RPM Fluctuation While Driving: These are the 12 most common causes

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RPM fluctuation is one of the most common problems among all kinds of cars, regardless of age. This topic has been widely discussed a million times on various automotive forums. However, no one wants to search through long discussions to find an answer to this problem.

That's why we decided to prepare an article with the 12 most common reasons that cause fluctuations in engine speed. To begin with, it should be noted that the RPM fluctuation does not have to be due to just one malfunction.


Sometimes a combination of various malfunctions is the root cause, so finding out which components are faulty might be challenging. Difficult starts often accompany fluctuations in engine speed, engine stalling while driving, loss of engine power, and high fuel consumption.

However, the driver cannot always recognize any of these other symptoms. Let's look at the 12 most common causes of engine speed fluctuations without further ado.

1. Throttle valve

Throttle body

The throttle valve might get clogged with various impurities. A clogged throttle valve is manifested by fluctuations while idling and irregular engine running, engine shutdown, or bad starts. However, the check engine warning light may not alert you to some of these problems.

The good news is that It might be pretty easy to eliminate the problem with a clogged throttle valve - if you consider yourself a tinker. The throttle valve must be dismantled and cleaned using suitable cleaning agents. However, with the engine running, you can clean the throttle valve without disassembly by applying a cleaning agent to the throttle valve body.

You have to take care not to mechanically damage the throttle body in any way, which is why cleaning a clogged throttle with, for example, a screwdriver or other similar tools is not recommended. If you do not dare to do it yourself, you will do better if you entrust this work to experts.

2. Lambda probe

Lambda probe

A lambda probe is a sensor in the exhaust pipe that compares the air in the exhaust pipe with the air around the engine and creates an electrical signal based on a chemical reaction. This signal is sent to the engine control unit, which then uses the throttle valve and injectors to adjust the ratio of fuel and air.

A typical manifestation of a damaged or non-functioning lambda probe is bad starts and the smell of gasoline from the exhaust. However, increased fuel consumption, irregular engine operation, and RPM fluctuations can also manifest the malfunction.

In some cases, it is possible to physically identify the lambda probe's malfunction because it is clogged with a large amount of soot. However, a lambda probe malfunction is not always detected by its physical inspection, so the best solution is to go straight to the repair shop, connect the car for diagnostics, and replace it if it's damaged. You will usually be alerted to a malfunction of the lambda probe by the check engine warning light.

3. Mass flow sensor

Mass flow sensor

Mass flow sensor malfunction might be another cause behind engine speed fluctuations. Its malfunction can also be behind the engine running erratically, jerking during acceleration, or shutting down. Other problems accompanying this phenomenon include loss of power and increased fuel consumption.

The mass flow sensor can malfunction if it is excessively polluted or damaged. It may still continue to function, but the changes in engine operation are significant and can be noticed even by a complete layman. If necessary, the damaged mass flow sensor must be cleaned and replaced, which is simple but not always the cheapest.

4. Spark plugs

Spark plugs

RPM fluctuations can also be caused by insufficient ignition of the mixture. In such a case, it is necessary to check the condition of the spark plugs. Spark plug electrodes wear out over time but can also be clogged with various deposits. These impurities can be removed, but since the spark plugs are not among the expensive parts, you will surely do better if you replace them.

If the spark plugs are excessively worn or clogged, the combustion of the fuel mixture is not optimal because the plug cannot ignite the mixture as it should. In this case, the engine works on fewer cylinders, which results in jerking, fluctuations in speed, irregularity of the entire engine, reduced engine power, and increased fuel consumption.

5. Ignition coils

A failure of the ignition coils manifests in the same way as a malfunctioning spark plug. The result is again jerky, irregular engine running, engine speed fluctuations, and a drop in performance. However, in this case, the solution to the problem is straightforward.

You simply replace the faulty coil, and the engine will work again. In addition to the mentioned problems, you will be alerted to the failure of the ignition coil by the check engine warning light.

6. Air filter

Air filter

The air filter has a significant effect on the operation of the engine. If the air filter is excessively clogged, it will cause insufficient air to enter the cylinders.

This problem manifests in a noticeable drop in performance, increased fuel consumption, and sometimes also uneven engine running or fluctuations in engine speed.

If you notice that your car's engine is not as powerful as it used to be and the fuel consumption has increased, you should first focus on checking and, if necessary, replacing the air filter. The price of the air filter is meager, and anyone can replace it.

7. Fuel filter

Oil and fuel filter

The problem with the fuel supply significantly affects the entire operation of the engine. There can be several reasons behind this problem.

However, as is the case when solving any problem, it is necessary to focus first on the absolute trifles and the cheapest things. This little thing is, for example, the fuel filter, which can be excessively clogged with dirt.

If the fuel filter is excessively clogged, it will not pass enough fuel. Replacing the fuel filter is a straightforward and inexpensive matter.

8. Intake manifold leak

Fluctuations in engine speed can often be caused by something completely trivial, such as a leak in the intake manifold. With a larger amount of air than recorded by the mass flow sensor, the engine cannot cope with it. In this case, checking the entire intake manifold and sealing the leaky place is necessary.

9. Loss of compression

Engine pistons

Loss of compression is a serious problem that can be solved, but only for a lot of money. Leaks in the combustion chamber cause loss of compression. These leaks can be caused by the following:

  • Worn or damaged piston ring

  • Worn engine cylinder

  • A damaged cylinder head

  • Damaged cylinder head gasket

  • A damaged valve

  • Damaged valve spring

  • A worn valve seat

All these problems cause a leak in the combustion chamber, which leads to the extrusion of the mixture during compression and a subsequent reduction in engine power. This can be accompanied by irregular engine operation or fluctuations in speed. Extruding the mixture reduces the pressure in the combustion chamber, which would otherwise act on the pistons.

10. Fuel injectors

Leaking or clogged fuel injectors can also, like the other problems mentioned, cause RPM fluctuations and irregular engine operation. However, this problem is also manifested by a noticeable decrease in performance.

The bad news is that solving this problem is difficult and quite expensive. There are only two options: an expensive repair or an even more expensive replacement of the fuel injectors.

11. EGR valve

Clogged EGR valve

The task of the EGR valve is to return part of the exhaust gases back to the combustion chamber. The EGR valve, therefore, opens and closes the connection between the intake and exhaust pipes. When the valve is opened, a certain amount of exhaust gas enters the intake manifold and mixes with the newly arriving fuel mixture into the cylinder.

Mixing the exhaust gases with the fuel mixture will cause the proportion of oxygen in the combustion chamber to be lower. This is undesirable from a performance point of view, but it will cause a decrease in the combustion temperature, which leads to a lower formation of nitrogen oxides. High temperatures and excess air increase the production of nitrogen oxides in exhaust gases.

However, the exhaust gases recirculated over time will clog the entire line with carbon, which will cause the EGR valve to fail. In this case, the EGR valve allows exhaust gases into the intake manifold even when inappropriate, such as during high revs or idling. Due to the incorrect function of the EGR valve, the engine may run unevenly, it jerks during acceleration, the RPM fluctuates, and the engine performance also decreases.

The problem with the EGR valve can be solved in two ways, either by cleaning it or replacing the EGR valve with a new one.

12. Engine control unit

ECU (engine control unit)

Another of these countless causes of engine speed fluctuations may be the engine control unit (ECU) or various sensors that send data to it.

The most important sensors include the following:

All these sensors send different information to the engine control unit. The engine control unit uses this data in order to operate the engine efficiently. These sensors, combined with the engine control unit, control the fuel injection timing, the fuel injection length, the ignition advance of the mixture, and many other things.

Suppose one of the sensors does not work properly. In that case, the engine control unit receives incorrect data, which can cause the incorrect amount of fuel to be injected into the cylinders or the incorrect timing of the ignition advance of the mixture.


Of course, countless other problems and malfunctions can cause the engine speed to fluctuate. However, in this article, we have focused on the 12 most common causes behind this problem.