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P0518 Idle Air Control Circuit Intermittent

OBD-II Trouble Code Technical Description

Article by
Stephen Darby
Stephen Darby
ASE Certified Technician

Idle Air Control Circuit Intermittent

What does that mean?

This diagnostic trouble code (DTC) is a generic powertrain code, which means that it applies to all 1996-newer vehicles (Dodge, Chrysler, Hyundai, Jeep, Mazda, etc.). Although generic, the specific repair steps may vary depending on make/model.

When your OBD-II equipped vehicle exhibits a malfunction indicator lamp (MIL) and a stored code P0518, it means that the powertrain control module (PCM) has detected an intermittent problem with the intake idle air control (IAC) circuit. If there are other, more specific, IAC codes these must be diagnosed and repaired first.

The IAC system revolves around a linear servomotor that controls a plunger which regulates the amount of air allowed to bypass the throttle plate and enter the intake manifold. Controlling the amount of air allowed to bypass the throttle plate is conducive to controlling engine RPM at idle. When the throttle plate is closed (at idle), very little air is allowed to enter the engine through the throttle body. The engine cannot continue to idle with this small amount of air and would certainly stall. An auxiliary air (vacuum) source must be provided so that the engine can continue to idle with the throttle plate closed. Of course, the auxiliary vacuum source would be detrimental to engine performance and fuel efficiency when the throttle plate is opened (upon acceleration). Hence the need for a fully adjustable vacuum source that is controlled by the PCM.

The IAC valve plunger housing is manufactured with two separate openings that allow air to pass through them. The IAC valve is usually bolted to the intake manifold, on or near the throttle body. It is positioned so that, when it is open (the plunger moves out of the opening), ambient air is allowed to flow through it and into the manifold and when it is closed (the plunger fills the opening) ambient air cannot flow through it.

An example of an Idle Air Control (IAC) valve:
IAC idle air control valve

The IAC is seldom completely open or closed but remains constantly in motion; controlled by the PCM. The PCM uses input signals from the oxygen sensors, the throttle position sensor, and various other sensors to determine how much air should be allowed to bypass the throttle plate. As voltage and ground signals to the IAC valve are adjusted (by the PCM) the valve reacts accordingly. If the engine RPM levels fail to coincide with programmed desired limitations, a code P0518 will be stored and a MIL may be illuminated.

Related idle air control system circuit trouble codes include:

Severity & Symptoms

A malfunction in the idle air control system may cause the engine to stall or idle at an unexpectedly high RPM. This code should be treated as severe and addressed with urgency. Symptoms of a P0518 code may include:

Causes

Possible causes for this code include:

Diagnostic and Repair Procedures

A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known issue with a known fix put out by the manufacturer and can save you time and money during diagnosis.

In order to diagnose a code P0518, I would require a diagnostic scanner, a digital volt/ohmmeter (DVOM), and a reputable vehicle information source (All Data DIY will do).

I might begin by examining all IAC system wiring and electrical connectors and repairing any that are damaged, corroded, or burnt. I would also check the engine for any vacuum leaks that might contribute to an uncontrollable engine RPM level. If you find any vacuum leaks, repair them and recheck the system.

Next, I’d connect the scanner to the vehicle diagnostic connector and retrieve all the stored codes and freeze frame data. I like to write this down in case this code is intermittent. Now, I clear the codes and test-drive the vehicle to see if it is reset. Obviously, if there are any other IAC related codes stored, I need to diagnose and repair those first. Also, check all technical service bulletins and safety recalls for the vehicle and malfunction in question. After performing any repairs, drive the vehicle normally and allow the PCM to enter readiness mode before declaring the problem rectified.

As a simple method for testing the IAC system, when engine idle RPM is very high, I carefully (to preserve the gasket or seal) unbolt the IAC valve from the engine and use a shop rag and my fingers to control engine RPM at idle. If I can control a high idle condition, after removing the IAC valve, then I know that the IAC system is not functioning properly. If I cannot control the RPM at idle, I suspect that a large vacuum leak is present. Large engine vacuum leaks may include PCV valve/hoses, vacuum hoses, vacuum brake booster, check valve, or hoses, EGR valve stuck open, EGR gasket leak, or an intake manifold gasket leak.

Consult your vehicle information source for the recommended method of testing the IAC motor with the DVOM. Replace the IAC motor/valve if it des not comply with manufacturer’s specifications.

Additional diagnostic notes:

Related P0518 DTC Discussions

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