When we release the accelerator, the engine’s speed slows. At a preset speed the engine idle circuit takes over and the engine runs at the lowest speed possible without stalling. Watching the tachometer shows an idle speed around 500 to 900 RPM. The engine’s idle speed also changes, based on outside conditions. A computer is adjusting the idle speed, based on complex calculations.
How do we adjust an engine’s idle speed?
In the past, adjusting an engine’s idle speed was easy. Manufactures have removed these adjustments on modern vehicles. The power control module or PCM now determines the speed at which an engine idles, based on several input-sensors. Manual adjustment of idle speed is NOT possible on modern vehicles and diagnosis of problems is a bit complex.
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.
They have eliminated the idle speed adjustment for many and involved reasons. Fuel injection carefully monitors the air flow, as it is directly relates to the fuel/air mixture. Carburetors set fuel/air mixture with jets. Air flowing through a venturi, draws a fixed amount of fuel based on the size of the jet. More air entering through the carburetor, means more fuel and an increase in engine speed. With this system, as we open the throttle the speed increases, because we draw in more fuel.
How does the computer adjust the engine's idle speed?
Early fuel injection normally uses an idle air control or IAC valve to control the speed of the engine's idle. 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. With this valve, the PCM increases duty-cycle to the magnetic coil. Increased duty-cycle moves the air valve against spring pressure. When input sensors tell the PCM to increase the engine's idle speed, the valve opens in the IAC, increasing air flow. The fuel injectors also open a fraction of a second longer, increasing fuel flow. We increase engine idle speed because additional fuel and air enter the engine.
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. The fuel injectors may also add additional fuel to help the cold engine run better, much as a choke does on older engines.
Electronic throttle control or drive by wire
More modern vehicles use a throttle that they have no longer physically connected to the accelerator. Pressing the accelerator causes the accelerator-pedal position sensors to move, which notify the PCM of the request for increased vehicle speed. The PCM sends a digital signal to a stepper motor, in the throttle body, which opens to increase the vehicle's speed. Simultaneously, throttle position sensors let the PCM know how much the throttle has moved. A mechanical connection between the throttle and the accelerator is no longer present.
With drive by wire, we no longer need the IAC system. The PCM directly controls an engine's idle speed by opening and closing the throttle. Each time we open the throttle, the PCM increases the fuel injector's pulse width to keep fuel/air mixture correct. The air flow meter, air intake temperature sensor, engine temperature sensor, barometric pressure and other sensors provide data the PCM uses to calculate injector pulse width. Fuel/air sensors also monitor the exhaust and make further corrections to the fuel/air mixture as needed.
Sometimes we abruptly release the accelerator. This may occur when braking at high speed. An engine must go from high RPM operation to idle, almost instantly. The idle circuit eases the transition and allows a more controlled reduction of speed. When the idle circuit fails, the engine may stall when we suddenly release the throttle. A driver often sees this as the engine dying, but failure to idle is very different from a standpoint of diagnosis.
Idle speed varies widely, depending on engine requirements. The power control module constantly monitors idle speed and learns the optimum setting for different circumstances. Idle-memory, along with many other factors are stored and constantly tweaked by the PCM. For example, a slight vacuum leak or worn spark plugs are compensated for in memory. This is one reason why there are seldom symptoms, such as a rough idle, even with an engine badly out of tune.
If battery power is disconnected, the PCM will lose idle memory. This may reveal several problems that were being disguised. Idle speed may fluctuate and the engine may run rough and stall. Sometimes these problems cannot be overcome by the PCM and the check engine light will signal a problem.
A dirty throttle body may keep the computer from relearning idle. Idle memory adapted to the building layers of debris, in the throttle body, over time. If idle-memory is lost, the PCM may no longer be able to obtain the necessary throttle setting. The engine may stall or idle very low. Correction involves cleaning the throttle and resetting base-idle. Resetting idle may require a scan tool or may occur on its own, after the throttle body is cleaned.
Some cable operated throttle bodies have a small screw, near the throttle plate, that may appear to be an adjustment. This screw is NOT an adjustment, only a throttle blade stop. The purpose is to keep the throttle blade from sticking in the fully closed position. Turning this screw will NOT affect idle speed and may cause the throttle to stick, damaging the throttle body.
When idle speed changes significantly or stalling on throttle release occurs, a problem is indicated. Most often an input sensor has failed or the throttle body has a problem. On later model vehicles, a check engine light may accompany the issue.
When a vehicle begins to idle rough or stall, the idle system requires service. With the proper knowledge and equipment, most idle problems can be quickly diagnosed and corrected. Unfortunately, there is not much that can be done without both.