Why does the engine rpm increase and decrease erratically even though the car maintains a stable speed?

Engine rpm (symbolized as “RPM”) is the total number of revolutions of the engine in 1 minute. The speedometer cluster displays the engine rpm as a traditional analog or digital clock.

When the vehicle is running, the engine rpm only increases or decreases with the driver’s influence. Specifically, when the driver presses the accelerator pedal to open the throttle, the engine rpm increases and vice versa.

If you run the car at a constant speed, and keep the accelerator steady, but the engine rpm suddenly increases or decreases, you need to think about the risks related to the operating machine immediately. Here are the main causes:

1. Vacuum leak

One of the common causes of abnormal increases and decreases in engine rpm is a vacuum leak. This “disease” originates from an “uncontrolled” airflow being introduced into the engine’s intake, creating a substandard fuel mixture that causes the engine to operate unstable.

2. The throttle position sensor has an error

The throttle position sensor on a car is responsible for providing feedback signals to the engine’s PCM controller about the throttle position.

If that feedback signal is not consistent, it will cause the engine to operate unstable and the vehicle’s engine rpm will immediately fluctuate abnormally.

3. Ignition is broken

When the engine’s ignition system cannot distribute sparks evenly and sufficiently, it is easy for the engine speed to fluctuate up and down abnormally.

This problem often originates from causes such as broken spark plugs/plug wires, ignition coils, etc. In addition, installing a damaged distributor also hinders the spark distribution process. Old cars and cars that have been used for a long time are susceptible to this phenomenon.

4. Fuel supply is unstable

The combustion chamber inside the engine does not receive enough fuel, causing the engine’s rotation speed to be uneven. This problem only occurs when the fuel filter is clogged, the fuel pump is broken, or the vehicle has cylinder problems related to injector failure.

5. Engine speed sensor error

Most vehicles with modern engines use one or more specialized speed sensors. Cars often have camshaft and crankshaft sensors of the engine.

When the sensor is not working well, a series of strange symptoms will arise leading to erratic increases and decreases in engine rpm.

6. IAC valve is defective

Suppose the engine rpm increases suddenly even when the vehicle is operating in idle mode, in addition to the errors mentioned above. In that case, you can think of an error in the idle air control valve (IAC). The idle air control valve is the part responsible for regulating the intake flow when the engine’s throttle valve is closed.

If this part is covered with soot or damaged, the air control valve will not operate smoothly, leading to error codes P0505, P0506, and P0507 appearing when checking. These are signs that the IAC valve is faulty.

7. Powertrain control box error

Sometimes some circuits on the vehicle’s PCM transmission system control box may be damaged, causing problems related to drivability and affecting rotation speed and rpm stability. machine.

If the vehicle is operating and detects abnormal fluctuations in engine rpm, your priority right now is to take the vehicle to a reliable auto repair service center to eliminate the risk of engine damage. opportunities and minimize costs incurred.