Why do we need to adjust idle speed?
If the engine is operating a large number of accessories, particularly air conditioning, the idle speed must be raised to ensure that the engine generates enough power to run smoothly and operate the accessories.
What makes engine idle high?
In modern vehicles, engine idle speed is most often controlled by the idle air control (IAC) motor. When your car’s throttle is malfunctioning, this can cause your car to stall or idle high. Often this is a result of dirt buildup in the air intake. Other times it can be a problem with a cracked intake tube.
How do you adjust the idle speed of an engine?
They use several designs, but most work in a similar manner. On early vehicles, General Motors often uses a stepper motor that turns a thread valve. Rotation of the armature moves the air valve in and out, adjusting the idle speed of the engine. The older Ford products more often use a spring-load plunger for the idle control valve.
How does the IAC system affect the idle speed?
For instance, switching the air conditioning on, increases the engine load. With the IAC system, the computer immediately increases the idle speed to compensate. A cold engine runs with less efficiency and the PCM also commands a higher idle speed. This helps to circulate engine oil and prevents stalling while the engine warms.
How does electronic fuel injection affect idle speed?
When engines had carburetors, they always provided a means of adjusting the idle speed. A simple turn of a screw corrected the problem if the engine’s idle speed was too slow or too fast. With electronic Fuel injection they eliminate the manual adjustment and the computer makes all corrections to the engine’s idle speed.
Why does a cold engine have a higher idle speed?
A cold engine runs with less efficiency and the PCM also commands a higher idle speed. This helps to circulate engine oil and prevents stalling while the engine warms. The fuel injectors may also add additional fuel to help the cold engine run better, much as a choke does on older engines.