Why does the engine have RPM fluctuations?

Engine RPM fluctuations can have many causes and can be very disturbing for the driver and bad for the engine over time.

Having various reasons for their appearance, the diagnosis can be difficult. Most problems, whether the car is petrol or diesel are the same. However, there are also some particularities for each engine type.

Main causes and symptoms of engine RPM fluctuations

Extra air in the engine

Abnormal RPM can be due to extra air in the engine, this can happen because of cracked elements of the air system, leaking vacuum hoses, bad hose, or pipe connections. It often sucks in the air where the throttle valve is in contact with the manifold. This symptom can manifest especially when the engine is cold, but it’s not always.

The parts in the air system that are responsible for RPM fluctuations are:

  • A bad throttle gasket. The throttle valve has the role of controlling the amount of air entering the engine, if the gasket is bad, excess air will pass through it and you will feel an engine power loss
  • Cracked or worn pipe that goes from the air filter to the throttle assembly. This situation is the same as above, excess air can enter through cracks and this can cause engine RPM fluctuations.
  • Bad or worn injector o-rings. Bad injector o-rings are a natural cause. They wear out over time and need replacement. Because of worn o-rings, excess air enters the combustion chamber, which causes the engine power to drop. This symptom is best seen when the engine is cold because the o-ring diameter is smaller, as the engine warms up, the o-ring increases in diameter, and engine RPM fluctuations are not present anymore.
  • Worn or cracked vacuum hoses. Extra air can enter through cracks in the vacuum hoses, causing abnormal RPM. Checking them is not that easy, so be patient 🙂
  • Bad absorber solenoid valve. This valve can also be responsible for extra air. The valve is controlled by the ECU based on information from a large number of sensors. Usually, the “check engine” light appears and you need an OBD scanner to be sure.
  • Bad idle speed regulator. Designed to maintain idle speed at fully closed throttle. If it fails, then with the throttle closed, the engine will simply stall. If the idle speed regulator is unstable, then the engine speed will fluctuate.
  • Bad intake manifold gasket. A very common fault of engine RPM fluctuations, if the intake gasket is worn, extra air enters the system which causes this problem. You should check this problem first if you see these abnormal fluctuations.

Low fuel pressure

Low fuel pressure is another cause why the engine has fluctuating RPM. Low fuel pressure can be caused by:

  • A bad fuel pump. A damaged fuel pump is the most common cause of low fuel pressure. The pump can fail due to a bad bearing, damaged electrical wires, and worn-out blades.
  • Dirty fuel filter. Another common cause of fluctuating RPM is caused by a dirty filter. This is a good reason to change it constantly when you change the other filters and oil 🙂
  • A bad fuel pressure regulator. A failing fuel pressure regulator will cause engine RPM fluctuations, however, if this fails, then the engine will stall.
  • Uneven fuel flows through the cylinders. Causes can come from dirty injectors and end with a failure in the ignition system. For example, misfires may occur.
  • Bad or worn crankshaft position sensor. This will cause inconsistency between injection timing and spark supply, thus it’s better to pay close attention to it.
  • Low compression in cylinders.

Bad idle speed regulator

The role of the idle speed regulator is to increase or decrease the fuel supply to the combustion chamber. One of the bad symptoms of idle speed control is the fact that the engine can operate normally under load, but stall at idle. Likewise, the driver is forced to press the gas pedal while starting the engine.

The idle speed value may change after an increase in the load on the engine, for example when turning the headlights, and other consumers.

Engine sensors

Engine sensors are also responsible for engine RPM fluctuations. We are talking about:

The sensors listed above, if worn or bad, can send false signals to ECU which, in turn, will manifest in the engine RPM fluctuations.

For example, a bad engine temperature sensor can manifest in the case of a failing idle speed regulator. The engine starts but it will stall if the driver does not press the gas pedal for a boost.

After a second start, the engine will work fine, but the RPM will fluctuate between 1000 RPM and 700-800 RPM, and it will sometimes pass the 1000 RPM but go back.

This can happen especially when the engine is cold because false information is being sent to the ECU or no information is sent. ECU sees that and it switches to “emergency mode”.

In this case, instead of a signal from the sensor, the ECU takes averaged data for calculations, and in parallel forms an error in the memory. For example, if the engine temperature sensor is out of order, then the ambient temperature will be taken into account instead of the data from it.

Everything will be back to normal after the engine warms up, and the RPM will be within normal limits.

A weak battery charge

A weak battery charge can cause engine RPM to fluctuate especially when cold. The battery has enough energy to start the engine, but then the power drops suddenly and is not enough for normal supply to other consumers. As the engine warms up, the battery is recharged and the RPM is back to normal.

To avoid this symptom is better to monitor its condition, and if the battery wakens, to change it if necessary. Most often, changing the battery is the best solution as the RPM goes back to normal if the battery is the fault.

You should note that RPM fluctuations due to a failing battery will be more likely for cars equipped with a large amount of additional electrical equipment, such as electric power steering, a lot of additional lighting, a powerful audio system, and so on.

A bad EGR

When the EGR valve is clogged, it can jam, which will automatically lead to a decrease in engine power and unstable engine RPM when idling, most of the time on diesel engines.


High engine RPM is a normal thing if the engine is cold, even fluctuations of RPM in the first few seconds when starting the engine. However, if the fluctuations continue and are followed by car vibrations, then you will have to diagnose the problem.

You could start by checking the errors in the ECU if there are any. If no errors are found, then you should check the above parts that I have mentioned.

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