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23 Idle Control / Idle Air Control (IAC) Valve

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 carbu-
retor. 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

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Module 23
Idle Control / Idle Air Control (IAC) Valve
Author: Grant Swaim IMPORTANT - READ !

E-mail: sureseal@nr.infi.net Do not read or study this information unless you agree to the
following conditions:
URL: www.tech2tech.net The information in this training module is the intellectual
property of N. Grant Swaim and is copyrighted by Sure Seal
Phone: (336) 632-9882 Products Inc.

Fax: (336) 632-9688 Subscribers to the Tech-2-Tech website, and persons partici-
pating in Tech-2-Tech’s on-line training program are entitled
to read this material on-line.
Postal Address: Tech-2-Tech Website
PO Box 18443 You may also click on the “save” icon on the Acrobat viewer
Greensboro, NC 27419 and save a copy to your local computer. You may save a
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the printed copy becomes illegible, or lost, an additional
Last Update: April 2000 copy may be printed.

Tech-2-Tech offers the following training modules in printed manual, CD-ROM, and on-line formats.

PGMFI Training Modules OBD-II Training Modules

• The PGMFI System Overview—Part 1 • On Board Diagnostics—General Overview

• The PGMFI System Overview—Part 2 • Diagnostic Trouble Codes
• PGMFI Flash Type DTCs • MIL / Freeze Frame
• Inputs / Outputs—Part 1 • Scan Tool
• Inputs / Outputs—Part 2 • Scan Tool—Advanced
• Engine Control Module • Monitor Tests—Overview
• Air Flow / MAP Sensor—Base Inj Pulse Width • Comprehensive Component Monitor
• Fuel Delivery System • Catalyst Monitor
• Closed Loop Strategies—Theory • EGR Monitor
• Closed Loop Strategies—Case Studies • Evaporative Monitor
• Thermistor Inputs • Fuel System Monitor
• Throttle Position Sensor • Misfire Monitor
• EGR Valve Lift Sensor • Oxygen Sensor Monitor
• MAP / BARO Sensor • Oxygen Sensor Heater Monitor
• Ignition Inputs • “P” Codes
• Vehicle Speed Sensor
• Oxygen Sensor Miscellaneous Training Material
• Lean Air Fuel Sensor
• Glossary of Terms
• Miscellaneous Input Signals
• Fuel Injectors—Multi-Port Injection
• Fuel Injectors—Dual Point Injection
• Ignition System—Outputs
• Idle Air Control Valve
2000 © - All Rights Reserved Sure Seal Products Inc - Greensboro, NC
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ECM would typically widen the injector pulse width (PW) slightly, and increase
timing, when any type of load was sensed.

23.2 Methods Used To Control Idle RPM

Honda has used at least
Image 23-1 Idle Controllers five different methods to
control idle speed. In
many cases you will find
several different meth-
ods used on the same
car. The engine shown
in Image 23-1 utilizes
three of the main idle
control methods.

Honda used all these

methods over the years.
Sometimes the same
task would be handled
using one method on
one model and another
method for the same
task on another model within the same year. This training module will not attempt
to identify exactly which method is used on what model. The training module will
explain all the idle control tasks, but will not identify exactly which method is
used to perform the tasks on each model.

With a little practice, a tech should be able to identify all the methods used to per-
form idle control tasks, on any given Honda model by performing an underhood
inspection. This is helpful since most specific idle problems can be associated
with specific idle control task and the method used to perform the task.

The six main methods used to control idle speed are summarized below:

23.2.1 IAC Valve

The left most arrow, in Image 23-1, highlights an IAC valve. The IAC valve was
added in 1988 to all models and it gave the ECM total control over the idle. The
IAC valve is covered in detail in the "Basic Idle Speed" section.

23.2.2 Fast Idle Thermo Valve

The middle arrow, in Image 23-1, highlights the fast idle thermo valve, which is
an all mechanical valve. The valve is heat sensitive and it will let air bypass the
throttle plate when the engine is cold. The fast idle valve is covered in detail in
the "Fast Idle" section.

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23.2.3 Starting Air Valve

The right most arrow, in Image 23-1, highlights a vacuum activated mechanical
valve that is used to add air at each start-up to temporarily increase idle speed.
The valve is covered in more detail in the "Extra Air at Start-Up" section.

23.2.4 Air Dump Solenoid

A fourth technique used to add air to the intake manifold under certain circum-
stances is the use of an air dump solenoid. The solenoid is usually located in an
emission box. The solenoid would be activated by the ECM to add a fixed amount
of air to the intake manifold.

Air dump solenoids have been used by different models to compensate for a num-
ber of different loads. They have been used to offset for A/Cs, automatic trans-
mission loads, and to add air at initial startup.

23.2.5 Throttle Kick-Up Vacuum Diaphragm

The fifth technique used to control idle was the use of a simple vacuum dia-
phragm to actually open the throttle valve a set amount. This technique is covered
in more detail in the "Air Conditioning Idle Boost" section.

23.2.6 Pulse Width and Ignition Timing

When the ECM senses a load on the engine, it will typically enrichen the air/fuel
mixture and add several degrees of ignition timing. This helps to stabilize the idle.
This method is covered in more detail in the "Load Event" section.

23.3 The Idle Control Tasks

On all models, certain idle control tasks must be performed. Over the years sev-
eral different methods were used to perform these tasks. Following is a listing of
idle control tasks that have to be performed on every Honda by using some
method or a combination of methods.

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 con-
trolled 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.

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To adjust the idle follow the instructions

Image 23-2 Air Bypass Screw
from the underhood label or the service
manual. The technique used to set idle
for models with an IAC valve is differ-
ent 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 elec-
trically 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.

23.3.2 Fast Idle

When a Honda is started cold, the idle must be increased until the engine warms
up. This is accomplished by the use of a fast idle thermo valve on most models.
The fast idle valve is located at the middle arrow in, Image 23-1.

The valves were used by the first PGMFI systems in 1985 and have been used on
most models until the late 90s. When the IAC valve was added to all models in
1988, the fast idle thermo valve was kept. Many newer systems have dropped the
fast idle valve and use the IAC valve for all idle control. When this is done the 3-
wired systems are usually used.

The valve is open when cold (below 86 Deg F) and will gradually close as the car
warms up. When the valve is open it al-
Image 23-3 lows air to by-pass the throttle blade. The
valve is closed by an expanding wax pel-
let that is heated up by the engine's cool-
ant. The valve is opened by a spring that
opposes the force from the wax pellet.
When the engine has reached a certain
temperature, the fast idle valve will be
completely closed and the idle will be un-
der the control of the IAC valve (on 88
and later models).

The fast idle valve, being an all mechani-

cal device, does tend to give some problems. The most common problem is that
the expanding wax pellet gets "lazy" and the valve will not completely close.

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You can easily check the valve by removing

Figure 23.4
the two screws shown inside the circles in
Image 23-3, and removing the lid.

If the valve does not completely close down

(after the coolant has reached 86 Deg F), the
IAC valve will not be able to control the idle
speed with this additional air entering the in-
take.

You check for the valve closing down com-

pletely by sticking your finger in the hole in
the middle of the seat (shown in Image 23-4).
If the plunger is not sealing off the air on the seat, you will feel suction. You can
usually hear air rushing also.

If a fast idle thermovalve is not closing down when the engine is fully warmed the
valve should be replaced. While some techs readjust the valve seat when the IAC
valve does not close off at full operating temperature, it is not a good practice.
When the wax pellet gets "lazy" and the plunger does not fully close, adjusting
the valve seat down to meet the plunger cuts RPMs off the fast idle on cold start-
ups.

Also note that a Honda with a coolant system that is not heating up properly may
not cause the thermovalve to close completely. Before replacing a thermovalve,
make sure the cooling system is working properly and a good supply of coolant is
being supplied to the valve.

23.3.3 Extra Air at Start-Up

When a Honda is initially started up, it will run better right after start up if addi-
tional air is added to the intake manifold. This is accomplished either by a me-
chanical starting air valve, electronically by the ECM activating an "air dump"
solenoid, or handled by the IAC valve. A mechanical starting air valve is shown at
the right most arrow, in Image 23-1.
Image 23-5
The mechanical style uses a starting air valve, which is
normally open and allows air to enter the intake manifold.
While the engine is cranking this valve is open and adding
air. Once the engine has started the rising intake manifold
vacuum will close the valve down and no more air will be
added to the manifold from this valve.

Some models do not use a mechanical valve but do dump

additional air into the manifold by activating a solenoid
(shown in Image 23-5). The ECM activates a solenoid

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(usually located in an emission box), which adds air to the intake during starting.

When the IAC valve was added in 1988, many models added extra air at
start-up with it. Some models still retained a starting air valve or an air
dump solenoid even after an IAC valve was added.

23.3.4 Air Conditioning Idle Boost

All Honda PGMFI systems have a way to compensate for the load of the
air conditioning. At least four different methods have been used over the
years to do this. Following is a closer look at the methods used by the
PGMFI system.

The earliest systems used a vacuum diaphragm to actually open the throttle much
like the earlier carbureted systems did. This sys-
Image 23-6 A/C Boost tem was only used for a couple of years.

When the A/C was turned on the ECM

would activate a solenoid that applied
vacuum to the idle boost diaphragm (in-
side the large circle in Image 23-6). The
idle speed, for when the A/C was acti-
vated, was set by a screw in the linkage
(inside the small circle).

All the A/C idle boost methods used

since the diaphragm method do not open
the throttle, they add air to the intake
manifold.
Image 23-7 A/C Boost
One of the earlier methods used to boost the idle
was a valve like pictured in Image 23-7. When
the A/C was turned on, the ECM would activate a
solenoid that would apply vacuum to the top
hose. This would open a valve that would let air
enter the intake manifold. The amount of air that
was added was controlled by the white plastic
knob (shown inside the circle).

Air dump solenoids were used on a few models to

add a specific amount of air to offset the load of
the A/C.

The latest method used is to simply use the IAC valve to offset the load of
the A/C. When the IAC valve was first added, in 1988, many models still

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used some of the other methods to control A/C load. Later models use the
IAC valve only to control operational idle.

23.3.5 Load Events

The Honda engines run so lean at idle that virtually

Screen Capture 23-1
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 electri-

cal load on the PW and ignition timing. This tech-
nique 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 strat-

egy 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 sig-
nificant 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 sig-
nal.

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.

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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 diagnos-
tic 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 com-
pensate 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

Illustration 23-1
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 moni-
tor (OSM) inside the ECM. The current needed to operate the IAC valve is com-
pared 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.

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In Screen Capture 23-2 you can see the voltage

Screen Capture 23-2
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 flow-
ing 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 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
Screen Capture 23-4
"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:

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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 com-
pletely, 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 tempo-
rarily 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.

23.6 Rotary Type IAC Valves (3-Wire Units)

The 3-wire IAC valve is a rotary type valve and can handle larger volumes of by-
pass air. Usually a model using a 3-wire IAC valve will not utilize a fast idle
thermo valve. You will find a 3-wire IAC valve on 96-98 Civics (with A/T), 98
Accords, and 98 Odyssey.

Very little theory of operation or testing information is revealed in the Honda ser-
vice manuals about the 3-wire IAC valves. I will be doing further field research
and will have more theory of operation information and testing information avail-
able soon !

You can check the 3-wire IAC valve with an ohm meter. The center terminal is
the power feed to two windings. The two outside terminals are grounds for each
of the two windings and they are connected to the ECM. You should read 16-28
ohms between the center terminal and each outside terminal. As usual, you should
have no continuity between any windings and the body of the IAC valve.

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.

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Following is some of the more common service problems that you may experi-
ence in the field.

23.7.1 IAC Valve Not Responding to a Load

Image 23-8
If you have a Honda that is equipped
with an IAC valve and it does not ap-
pear to be responding to a load, check
for a blocked inlet screen (shown in-
side 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 pat-
tern. 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

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.

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.

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The first thing to check for is a tight

Image 23-9 Throttle Stop Srew
throttle cable. This is fairly common,
especially on Civics. Make sure the ca-
ble has slack when the car is at idle.

If the cable is not too tight, but you sus-

pect the throttle is being held open,
check the throttle stop screw. The throt-
tle 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.

Images 23-9 and 23-10 show a typical

throttle stop screw on a multi port injec-
tion system and a dual point injection
Image 23-10 Throttle Stop Screw 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 uncom-
mon 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 fac-
tory setting. If the throttle stop needs
setting simply unscrew it until the throt-
tle 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

Screen Capture 23-5 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

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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 tempera-
ture) 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.

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