Idle surge can be annoying as well as difficult to defeat, especially in an older Honda that looks like it’s in perfect working condition otherwise. When your Honda has an intermittent idle surge, or the idle fluctuates when the vehicle is warmed up, chances are you have a problem with your fast idle valve.
The Fast Idle Valve or (FITV) in your Honda is responsible for idle control, having replaced the older carbureted mechanical fast idle system. Like the older mechanical fast idle system, the FITV can become clogged and fail to carry out it’s duties. Because it can be expensive for a mechanic to diagnose your problems, we have our How To Fix Honda Idle Surge DIY article to save yourself some serious money.
In most EFI Hondas there’s a passage in the intake manifold that helps control idle. This passage is completely independent of the air coming into your engine past your throttle blade. This passage is simply unmetered air that your idle controller can use to level out your idle once your EFI Honda has warmed up.
The FITV is a valve that helps control this stream into your engine and control your idle. To test your FITV, simply locate it and cap the nipple at the top of the valve to see if the surging stops. You must make sure your Honda is at proper operating temperature to test the FITV.
Fast Idle Valves have a thermowax pellet inside of them, not unlike the wax pellets found in thermostats. Once this pellet is worn out or the seal fails, you will undoubtedly have FITV issues. When your engine is at proper operating temperature, try to feel the outlets from your FITV which should be warm to the touch.
The Honda FITV should be normally open and gradually close as the engine warms up. When you experience idle surge or your Honda fails to idle when warm, chances are your thermowax pellet is damaged or simply worn. Not being able to properly seal, this causes the irregular idle and the fluctuations in RPM as your engine idles.
Your Honda FITV could also fail to open, or be stuck closed. This would not cause your Honda idle to surge, but instead would cause it to turn off or stall immediately. To correct this issue, you will need to remove the screws on top of the FITV and removing the cover. You should feel a suction at the top nipple of your air bypass valve to check if your FITV is working correctly.
While we’ve seen this problem before, the much more common issue is the idle surge that many Honda owners find themselves stuck with when they have problems with their FITV. The FITV should not be confused with the OBDII IAC or Idle Air Controller, which is shown below.
The Honda Idle Air Controller or IAC is also known as the EACV or Electronic Air Control Valve. If you’ve checked your IAC and your Honda still suffers from a surging idle, check your FITV.
The FITV is found under the throttle body usually and is incorporated into the throttle body passages. To locate your Fast Idle Thermo Valve, remove the intake snorkel and locate the FITV beneath your throttle body opening.
Remove the FITV by undoing the three 10mm bolts that hold it to your throttle body. Although there’s some places online that want you to take this FITV apart, there isn’t much good you can do by disassembling it.
Honda calls for you to simply replace this part, and unless you have a replacement thermowax pellet there’s nothing you can service inside the actual FITV unit itself.
Once the FITV unit has been removed, inspect the sealing orings to make sure there’s no damage to the seal or the way your FITV mounts. Carefully transfer over the gasket to your replacement FITV for service.
Shown below is another diagram of the Honda FITV and how it’s mounted onto the intake manifold / throttle body.
Once you have your FITV replaced, reconnect the vacuum lines and warm up your engine. At the proper operating temperature your idle surge should now be gone and your Honda should have a correctly working brand new FITV. You now know How To Fix Honda Idle Surge by replacing your FITV. If you have any questions or comments please leave them below!