The Idle Air Control Valve, will let you breath or not ...

The IAC or Idle Air Control Valve has a simple job, to let a measured amount of air pass the intake throttle to thereby adjust the idle speed. A lot goes behind the scenes in this system, as the ECM has to take this air leak into account and adjust for at the injector pulse width. Here's how this works...


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 internal pre-programmed 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 problems.

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 control.

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.

Component Testing

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.


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