If the engine speed fluctuates when idling – possible causes!

Recently updated on January 27, 2022 at 10:20 am

When "saws"Or"rough idle" is what you call it engine speed fluctuations, without the driver doing anything. It also happens that the engine generally does not run smoothly. It may be that only when the engine is cold or only when the engine is warm speed fluctuations appear. It can also happen that the motor loses power and even stops because of too little speed. It is therefore important for the workshop and the pending troubleshooting that one closely observes when the problem arises. This enables the cause to be identified more quickly. In most cases, reading out the vehicle's fault memory helps. This usually allows the cause to be narrowed down. A fluctuating speed when idling can have the following causes!

Dirty or defective throttle valves

The throttle valve is located in the intake tract. The throttle valve regulates the amount of air that is sucked in by the engine due to the vacuum. So that the fuel supply can be adjusted correctly, the intake air is determined by the air mass meter. They only have petrol engines right one throttle bodies. In diesel engines are often so-called storage flaps installed. A well-known throttle valve problem is the very slow drop in engine speed as soon as you take your foot off the gas pedal. Speed ​​fluctuations when idling are also common. It is often sufficient to clean the throttle valve. If it is a mechanical defect, then a new part be installed.

ignition system problems

In the case of a petrol engine or Otto engine, it can be due to the Ignition problems occur, which cause the speed fluctuations at idle. A broken ignition cable or a defective ignition coil can cause such a problem. If an ignition cable is defective, it can also be damaged by a marten bite. Also defective spark can cause speed fluctuations.

Defective idle actuator

So that the engine speed can always be kept constant when idling, the idle adjuster, also known as the idle controller, installed. Without the idle actuator, the engine may stall as soon as you Air conditioning turns on. If the idle speed controller is defective, this can be a reason for the speed fluctuating when idling. Then you can't avoid an exchange. This means a workload of about 20 to 80 minutes. Assuming an hourly rate of 100 euros and material costs of around 40 to 200 euros, the total cost of the repair is between around 60 and 300 euros.

Fuel supply causes problems

Idle speed can also fluctuate when the engine is not running the right amount Fuel type receives. This can happen if a pressure valve of the high-pressure pump or the fuel pressure regulator is defective. A clogged fuel filter can also reduce fuel flow.

Defective air mass meter

In order to be able to record the exact air mass, there is the Air flow sensor. This is a sensor that registers the exact air mass of the air sucked in by the engine. This makes it possible for the engine control (the engine control unit) can be calculated and injected. If the car has limited performance under load, then the engine may be in the doldrums Emergency run went. As a rule, then lit up Engine control lamp (MKL). This is a typical sign of a defective air mass meter. Here, too, the idle speed can fluctuate. Normally it is sufficient to clean the air mass meter. However, if the sensor is defective, it must be replaced with a new one.

Defective camshaft adjustment

The efficiency of the engine can be determined by the camshaft adjustment increased (Keyword efficiency increase) will. This is possible because the timing of the engine can be varied as required. If the vehicle runs rough when idling, this can be a sign of a defective idle adjustment. Depending on the car, the work can take between one and six hours. Assuming an hourly rate of 100 euros and adding the costs of a new camshaft adjuster, which is often between 100 euros and 500 euros, the total costs are around 200 euros to 1.000 euros.

Other causes can also be a reason for a fluctuating speed: In addition to the causes mentioned above, the following reasons can also occur.

1. Defective injector(s)
2. EGR valve not working properly
3. elongated timing chain
4. defective lambda probe
5. Toothed belt has incorrect control times (Defective idler pulley or incorrectly fitted belt)
6. as well as many other causes

Despite fluctuating idle speed for the main inspection?

Before the car goes to the upcoming general inspection, the fluctuating idle speed problem should be fixed. It is possible that during the emissions test, which is included in the general inspection, irregularities appear. Then the exhaust gas values ​​may be incorrect. Due to the problem, it is possible that the badge will not be granted. Failure to pass the emissions test is considered a serious defect.

Drive on anyway if the idle speed fluctuates?

If the engine idles rough, the engine can stall. This is also the Braking and steering assistance without function. This in turn means that a dangerous situation cannot be ruled out. It is therefore strongly advisable to visit a workshop.

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