The operation of vehicles is prohibited if:

Explanation

In both cases — whether the sealing of the crankcase ventilation system is broken or that of the fuel supply system.

Both items appear in the engine section of the list of defects, and each is sufficient on its own for a prohibition.

A leak in the fuel supply system is a fire risk, nothing less. Petrol or diesel escaping from a loose fitting or a cracked hose sits centimetres from the hot manifold and exhaust, and fuel vapour will ignite from a single electrical spark. Fuel dripping onto the asphalt also makes the road behind you slippery for motorcycles; a fuel patch on wet asphalt is one of the commonest causes of a motorcycle sliding out.

A leak in the crankcase ventilation system is a different matter: blow-by gases past the piston rings, which should be returned to the engine under control, escape freely to the outside and often into the cabin. These gases contain hydrocarbons and carbon monoxide; in heavy traffic with the windows up, drowsiness and slowed driver reaction are the direct result, quite apart from air pollution.

Exam note: in this chapter, wherever both items belong to the same subsection of the list (engine, brakes or wheels, say) and both really are on the list, the answer is "in both cases". By offering two similar-sounding phrases the question-setter wants to push you into choosing one.

• Correct but incomplete; you have set aside a fuel supply system leak, which carries a direct fire risk. • This too is a prohibiting defect on its own, but choosing it means ignoring the crankcase ventilation system leak, which is a separate item on the list.

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