Idle Surging

Three things I know and have seen to cause a surging idle:

1) air in the cooling system - bleed the system to purge all the air out. Depending on the engine, the bleeder screw may be on the thermostat housing or on the head where the upper radiator hose connects.

2) bad or clogged Idle Air Control Valve - on the back of the IM and should have a blue w/yellow and a black w/yellow wire going to it. Cleaning the filter screen inside the AICV will help restore a more steady idle. To do so, you must remove the valve assembly from the throttle body and clean out with brake cleaner.

3) A leak in the throttle body or intake manifold gasket - with the engine running, carefully spray brake cleaner on the mating surface edge by the gaskets. If the engine bogs when you spray, there is a leak where you sprayed.

There is of course the possibility of leaking vacuum lines. If you don't hear or see any obvious leaks, unplug all the vacuum hoses one at a time. Each time after you unplug one, plug the exposed nipple at the manifold or throttle body with your finger. If the surging goes away you know it involves that line and or any component that line goes to. After you do this, put that hose back on and proceed to repeat that for each and every vacuum hose and it may uncover the problem. Another way to help isolate a pinhole leak in a vacuum line is to put your hand over the throttle body with the engine running. A leak will give itself away by the whistling it will make. Note* cover up only as much of the throttle body opening as needed to allow the engine to stay running yet force the leak to reveal itself.

Since this thread is sticky'd in the FAQs, I want to repost some great info that is buried on the Hondata web site:

The idle speed is controlled by three methods:

1. The cold idle valve (FITV), which is located under the throttle plate. Late model engines may not have the cold idle valve. The valve consists of a air channel bypassing the throttle plate which is blocked once coolant heats up a wax element. When the engine is cold the cold idle valve will allow a lot of air to bypass the throttle plate, and the idle will be around 2000 rpm. As the coolant warms up the cold idle valve will slowly close, bringing the idle speed down.

2. The idle air control valve (IAC or EACV), which is located on the back of the intake manifold. The IAC valve is a solenoid which is controlled by the ECU using pulse width modulation. Thus the ECU can control how far the solenoid is open and how much air bypasses the throttle plate. There are limits, both upper and lower, to the amount of air which passes through the valve because the solenoid will only work from 20% to 80% duty cycle.

3. The throttle air bypass screw, which is located in the top of the throttle body. Unscrewing this will allow more air to bypass the throttle plate, and so will increase the idle speed. The purpose of the bypass screw is to adjust the amount of air going pass the throttle plate so that the IAC valve duty cycle is within the limits of what the ECU can control. The manual tells you how to adjust this screw by unplugging the IAC valve.

Click to expand...

Also, if you need to adjust your TPS but do not want to tear apart your throttle body and slot those rivets, you can try this:

1.) Tap your volt meter into the MIDDLE/RED wire on your TPS... some say to cut the wire sheathing and alligator clip it, and remember to tape it over. I use the probe-style end and jam it up inside the back of the plug where the wire goes in.

2.) Turn the ignition to the "ON" position

3.) Instead of trying to adjust the TPS, adjust the throttle return screw (very slightly) until the closed throttle voltage reads .45V . The throttle return screw is located near the return spring/cable assembly... its what stops the spring from rotating...push down on our throttle cable and release it, and the return screw will make a "click" noise when the rotor snaps against it.

On some engine/ecu setups it has been reported to idle better at ~.35V...but who knows if that sacrifices some other reading at WOT. You'll need to experiment to find out.

After adjusting the return screw, you'll most likely need to reset your idle per the factory manual, so the IACV is back to its normal operating range.

If you cant get the voltage you need from that, then good luck with the cutting wiggling bit.

Good luck!

Ok I had to add input here I have been fighting a surging idle for 2 months on a car I get to work on about 4-5 hours a week. I am a very busy person so I do not have the luxury to take even a Saturday or Sunday to set aside.

I would like to point out before we get started. I had no CEL this whole time.

First things to check as mentioned above. Burp the coolant system as bubbles in the system especially the IACV aka EACV (Idle Air Control Valve) and ECT (Engine Coolant Temp Sensor) will cause the improper Thermo readings and adjust idle and fuel incorrectly.

My Findings.
1.My vehicle would Idle fine until the engine hit 118F then my wide band would go from stoich and slowly run leaner and leaner the warmer it got, this of course raises RPM until the engine can no longer make power and idle drops. At the point the vacuum would cause the stock Honda fuel table to dump fuel at low RPM causing a buildup of fuel trying to raise the idle. This load of fuel of course fed the engine to speed up and burn the fuel... Hench Surging Idle! And repeat.

At first I checked all vac lines and replaced everyone! Still the problem persisted.

To Google!
clean IACV and FITV... Check, reassemble. Still the problem persisted.

Replace all gaskets on intake manifold to rule out vac leak.... You guessed it. still surging idle.

Pulled the plug on the IACV with the key in the on position engine not running. silence. I did not hear a Click. This car sat for 2+ years and when I put it back together the IACV was humming like it was locked up. :doh: Should have triggered a light bulb above my head there! But I had forgotten about it because it was months ago. It was quiet now.
Tested the IACV plug for voltage 11.6volts dc... I have power! must be the IACV is bad. So I built a test fixture and connected the IACV to my battery 12vt source. CLICK! OMG it works it must be the connector!... NOT!

Swapped the connector to no avail. Searched the Service manual looking at the trouble shooting. Nothing gave me a value to took at.

Dusted off my Snap-on Vantage 2400 (Which is a tool everyone should have if they can afford it. Picked mine up on eBay for $300)
It actually had an expected resistance value of the IACV motor! As well as duty cycle tests and expected voltage tests.

So how did I suspect the IACV? I used my finger. When the Idle was surging all over I took my finger and blocked the top port in the throttle body. This is the bypass hole for the IACV. By doing this I simulated the ECU sent 12vts to the IACV and closed the "air leak" or bypassed air.

This was not in the Honda Service manual:
IACV should read around 20 Ohms across the connector:
IACV should have 10-12Vtc DC sent to it with the key on engine running at operating regular Temp:
IACV should have 6-10Vts DC with lights and fans and blower motor/AC on in the vehicle:
Check for Continuity from each pin to ground. This means the motor is shorted internally:

My results.
Voltage is OK in all checks. Phew good ECU!
Ohm readings! 39.5 Ohms! :evil:

So some may not realize whats going on.
10-12vts the IACV motor closes and cuts off the bypassed Air.
When the car is first cold started extra fuel is dumped into the engine to warm it up causing a higher than normal idle normally 150-300 RPM. At this point the IACV is opened more to allow the fuel to burn but you are running rich. As the ECT warms up the fuel is slowly cut back and the IACV should close up slowly. It opens when accessory such as lights AC fans blower motor and other accessories are run... to boost the idle.

But wait, yes I did say that the IACV opened when connected to a battery. Without going into Ohms law I will explain that the 12vt signal coming from the ECU is weaker than the 12vt signal directly from the battery. You have a higher amperage direct source from the battery. This meaning that when I use battery juice the battery can work harder to force the 12vts into the IACV where the ECU is limited. Ohms law basically states if you run voltage through resistance on the other side of the resistor the voltage will be lower than what is was originally before the voltage hit the resistance in the circuit. Basically to reach the IACV with double the expected resistance I would need double the 12vt signal at 24vts at the same current reading from the ECU for the IACV to function.

So I hope these values will help someone in the future as again they are not listed in the Service manual. At least the IACV Ohm value is not.

So now that I may have helped can anyone tell me what IACV is the same size as a B16A? The bolt patterns are different. I have to hit the junkyard unless someone here has one.