clamp and disconnect the air tube from the valve inlet. This prevents air from running through the bad valve. Cap off the open intake tube to seal it.
Without restriction from the valve, air will flow directly from the throttle body to the engine and allow idling. You may need to monitor and adjust throttle stop screws to maintain proper idle speed. (Read Idle Car With AC On)
Install A Manual Air Bypass Screw
Some vehicles have a secondary air bypass screw installed near the throttle body that allows manual adjustment of bypass air when open. Turning this screw out adds more air which raises idle speed.
If your vehicle does not already have an air bypass screw, you can install one near the IAC valve. Drill and tap a hole in the intake tube and install a screw to control air volume manually.
Resetting The IAC Valve After Bypass
Once you install a new IAC valve or properly repair the existing one, you will need to reset the idle air control system. Here are the steps:
- Reconnect valve electrical connectors and air intake tubes properly. Double-check for leaks.
- Start the engine and allow it to warm up to operating temperature fully.
- Turn headlights, radio, and other loads on to put demand on the engine.
- Allow the engine to run for 10+ minutes before turning off the key. This allows the computer to relearn idle control.
- After restarting, run the engine for several more minutes under varying loads to complete the relearn procedure.
- Check for smooth, steady idle speed around 700 rpm once warm.
Resetting the IAC valve and computer memory allows the system to recalibrate for proper idle control and performance.
Conclusion
The idle air control valve regulates smooth idle operation in fuel-injected engines. If symptoms indicate it is not functioning properly, there are ways to bypass it temporarily and keep your car running.
Disconnecting the valve electrically, blocking air flow through it, or installing a manual air bleed screw can override the bad valve, and you can reconnect the electrical connector later. In addition, you can clean an idle air control valve without removing it.
Just be sure to reset the IAC system properly after repairing or replacing the valve. Monitoring your valve and addressing issues promptly will allow your engine to keep running at peak idle efficiency. (Learn How To Adjust An Edelbrock Carburetor)
FAQs
How do I locate the IAC valve?
The IAC valve is typically located on or near the engine’s throttle body. Consult your vehicle repair manual for the exact location. Look for a small round or rectangular component with an electrical connector and air hoses attached.
Can I drive with the IAC valve bypassed?
You can drive a short distance with the IAC bypassed to reach a repair shop. However, prolonged bypassing is not recommended as it can cause performance issues. Have the valve properly repaired/replaced soon.
Will a bad IAC valve damage my engine?
It’s unlikely a bad valve plays a part in engine damage in the short term. However, idle control problems can negatively impact engine operation, fuel efficiency, and emissions over time. Have the valve inspected and repaired soon.
What typically causes an IAC valve to fail?
Carbon buildup, contamination, electrical issues, and internal parts wear can make IAC valves fail over time. Prolonged air leaks near the valve can also impact its performance and calibration.
Does the IAC valve affect acceleration?
The IAC valve only controls engine idle speed, not acceleration performance. Issues accelerating are more likely from other components like the throttle body, fuel injectors, or engine sensors.
