Is erratic idling an engine or electrical problem?

A Straits Times reader wants to know if his car's erratic idling is an engine or electrical issue.

What are the possible reasons for erratic idling?

Also, I have noticed that at night with the headlights on, the idling speed slows down even more, while the lights stay very dim and brighten considerably only when I rev the engine.

Is this an engine problem, an electrical problem or both?

The most likely cause for the symptoms described above is a low voltage supply.

This will often cause the engine control unit to function a little abnormally, resulting in a lower than normal idle speed. Low voltage will definitely cause lights to be dim too.

The first thing to check in rectifying the problem is the condition of the battery. If it is weak, replace it immediately.

Sometimes, the problem may just be loose battery terminal connections. Make sure these are clean and tight.

Often neglected is the earth cable that runs from the negative battery terminal to a secure point on the car’s body or chassis. Many electrical problems are caused by a weak earth connection.

Have the cable checked and replaced, if necessary. It can deteriorate over time.

Finally, check the condition of your vehicle’s alternator, which should supply 14 volts to the battery when the engine is running at its normal idle speed.


Related story: Why is my engine idling so erratically?
Related story: What causes erratic engine idling?