Ask an expert

I have just bought a 2004 Subaru Legacy fitted with factory HID lights. When I switch them on the engine rev's increase and I can feel a light vibration in the car.

Is this normal?

Prateik

From the "Ask Jack" archives - 7 November 2009

The reason for the change in engine revs is due to the load being applied to the alternator when the lights are switched on.

To compensate for this, the on-board engine management system will automatically increase the engine idle speed to a predetermined level.

If this did not happen, the idle speed would drop to a point where the engine may stall with the car in gear especially if accessories such as air-conditioning or the rear screen demister were switched on at the same time.

If it worries you, get it checked out but I suspect it is a normal condition.