P0518 Idle Air Control Circuit Intermittent
OBD-II Trouble Code Technical Description
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:
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:
P0508 Idle Air Control System Circuit Low P0509 Idle Air Control System Circuit High P0511 Idle Air Control Circuit P0518 Idle Air Control Circuit IntermittentSeverity & 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:
High engine RPM level Engine stall, especially at idle Unusual noises from the throttle body area Other IAC related codesCauses
Possible causes for this code include:
Defective IAC motor/valve Engine vacuum leak Open or shorted IAC control wiring or connectors Carbon buildup in throttle body or IAC valve Faulty PCM or PCM programming errorDiagnostic 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 wil
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