The causes of engine rpm going up and down at idle
Two essential things significantly affect engine speed or rpm: the amount of fuel and air volume entering the combustion chamber. These two things will determine the level of engine rotation stability in each engine working condition, such as when idling, acceleration, deceleration, and when the engine load increase.
When the two things act unbalanced, for example, the amount of fuel is too much or less, then the rpm condition automatically changes also. The effect is that engine rpm going up and down at idle, and the rpm needle fluctuates, making the car's speed rough when driving.
The causes of this rpm going up and down at idle can be quite diverse, but generally, the root of the problem remains in the air and fuel supply lines. So what are the causes of engine rpm going up and down at idle? Here are four reasons why rpm going up and down at idle that often occur in gasoline injection cars.
1. Bad idle speed control (ISC) actuator
Idle Speed Control (ISC) is a component that functions to regulate the idle rpm, so it is always stable and constant based on orders from the Engine Control Unit (ECU). Yes, idle speed control is an actuator in the injection system used to stabilizing idle RPM.
As we already mentioned above that the engine working conditions will always change (especially when driving). The idle rpm must stabilize at standard idle value after the engine acceleration, deceleration, or when the Air Conditioner is on (load). The goal is to achieve comfort while driving.
That is where idle speed control comes into play. ISC will maintain the engine rpm condition to always return to the idle rpm target set by the ECU regardless of the previous engine rpm conditions.
When the idle speed control is going bad, ISC can no longer maintain the idle rpm in a stable position. Car rpm needles tend to fluctuate up and down, especially when the AC is on.
Bad idle speed control generally occurs due to dirt blocking the throttle valve and causing an air leakage in the idle port or damage to the idle speed control component due to the service life.
2. Dirty throttle body
In addition to the problem with Idle Speed Control, the cause of the car's rpm going up and down at idle is due to the throttle body's condition, which is very dirty. An air filter that is rarely cleaned or even replaced is the first cause of a dirty throttle body.
As we know, the throttle body is a component that is used to regulate the air supply before it enters the engine. Inside the throttle body, several air holes are small enough to be easily clogged by dirt and dust.
One of them is the idle port, which is directly connected to the Idle Speed Control actuator. When these holes are clogged with dirt and dust, the air supply will also be disrupted. As a result, the car's rpm fluctuates up and down and becomes unstable.
Therefore, keeping the air filter and throttle body clean is a wise move that needs to be taken to anticipate car engine rpm's going up and down at idle.
3. An air leak in the intake channel
The next cause of the engine rpm going up and down is an air leak in the intake channel, especially the air leak that occurs after the Air Flow Meter Sensor to the intake valve.
Yes, the Air Flow Meter sensor functions to calculate the air volume that enters the engine. The amount of incoming air is calculated by the ECU with some data from other sensors according to engine working conditions.
This data is used to instruct actuators such as injectors, Idle Speed Control, EGR, and others so that the engine can work normally.
When the intake channel has an air leak, the data for calculating the volume of air entering from the Air Flow Meter Sensor will be different and not the same according to actual conditions.
The difference in the amount of incoming air with the count that occurs in the engine ECU will result in different commands to the actuator.
In effect, the ideal amount of air-fuel mixture to reach optimal combustion results will be difficult to achieve. When this condition occurs, the engine rpm going up and down at idle and makes a rough idling.
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4. The air conditioner (AC) system has a problem
The last thing we know is that the car's rpm going up and down at idle is a problem with the AC system. Yes, most of the AC system in gasoline injection cars is closely related to Idle Speed Control.
Please note that it will also increase the engine workload when we turn on the air conditioner. That happens because the engine has to rotate the AC compressor. For this reason, it is necessary to increase the idle rpm, which we often hear as AC Idle Up.
AC Idle Up is an increase in idle rpm that occurs after the compressor starts. For example, the normal idling rpm is 800 rpm. When the air conditioner is on, the engine rpm will automatically increase from 850 rpm to 900 rpm.
This condition is normal because it aims to prevent the engine from falling due to engine load (turning on the AC compressor).
In several cases, it was found that there was an improper repair of car air conditioning that affected the work of the idle speed control (often found there was an overhaul of the AC harness and the addition of an AC rheostat).
Overhaul of the AC harness often causes the idle speed control to become an error so that the rpm fluctuates when the AC is on. To be sure, it takes a scan tool and experienced mechanics.