A vacuum leak will cause a high idle speed without a corresponding increase in power. Stalling when coming to a stop is typically caused by idle speed that is too low. That could be the result of a misfiring cylinder, or a problem with the idle speed motor and circuit. Electrical problems with the idle speed motor are not common. Also, this is not a normal motor with a spinning shaft. It is a "stepper" motor that has an armature that is placed at various positions by pulsing its electromagnetic coils with varying voltages and polarities. As the armature slowly rotates, it is on a threaded shaft that extends or retracts a pintle valve. That valve opens an air passage around the throttle blade. That is a controlled vacuum leak. At the same time, the engine computer shortens or lengthens the amount of time it pulses the injectors on to adjust the amount of fuel to go with the amount of air it is letting in. That way idle speed is controlled with a corresponding amount of power.
The idle speed motor, often called the automatic idle speed motor, (AIS), causes very little trouble. You are more likely to find the engine computer is responding to an incorrect sensor reading that would call for a lower idle speed. That can often be identified by observing the sensor readings on a scanner while the low idle speed is occurring. On GM engines, it is somewhat common to find the coolant temperature sensor reading incorrectly due to corrosion on the sensor's connector terminals.
Years ago it was real common to find low idle speed caused by carbon build-up blocking the air passage around the throttle blade on some engines. You could see on the scanner that the computer was commanding the idle speed motor to open more and more, but idle speed never increased. That problem has become very rare today thanks to better additives in gas.
The next step in identifying the cause is to observe on the scanner how far the idle speed motor is being commanded to open. On Chrysler engines, and some others, you look at the number of "steps" it is listed at out of 256 steps. For a properly-running engine, step 32 is typical. With one misfiring cylinder on a V-8 engine, the computer will need to bump it up to around step 50 to get idle speed back up. Many other manufacturers list the idle speed setting as a percentage. Your job is to see if idle speed goes up when the steps or percentage listed goes up, meaning the system is working, or if idle speed does not go up when the steps listed are higher, meaning the computer is trying to raise idle speed but is not having success. That is where a plugged passage or a sticking motor are the best suspects. The engine computer does not monitor the position of the idle speed motor. It only looks at the engine speed resulting from where it thinks it set the motor to.
Wednesday, January 31st, 2018 AT 11:08 PM
