Many newer cars with powertrain control modules (PCMs) have to relearn engine idle after a battery is disconnected. The removal of electrical power from the engine erases idle memory from the PCM, the computer where it is stored. The relearning process depends on the PCM.
Engine idle refers to the lowest rpm or speed at which the engine turns over when the driver is not depressing the gas pedal, according to 2CarPros.com. At idle speed, the engine generates its lowest emissions, thus conserving fuel. Idle speed that is too low can stall an engine. If it's too high, fuel is wasted and more emissions are produced.
During the relearning procedure, the PCM may change the idle speed until the correct speed is achieved. If the engine threatens to stall or idles too high, stepping on the gas slightly or taking the car for a short drive will help set the PCM correctly. But with some PCMs, the relearning procedure is not noticeable.
Engine idle is only part of the natural memory, or accumulated data, lost when the battery is replaced. Adjustments to the fuel mixture, transmission shifting and various habits of the driver may all have to be relearned. It may be necessary to use a scan tool to reset PCMs on newer cars and to reset the anti-theft system, according to AA1Car.com.
