Why Does My RPM Go Up & Down?

‍What are RPMs?

RPM stands for Revolutions Per Minute.

Every time you accelerate or decelerate, the engine’s crankshaft rotates. RPMs measure the rotations combined with the speed that which pistons are moving up and down in the cylinders. The Tachometer, which is next to the speedometer shows you the RPMs.

The information on the tachometer is what guides the driver as he shifts. The section marked in red at the end of the tachometer indicates that it’s time to shift. A driver should shift before the indicator gets to the red zone to make the drive smooth.

RPMs go up when you accelerate and they decrease when you release the accelerator. Most engines can rev up to 6,000 RPMs before shifting when running normally. However, this value isn’t fixed and will depend on your car model and the installed motor.

What is the Function of a Tachometer?

Although most drivers hardly pay attention to the tachometer, but the RPM readings are essential and can tell you more about your car’s engine and your driving patterns. Some of the functions of a tachometer include:

Increase the Engine’s Lifespan

An engine has multople parts made of metal. Friction is created when metal rubs against metal and this can be damaging to your engine. Although engine oil can help reduce the friction between the moving parts, it doesn’t eliminate the friction completely.

A tachometer comes in handy as it shows you how fast the engine is spinning. Keeping the engine at optimal range reduces the friction inside, which further reduces wear and increases the engine’s lifespan.

Helps With Accurate Shifting

The tachometer shows when it’s time to shift if you’re driving a manual car. Although as an experienced driver, you‘d know when to shift by the sound the engine produces and the vibrations of a car, using a tachometer can guarantee proper shifting.

Fuel Efficiency

Your car’s manual indicates the right time for shifting, and that is when the shifting will cause the least drag. When the engine load is less, there’s less fuel burned.

What are Normal RPM Values?

You should cruise at 1500-2000 RPMs when everything is functional and fine with your RPMs.

However, the optimal range will depend on the car model.

Why Does RPM Go Up and Down?

When you have an issue with your RPM, the indicator is not stationary. It fluctuates whether you’re driving at a steady speed. Sometimes it can also shift when your car is stationary.

Some of the reasons your RPM could fluctuate include:

Faulty Spark Plugs

Spark plugs are essential as they are responsible for firing the fuel into your engine. Worn-out spark plugs are a common cause of RPM fluctuations as they are unable to inject the fuel within the piston at the right time.

At this point, you’ll experience a sluggish acceleration. That’s because the engine tries to rev, but can’t get the right amount of power, which causes RPM to go up and down. Sometimes you may experience jerking and misfires.

You may have faulty or worn-out spark plug or ignition coils if you experience a strong vibration when you drive, poor fuel economy, and engine misfires.

The only way to solve this is to have the spark plug changed as a faulty spark plug can be dangerous and also damage your engine.

Engine Vaccum Leak

An engine vacuum leak can also lead to RPM fluctuations. Every time you accelerate, there’s a vacuum leak as the engine air supply is interrupted.

A vacuum leak occurs when there’s an airflow interruption in the intake manifold or exhaust system of your car. Apart from your RPM going up and down, you may also experience power loss, reduced fuel efficiency, or notice a hissing sound. Sometimes your check engine light will turn on or the vehicle may stall in the middle of nowhere.

You need to have the problem checked and the vacuum leak fixed.

Idle Air Control Valve

An idle air control valve can also lead to RPM fluctuations, especially while accelerating.

The IAC is on the throttle body of fuel-injected engines. It's here where it communicates with the car’s ECU to regulate airflow electronically and this controls the engine’s idle speed.

Communication issues between the idle air control and ECU mean there's no way to regulate airflow or maintain a steady RPM. That leads to RPM fluctuations.  A dysfunctional IAC can also lead to rough idling or cause your engine to stall.

Dirty Air Filter

The combustion chamber has fuel that needs to mix with enough air to create power. An air filter helps to trap any dirt and debris that might seep into the combustion chamber and contaminate the fuel.

Having a dirty filter means it can no longer trap contaminants and this stops air from getting into the engine. That might lead to a delay in accelerating and cause RPM fluctuations.

Changing the air filter will help clear this problem.

Timing Belt Issues

The timing belt synchronizes the camshaft and crankshaft of the engine.

In case the timing belt is worn off or not connected to the engine properly, you could have a problem with the RPMs. A faulty timing belt causes rough idling, which leads to RPMs fluctuation when accelerating.

You need to have the timing belt replaced to solve this problem.

Dysfunctional Crankshaft Position Sensor

The crankshaft sensor faces the timing rotor and is connected to the engine block. It controls the airflow by regulating the speed of rotations inside the eengine.

The sensor is what calculates the revolutions per minutes.

A faulty crankshaft sensor can cause your RPM to go up and down. Additionally, you could have problems starting the car or your engine will vibrate excessively.

Your mechanic can tell whether you have a faulty crankshaft sensor and he will replace it with a working position sensor.

Can You Still Drive With Fluctuating RPMs?

If you notice that your vehicle’s RPM keeps going up and down, this is an indication of an issue with your car.

Although there are some risks associated with driving around when your RPM is fluctuating, there are no immediate risks.  Nonetheless, you should have the problem fixed immediately.  Sometimes cleaning the air filter or changing spark plugs is the only thing required to fix the problem.