Idle Control Mode - For heavens sake let the the car idle for a while...

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Idle-speed control mode – is used to control the engine’s idle speed and prevent a stall. The ECM takes various sensors into account when in the idle mode, such as transmission range switch, power steering switch, A/C status, RPM, TPS and ECT. The engine’s idle speed is an adaptive feature of the ECM. This means that the idle air control valve position has to be commanded by the ECM just right, in order for it to maintain the correct idle speed. Therefore, the ECM has to take into consideration all the possible loads that can be present at idle. A good example would be the power steering pressure switch signal, which the ECM uses to increase the IAC steps and injector pulse to compensate for the extra load. These operating conditions must be learned by the ECM and stored in RAM/KAM memory for future reference. The same can be said for drive-by-wire systems where the throttle plate motor position must be learned at different operating conditions. This mode may also be associated with excessive HC and CO emissions. In these cases, if one of the load signals (power steering, A/C, trans range) is shorted, the ECM will adjust idle speed, timing and increase injector pulse to compensate for the false load. A thorough PID analysis will reveal the faulty load signal. Yet, another faulty condition associated with the idle mode is a false idle switch input signal. As previously explained, in order for the idle mode to be activated either the idle-switch or TPS should indicate a closed throttle. The absence of the idle signal with a closed throttle plate will surely create an engine performance problem. On the other hand, a shorted idle signal at an off idle throttle position will create severe hesitation, since the system reverts back into injection cut-off mode.


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