An engine’s revolutions per minute (RPM) can fluctuate for several reasons, which can both annoy and damage it over time. The diagnosis can be difficult because there are so many possible explanations for their occurrence. Petrol and diesel cars have similar issues, therefore it doesn’t matter which one you have. However, each engine type has its own set of unique characteristics.
Main causes and symptoms of engine RPM fluctuations
Extra air in the engine
Extra air in the engine can create abnormal RPM, which can be caused by broken pieces of the air system, leaks in vacuum lines, or faulty connections in the air lines and hoses. It’s common for the throttle valve to suck in air when it comes into contact with the manifold. If the engine is cold, this problem is more likely to occur, but it doesn’t usually.
Air system parts responsible for varying RPMs include:
- A bad throttle gasket. The throttle valve has the role to control 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 of 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
Another reason why the engine’s RPM fluctuates is due to low fuel pressure. It is possible that low fuel pressure is due to
- 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 flow through the cylinders. Causes can come from dirty injectors and ending 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 idle speed regulator’s job is to regulate the amount of fuel that is delivered to the engine’s combustion chamber. One of the drawbacks of idle speed control is that the engine can run normally while under load, yet stall when idled. While starting the engine, the driver is also obliged to press the gas pedal.
When the engine is put under more strain, such as when the headlights are turned on, the idle speed may need to be adjusted.
The RPM of the engine is also influenced by sensors in the engine. Here’s what we’re getting at:
- MAP sensor
- MAF sensor
- Engine temperature sensor
- Manifold intake air temperature sensor
- Throttle position sensor
The engine RPM will fluctuate if the sensors indicated above are worn or defective, and this will be reflected in the ECU’s output.
Idle speed regulator failure might be a symptom of a malfunctioning engine temperature sensor. If the driver does not press the gas pedal, the engine will stall. A second start of the engine works OK, but the RPM will fluctuate between 1000 RPM and 700 800 RPM, and it will occasionally pass 1000 RPM then return to 700 800 RPM.
This is more likely to occur when the engine is cold since the ECU is receiving incorrect or no data at all. Emergency mode is activated by the ECU when it detects this.
To calculate the error rather than relying on a signal from a sensor, the ECU collects averaged data and makes a mistake in the memory. The ambient temperature will be used instead of the data from the engine temperature sensor, for example.
After the engine warms up, the RPM will return to its typical range.
A weak battery charge
When the engine is cold, a low battery charge might cause the RPM to vary. To start the engine, the battery has enough energy, but after that, the power declines dramatically and is no longer sufficient for the typical supply of other end users. Battery recharges and RPM is restored as the engine heats up.
You should keep an eye on the battery’s status and replace it if necessary if it wakes up. If the battery is the problem, the RPM will usually return to normal when a new one is installed.
For cars with a lot of additional electrical equipment, such as electric power steering, a lot of additional lighting, a powerful music system and so on, RPM changes due to a failing battery are more frequent.
A bad EGR
Diesel engines, in particular, suffer when the EGR valve becomes clogged, resulting in decreased output and unsteady idle RPM.
When the engine is cold, it is common for the RPM to fluctuate during the initial few seconds of the engine’s operation. You’ll need to investigate more if the oscillations continue and are accompanied by automobile vibrations.
If there are any issues in the ECU, you may want to check them out first. If no errors are identified, go back and double-check what I just said.