What is the cause of erratic engine idling? When the engine is warmed up, I notice that the RPM needle varies from 700 to 900rpm. It started recently and it seems to be getting worse. Is this a serious problem and will it be costly to repair?
Erratic engine idle can be caused by various factors, but the most common and likely cause is a dirty or malfunctioning Mass Air Flow (MAF) sensor.
This is a device found in fuel-injected cars and measures air flow so that the appropriate amount of fuel can be injected.
The MAF sensor is located in the intake manifold, close to the throttle valve. Although it is positioned after the air filter, over time, contaminants can build up on the sensor. This causes erroneous readings by the engine control unit, leading to incorrect fuel injection quantities.
If you prefer to do it yourself, you can try cleaning it with spray contact cleaner. If that does not work, you will probably need to replace it.
Neither is costly. For example, the price of a new MAF sensor for a 2008 Golf GTI is less than $300 from Volkswagen Singapore.