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Old 02-09-2008, 12:32 AM   #1 (permalink)
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The VX - '92 Honda Civic VX
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Exclamation Honda IACV explained

This thread will deluge you with more info than you've ever wanted to know about the IACV in your Honda. This information was copied out of the Honda OBD training manual as I didn't feel like paraphrasing it all.

23.1 General Overview

Idle control, along with fuel control are two of the most important sub-systems of the PGMFI fuel injection system. These two systems have a large impact on the driveability of a car. If the idle control system is not operating correctly, many driveability symptoms can occur, some of which are:
  • Idle RPM too high
  • Idle fluctuating wildly
  • Idle RPM too low and/or erratic
  • Idle fluctuating when loads occur to the engine
  • Fast idle too low for cold starts

The main output device that controls engine idle is the idle air control (IAC) valve. This valve was added to all models in 1988 and is controlled by at least the following inputs:
  • Air Conditioning Switch
  • Brake Light Switch
  • Clutch Switch (Manual Transmissions)
  • Electric Load Detector (ELD)
  • Engine Coolant Temp (ECT) Sensor
  • Engine RPM
  • Gear Position Switch (Automatics)
  • Power Steering Pressure (PSP) Switch
  • Starter Signal Input

The Honda PGMFI system is a speed/density type fuel injection system. It does not measure actual mass airflow, but calculates it from the engine RPM and the manifold absolute pressure (MAP) sensor input. On speed/density fuel injection systems, idle is controlled by simply controlling the air that bypasses the throttle plate.

The idle control system is one of the PGMFI sub systems that has seen a lot of change since the first 1985 models. The early idle control systems were primitive by today's standards. The curb idle was set by an air by-pass screw in the throttle body. This screw controlled how much air bypassed the throttle blade. Except for the fast idle controls the only other feature was a vacuum diaphragm that opened the throttle to offset the air conditioning compressor load.

The idle control systems of these earlier models had the characteristics of a carburetor. The engine control module (ECM) had no control over the idle other than the air conditioning vacuum diaphragm. To help make the idle more stable, the ECM would typically widen the injector pulse width (PW) slightly, and increase timing, when any type of load was sensed.

23.3.1 Idle Speed Control

The idle control systems on the earliest PGMFI systems were very basic. Other than a fast idle system and an air conditioning idle boost, the idle was totally controlled by how much air bypassed the throttle plate. This bypass air was controlled by the idle air bypass screw. The idle air bypass screw is a large brass screw with a flat slot and is located on the throttle body. The idle air bypass screw in Image 23-2 is shown inside the circle.

To adjust the idle follow the instructions from the underhood label or the service manual. The technique used to set idle for models with an IAC valve is different than the earlier models.

In 1988 Honda added an IAC valve to the PGMFI system. By adding the IAC valve the ECM now had full control over the idle. The IAC valve is an electrically activated valve that controls the amount of air that bypasses the throttle blade. Now the ECM could increase the idle to offset the load from any event that occurred. An IAC valve is shown at the left most arrow, in Image 23-1.

23.3.5 Load Events

The Honda engines run so lean at idle that virtually any event will cause the idle to "bobble". When a load event occurs, more than just the IAC valve makes a correction. Small changes are also made to the PW and the ignition timing.

Screen Capture 23-1 shows the effect of an electrical load on the PW and ignition timing. This technique was used prior to the IAC valve in an attempt to help stabilize the idle when loads occurred.

Even with the addition of the IAC valve, this strategy continued. When a load occurs on a late model car, the IAC valve opens, the PW widens, and the timing is increased.

The ECM monitors the idle and will compensate for any load that attempts to pull the idle down. The disadvantage with this approach is that the idle must start dropping before the ECM can make a correction. This would result in the idle "bobbling" on every load event.

To help prevent this, the ECM monitors most components that could have a significant effect on the base idle. It can then begin making a correction before the idle has a chance to drop. The PCM is programmed to apply a specific package of adjustments to the IAC valve, PW, and ignition timing for each load event signal.

Some of the inputs that are used by the ECM to anticipate a load are:
  • A/T Gear Position Switch
  • Air Conditioning Switch
  • Brake Light Switch
  • Clutch Switch
  • Electrical Load Detector (ELD) Sensor
  • Power Steering Pressure (PSP) Switch

23.4 Idle Air Control (IAC) Valve Overview

Honda used two different types of IAC valves. The most popular is the 2-wire
unit. The two wire IAC valve uses current through a winding to open a plunger against a spring. On some of the later model Hondas, a 3-wire IAC valve is used. The 3-wire is a rotary type valve. The following information applies to the 2-wire IAC valve. The 3-wire IAC valve is covered at the end to this section.

The models with IAC valves still have an idle air bypass screw that allows the
base idle to be set. The base idle speed is set by unplugging the IAC valve and adjusting the idle air bypass screw. Unplugging the IAC valve will set a diagnostic trouble code (DTC). It is best to clear the DTC with scan tool so that adaptive learning will not be lost.

The base idle speed is important. If the base idle is set too high, the IAC valve will not be able to bring down the idle speed by reducing the bypass air. If the base idle is set too low, the IAC valve will not be able to add enough air to compensate for large loads.

When the base idle speed is set correctly, the IAC valve will need to open some to supply additional air for the engine to reach correct idle speed.

23.5 Two Wire IAC Valve

The 2-wire IAC valve is supplied battery voltage on one side of its winding and the ECM supplies an electronically simulated ground on the other side, as shown in Illustration 23-1.

The ECM controls the current in the IAC valve winding by controlling the amount of time the ground is supplied to the IAC valve. The amount of current that flows through the IAC valve windings control how strong the magnetic force is that opens the plunger against spring tension.

On OBD-II equipped Hondas, the functionality of the IAC valve is checked by an output state monitor (OSM) inside the ECM. The current needed to operate the IAC valve is compared to a standard. If the current requirements deviate significantly from the normal level a DTC could be set.

The ground signal supplied by the ECM is a duty cycle type signal. The current is controlled in the IAC valve winding by controlling the amount of time the ground is supplied.

The valve does not fully open and close, but is held open a certain amount by the current in the valve windings. The ground is turned on and off so fast that the plunger does not have enough time to fully closed.

In Screen Capture 23-2 you can see the voltage reading of an IAC valve ground wire taken with a digital storage oscilloscope (DSO). When the volt- age is at 0v (at the bottom of the signal) a ground is being supplied to the IAC valve and current is flowing through the IAC valve winding.

The DSO Screen Capture 23-2 was taken on a cold engine. You can see that the ground time (0 volts) is more than the no ground time (battery voltage). This causes the IAC valve to be more open and increases the idle.

Screen Capture 23-3 is the same car but just warmed up to normal operating temperature. You will notice now that the on/off time is virtually the same.

Screen Capture 23-4 shows the IAC valve current as loads are created. Turning on the defrosters made the first "hump" and turning on the air conditioning created the second one.

This is the information that is available by using a Mastertech with the Honda/Acura software on some models as early as 1992. Many scan tools cannot provide this information prior to OBD-II. Since "IAC Command" is an OBD-II defined parameter, it is available on all OBD-II equipped models with a generic scan tool (GST).

When you use the Mastertech unit with the Honda/Acura software on pre OBD-II Hondas, the unit of measure is in amperes. The unit of measure for IAC valves since OBD-II is in "counts". It is a relative number that increases when the IAC valve current increases and decreases when the IAC valve current decreases.

The IAC valve is supplied heated water, via two small coolant hoses, to keep the valve from developing ice during cold operation. The IAC valve is not sensitive to the temperature increase from the heated water. The position of the IAC valve's plunger is totally dependent upon the amount of time the ground is on, which is controlled by the ECM.

IAC valves traditionally give very little trouble. You can test them for full open and full close operation as follows:

23.5.1 To Cause the IAC Valve to Fully Close

If you want to test the IAC valve to make sure it will fully close down, you can temporarily unplug it. When you unplug the IAC valve the idle should drop to the base idle. When the car is fully warmed up, and is on base idle, all the air is being controlled on the idle bypass screw. If the idle air bypass screw does not seem to have total control over the idle, the IAC valve may not be shutting down completely, or there is another source of unmetered air.

23.5.2 To Cause the IAC Valve to Fully Open

If you want to test the IAC valve to make sure it will fully open, you can temporarily ground the wire that goes from the IAC valve to the ECM. One wire will read steady battery voltage, and one will have varying volts. The varying volts wire is the one you should ground. When you ground this wire, the idle should increase significantly.

You can also test the 2-wire IAC valve winding with an ohmmeter. Make sure there is no continuity between the winding and the case. Check for approximately 11.5 ohms on the winding itself.

23.7 Service Issues

The idle control systems on Hondas are more prone to malfunction since they are a blend of mechanical and electrical devices. Also, older Hondas may use many of the different idle control methods on one car. Later model Hondas are less prone to idle control problems since virtually all the idle control is done with one component, the IAC valve.

Following is some of the more common service problems that you may experience in the field.

23.7.1 IAC Valve Not Responding to a Load

If you have a Honda that is equipped with an IAC valve and it does not appear to be responding to a load, check for a blocked inlet screen (shown inside the circle in Image 23-8). The inlet screen can become clogged, and stop bypass air even if the IAC valve is open.

The inlet screen had been used on some models, but there seems to be no pattern. Most IAC valves do not have this screen, but you should be aware if it.

23.7.2 Engine RPM "Dips" When a Load Event Occurs

If the RPM dips or bobbles when a certain load is introduced, it is probably not getting an "advanced warning" about the load. For instance, if every time you turn the steering wheel the idle fluctuates, the ECM is probably not getting a signal from the PSP switch.

23.7.3 Idle Speed Too High

This seems to be one of the more common problems with the idle control system. Since the PGMFI is a speed/density fuel injection system, any additional air will cause the idle to increase. Some of the sources of additional air could be:
  • Intake manifold to cylinder head vacuum leaks
  • IAC valve not closing down
  • Base idle speed set too high
  • Throttle blade not closing completely
  • Vacuum leaks from vacuum lines
  • Vacuum leak from a vacuum operated component
  • Fast idle thermovalve not closing (VXs do NOT have a FITV, so this will not be a cause for fluctuating idle on a VX)

The most common reason for a high idle is a defective fast idle thermovalve. The testing of this valve was covered earlier in this module. (VXs do NOT have a FITV, so this will not be a cause for fluctuating idle on a VX)

The second most common problem is that the throttle plate is not shut. The PGMFI system is designed to run with the throttle shut and all the air that enters the engine at idle bypassing the throttle plate.

The first thing to check for is a tight throttle cable. This is fairly common, especially on Civics. Make sure the cable has slack when the car is at idle.

If the cable is not too tight, but you suspect the throttle is being held open, check the throttle stop screw. The throttle has an external stop that keeps the throttle blade from actually hitting the bore of the throttle body. It is set at the factory and should never need setting.

Image 23-9 shows a typical throttle stop screw on a multi port injection system and a dual point injection system. The screws are usually hard to
find since you should not be adjusting on them. Look and see if the yellow
paint has been broken. It is not uncommon to find that somebody has used this screw to set the idle.

Honda does not offer an adjustment procedure for this screw since it is a factory setting. If the throttle stop needs setting simply unscrew it until the throttle is resting against the bore. Then turn the stop screw in until it starts moving the throttle. Go an additional 1/2 turn or so.

23.7.4 Idle Fluctuates Wildly

This situation is also common. This is not really a malfunction, but more of a symptom. It is symptom of an idle that is too high after the car is fully warmed.

What is actually going on is the ECM's fuel cut on deceleration strategy is cutting the injectors off at 1100 RPM. You can see this from the scan tool Screen Capture 23-5. When the solid line (RPM) goes up to 1100, the dotted line (PW) goes to zero. When the RPM drops to below 1100 the injector turns back on. This cycle will repeat until the idle is brought below 1100 RPM.

The ECM determines that the car is decelerating if it sees the throttle closed and the RPM above 1100 RPM. During deceleration (at normal operating temperature) the ECM cuts the injectors off for fuel economy and emissions control. This same situation occurs if the idle RPM is too high (throttle closed and over 1100 RPM).

To correct this problem look for the source of air that is causing the engine to idle so high.


Last edited by TomO; 05-27-2009 at 03:45 PM..
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Old 01-14-2009, 02:58 PM   #2 (permalink)
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Holy cow. Thanks for all the great info! Is the OBD training manual available online?
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Old 08-22-2009, 09:27 AM   #3 (permalink)
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where did u get so much info.for years I have been seraching and never found anything like this.u hit the nail on the head(concise).u hav a contact need as much info as possible.automechanic.
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Old 08-22-2009, 11:06 AM   #4 (permalink)
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Thanks for the geat info! I needed this.
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Old 08-22-2009, 12:03 PM   #5 (permalink)
That VX guy!
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The VX - '92 Honda Civic VX
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You all are welcome for the info. I actually have a copy of the OBD Training Manual that I source the info from when I have time to read it.

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Old 08-22-2009, 02:06 PM   #6 (permalink)
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I have to say, the PGM-FI system on my '90 Accord is far better than a lot of newer vehicles I've driven. Only the A/C toggle makes any appreciable change in the idle, and then only for a fraction of a second. The fact that I can crank the steering wheel as fast as possible and the idle doesn't even bobble bests even my OBD-II '96 Ford Ranger.

Is there anything in there that indicates a sensor that I can interrupt that will kill the engine instantly without either powering off the computer or triggering the Check Engine light?

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Old 02-02-2011, 01:22 PM   #7 (permalink)
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Hi. TomO - once again great info! Thanks!

Last edited by cnc; 02-04-2011 at 02:16 PM..
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Old 02-02-2011, 01:59 PM   #8 (permalink)
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Great information! Just wondering if any of this applies to the HCH II. I have an bobble issue at idle. Appears to be the a/c compressor cycling. Not a major problem for me, but annoying to other drivers of the car.
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Old 03-13-2011, 10:51 PM   #9 (permalink)
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bumping this from the dead.

I have a VX engine D15Z1 i am working on getting into an 89 civic wagon.

I do not have a IAC valve. Is the IACV very specific for a vx?

I see that the vx does not have the fast idle thermovalve. It does have a bolt pattern similar to the one shown in image 23-8 shown above. I am thinking about going to a yard and finding something with a similar bolt pattern and 2 wire plug.

Any one else run into an issue replacing the IACV on a vx?
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Old 06-01-2011, 07:47 PM   #10 (permalink)
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Thanks a lot, this is exactly what I have been looking for. I have an issue with my Honda, and I suspect that it has something to do with the fuel injection system, but I'm not sure. This information should be very helpful in helping me to isolate the problem.
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Last edited by BigGreenie; 06-06-2011 at 01:01 PM..
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