Error Code P0518 is defined as Idle Air Control Circuit Intermittent, meaning there’s been a registered inconsistency in the engine’s idle RPM.
This is a generic trouble code, meaning it applies to all vehicles equipped with OBD-II system, especially those made since 1996 up to present. It is more common among Chrysler, Dodge, Hyundai, Jeep, and Mazda makes, though it also appears in every brand. Specifications on the definition, troubleshooting, and repairs, of course, vary from one make and model to another.
Revolving around a linear servomotor which controls a plunger, the IAC system works by regulating the amount of air allowed to bypass the throttle plate and enters the intake manifold. This way, the engine RPM at idle is also controlled. When the throttle plate is closed (at idle), the throttle body only allows a tiny amount of air to pass through. The engine will not be able to continue it’s idle and will eventually stall. Thus, there’s an auxiliary air (vacuum) source provided for the engine to continue it’s idle while the throttle plate is closed. Obviously, this auxiliary vacuum source is detrimental to both the engine performance and fuel efficiency when the throttle plate is opened (upon acceleration). Hence, there a fully adjustable vacuum source is controlled by the PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes).
Vehicle manufacturers set expected idle RPM for the engine. The PCM (powertrain control module, also known as ECM or engine control module in other vehicle makes) reads this RPM during idle. If the reading is too high or too low than the specified digits set by the manufacturer, the PCM will automatically try to correct the problem by sending specific commands to the various parts of the engine transmission. If this PCM is unable to correct the RPM, then the Error Code P0518 will be set.
Other related error codes include:
- Error Code P0508 Idle Air Control System Circuit Low
- Error Code P0509 Idle Air Control System Circuit High
- Error Code P0511 Idle Air Control Circuit
- High engine RPM level
- Engine stalls, especially at idle or low speed
- Intermittent stalling when the throttle is released
- Unusual noises coming from the throttle body area
Many components could go wrong and lead to this error code, such as:
- Engine vacuum leak
- Faulty AC motor/valve
- Open or shorted wirings or connectors in the IAC
- Carbon buildup in the throttle body or IAC valve
- Defective or programming error in PCM
How to Check
A good starting point to diagnose this code is always checking with the TSB (technical service bulletins) for the specific model of the vehicle. Manufacturers provide step-by-step guides to address most problems.
Diagnosing this code would require a diagnostic scanner, DVOM (digital volt/ohmmeter), and a quality information source.
Specific diagnosis should start with a visual inspection of the wirings and electrical connectors in the IAC system. Any burnt, corroded, and damaged problems must be fixed, if not replaced.
Then, the engine must be checked for leaks, as leaks can contribute to uncontrollable engine RPM levels. Any vacuum leaks must be fixed as well.
Next, hook the scanner the vehicle diagnostic connector to retrieve all stored codes and their freeze frame data. Take note of these codes. Then, clear the codes and then take the vehicle for a test drive to see if the codes reset.
If there are other IAC related codes stored, then they should be diagnosed and repaired first.
Then, allow the PCM to enter readiness mode before declaring the problem is fixed.
A simple method to test the IAC system is that, when the engine idle RPM is extremely high, carefully unbolt the IAC valve from the engine (make sure you preserve the seal or gasket) from the engine. Then, use a shop rag with your fingers to control engine RPM at idle. If you can control the high RPM this way, then that means the IAC system is in good condition and working properly. If you CANNOT control the RPM idle, then there must be a large vacuum leak. Large engine vacuum leaks may include PCV valve/hoses, vacuum brake booster, vacuum hoses, hoses, check valves, gasket leak or stuck open EGR valve, or intake manifold gasket leaks.
How to Fix
Depending on your diagnosis, common repairs for this code include:
- Cleaning of the throttle body bore
- Vacuum leak replacement
- Idle Air Control valve replacement
- Replacement of defective PCM
A malfunction issue in the IAC system can cause the engine to stall or idle at unexpectedly high RPM, which could pose a serious problem to the engine. Thus, it’s important to have this problem addressed as soon as possible.