The crankcase breather valve and its four lines often fail. The valve sticks open and the lines break, and both let false air in. The valve and the lines last between 110,000 and 200,000 km and typically fail around 150,000. In cold weather the fault develops faster, because the condensing vapour freezes in the valve and blocks it. The symptoms are oil being used up, a rough idle and blue smoke, and a dense vapour can also appear at the front under the bonnet. If the drain of the valve blocks, the burnt oil mist also leans out the mixture and the engine fault light comes on. Left untreated it can end in engine damage. There are two tests for it, and both can be done at a viewing. With a warm engine at idle, lay a flat plastic bag over the oil filler opening. If the bag stays up or is only drawn in by 2 to 3 cm, the valve is fine. If it is drawn in strongly, the valve has stuck open. With a cold, stopped engine take the breather line off at the front corner of the cam cover and blow into it: you should hear bubbling in the sump. If you do not hear it, one of the lines has broken or the drain in the dipstick tube has blocked.