Theory of Operation
The idle air control motor or valve is a device in charge of
maintaining an acceptable engine idle speed by letting a controlled amount
air passed the throttle plates. This acceptable idle speed is determined by
what the ECM considers to be the “DESIRED IDLE”, which is an
actual PID on the scan tool. The ECM is constantly comparing the actual idle engine RPM to its
desired idle speed. The inability of the ECM not to be able to match
these two values creates a problem for proper idle operation. Not all
scanners provide the desired idle PID, and there are variants of idle air
control valves. In essence, they all accomplish the same thing. The IAC
motors/valve is directly controlled by the ECM, regardless of the type.
The first variant is the stepper motor type. Stepper
motors are special types of electric motors that do not turn continuously
like regular electric motors. These devices turn only a couple of degrees
when power is applied to them. In actuality they require a series of
electric pulses to operate. Depending to the particular stepper motor
design, it will turn X amount of degrees for every electric pulse. For
example, if a specific type of stepper turns 10 degrees (X=10 º) for every
electric pulse, then by the ECM applying 3 pulses the IAC (stepper motor)
will turn 30 º. This
pulse-step-count value is seen on the scanner as the IAC COUNT. The higher the IAC
count more air is bypassed and the higher the idle speed.
The second type is the duty cycle controlled valve.
This type of IAC is fairly simple in operation. It consists of an electric
motor coil winding and operates by receiving continuous rapid pulses by the
ECM having a specific duty cycle (ON TIME). The longer the duty cycle (ON
TIME), the more the IAC valve opens and hence the more bypassed air that the
valve lets through, therefore raising the idle speed. This type of IAC
operation closely resembles that of an injector. It is referenced on a scan
tool as IAC %, not
count. A high percentage indicates a longer duty cycle (ON TIME).
A third type of IAC is
a regular DC motor
attached to a rod like actuator that actually opens and closes the throttle
plates. These usually have what is called a nose switch, which is an actual
switch inside the DC motor that senses when the throttle plate lever is
pushing against it. Every time the IAC actuator rod touches the throttle
plate lever, the switch closes and it is indicated by a particular scanner
PID (NOSE SWITCH or THROTTLE
SWITCH) as an ON/OFF value. This type of IAC is found among others in
CADILLACS and some Asian imports.
Still another type of
IAC is the mechanical
coolant temperature activated type. This device works like a coolant
thermostat, but is not considered an actual full time idle controller, since
it is used only for warm up. Therefore it is not covered in this article.
However this type of valve, if not operating properly, will cause idle
IAC motors are very closely related to the minimum air rate
adjustments. If this adjustment is not performed properly, there will be
problems with the idle speed. If the IAC can not control the idle speed, it
will set a CODE and usually light up the CEL. Typical IAC counts are between
6 and 15 at idle, and percentages at close to 25%. As loads are placed on
the engine (power steering, A/C, electrical loads) the IAC count or
percentage also increases. This is indicative of the ECM trying to raise the
idle to compensate for the extra load.
Conditions that Affect Operation
The minimum air rate is the first area to check for when
encountering IAC problems. This adjustment is the lowest possible engine RPM
that it can operate at, right before stalling. It is basically accomplished
by setting the IAC valve to its closed position. In other words, so that it
is not controlling idle at all, therefore, eliminating it from the
adjustment process altogether. With the IAC closed, the idle screw is
adjusted to a minimum RPM, according to specs. A rule of thumb is to go
slightly lower than the desired idle RPM seen on the scan tool, but not
letting the engine stall (about 600 RPM). After this operation, further TPS
adjustments are necessary, which can be found on the minimum air rate
article. What the minimum air rate adjustment does is to set the throttle
plate screw to the lowest RPM possible. By doing so, the IAC will always be
in control, since idle RPM will never go as low as the throttle screw
setting to begin with. For example, if the throttle screw is set to 600 RPM
the IAC will tend to maintain a higher idle and therefore will always be in
It is also obvious that an engine with mechanical problems
will have a hard time running at such low RPM speed. That is why engine
mechanical problems could also set codes for the IAC motor (IAC count too
high/low). Vacuum leaks will render the IAC valve operation useless, since
it would not be able to control idle. The IAC motor is in effect a
controlled vacuum leak and any other uncontrolled vacuum leaks will usually set the IAC counts to 0 on
the scan tool. Throttle body carbonization is also detrimental to
maintaining proper idle speed. This problem will usually raise the IAC count
to above normal, since the ECM has a clogged throttle bore and needs to
raise the idle speed. Improper TPS adjustment will skew the IAC operation.
This condition will signal the ECM that the throttle plate is parked at the
wrong position. The ECM looks at the TPS reading very closely when operating
the IAC valve or motor. This is why the minimum air rate adjustment is of
tremendous importance for correct IAC operation and idle adjustment.
Regardless of what type of IAC we are dealing with, the same
functional tests apply to all of them.
• First, analyze the IAC step count seen on the scan
tool. A low count or percentage indicates a throttle screw set excessively
high or a vacuum leak. A high count/percentage indicates an overly closed
throttle plate screw setting or an air intake obstruction (clogged air
filter or throttle body).
• Second, watch for correct TPS readings at idle. An out of
adjustment TPS base reading will throw-off the IAC operation. If the TPS is
out of adjustment, perform a minimum air rate adjustment.
• Third, using either a scan tool or an IAC actuating
electronic tool set it so that it is fully closed. Determine that the engine
is at the lowest possible idle speed setting (600 RPM) and that the engine
does not stall. If the engine stalls it is an indication of a misadjusted
minimum air rate. Adjust it accordingly.
• Fourth, make sure that there are no uncalled for engine
loads or vacuum leaks. A shorted power steering switch, low engine vacuum,
A/C on/off switch, etc will make the ECM think that there is a need for a
higher idle speed when in fact, this is not the case. A quick glance at the
engine load PID on the scanner will alert you to this. Vacuum leaks
will also raise the LT FUEL TRIMS PID value on the scanner. Always
remember that any uncalled for vacuum leaks, engine loads or out of
adjustment minimum air rate will cause IAC operation problems. The minimum
air rate, engine load and TPS adjustment go hand-in-hand when it comes to
the IAC valve. These procedures give the technician a general idea for a
good game plan of attack in diagnosing idle control and IAC problems.