Rough Idle of the Engine: The 10 most common causes

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Rough idle troubles many drivers of both old and new cars. However, some drivers ignore this problem and do not even realize how big of a mistake they are making.

Neglecting this problem can cause a whole series of other, much more expensive problems. But what kind of malfunction can cause the engine to run erratically? What to check and what to watch out for? Let's take a look at it.



Before we begin, it should be noted that rough idle does not have to be due to just one malfunction. Sometimes a combination of multiple malfunctions might be behind this problem, so finding out which components are faulty is challenging.

Rough idle is often accompanied by difficult starts, RPM fluctuations, engine failure while driving, loss of engine power, and high fuel consumption. Let's look at the ten most common problems causing rough idle without further ado.

1. Spark plugs

Spark plugs

Incorrect and, therefore, insufficient ignition of the mixture can be one of the causes of irregular engine operation. With this problem, 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 spark plugs are not expensive parts, it is recommended to replace them.

If the spark plugs are excessively worn or clogged, the combustion of the fuel mixture is not optimal because the spark plug does not work properly or not at all. In this case, the engine works on fewer cylinders, which results in jerking and rough idle of the entire engine and thus reduces engine performance and increases fuel consumption.

2. 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 operation, and a drop in performance.

However, in this case, the solution to the problem is straightforward. Just replace the defective coil, and the motor will work properly again. In addition, you will be alerted to this malfunction by the illuminated check engine warning light.

3. Throttle valve

Throttle body

The throttle valve can become clogged with various impurities. A clogged throttle valve manifests in several ways, such as erratic engine running, fluctuating idle speed, engine stalling, and poor starts. However, the check engine light may not alert you to some of these problems.

Even in this case, it is not difficult to eliminate the malfunction. 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 body.

4. Mass flow sensor

Mass flow sensor

Mass flow sensor malfunction might be another cause behind the engine's rough idle. 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 quite easily. If necessary, the damaged mass flow sensor must be cleaned or replaced, which is simple but not always the cheapest.

5. 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.

You can notice a faulty lambda probe by experiencing bad engine starts and the smell of gasoline from the exhaust. However, increased fuel consumption, irregular engine operation, and RPM fluctuations can also hint at the lambda probe's 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, this is not always possible, so the best solution is to go 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.

6. Loss of compression

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, leading to the fuel mixture's extrusion during compression and a subsequent reduction in engine power. This can be accompanied by irregular engine operation or fluctuations in speed.

Extruding the fuel mixture reduces the pressure in the combustion chamber, which would otherwise act on the pistons. It should also be noted that fixing these problems is laborious and costly.

7. 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, manifested by a noticeable decrease in power, increased fuel consumption, and sometimes uneven engine running or fluctuations in engine speed.

So, 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 even a monkey can replace it.

8. Fuel filter

Oil and fuel filter

The problem with the fuel supply significantly affects the entire operation of the engine. This problem can often be hidden behind a tiny thing, and that is a clogged fuel filter.

If the fuel filter is excessively clogged, it will not pass enough fuel, resulting in a rough idle. However, replacing the fuel filter is a straightforward and inexpensive matter.

9. Fuel injectors

Rough idle can also be caused by leaking or clogged injectors. However, in addition to an irregular engine running accompanied by RPM fluctuations, this problem manifests in a noticeable decrease in performance.

Solving this problem is quite difficult and quite expensive. There are only two options: an expensive repair or an even more expensive replacement of the injectors.

10. Engine control unit

ECU (engine control unit)

Another of the countless causes of rough idle may be the engine control unit or various sensors that send data to it. The most important sensors include the lambda probe, mass flow sensor, crankshaft position sensor, knock sensor, coolant temperature sensor, or camshaft sensor.

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.

The engine thus does not work properly, resulting in rough idle accompanied by lower performance and increased fuel consumption.


Of course, countless other problems and malfunctions can cause the rough idle. However, in this article, we have focused on the most common issues behind this problem.

In conclusion, we would add that the easiest way to solve rough idle is to diagnose the engine control unit.

Suppose the error does not show up in the diagnostics. In that case, it is necessary to go about it in the old guaranteed but laborious way by gradually checking every single component that could be behind this problem.