A power steering system is a hydraulic system in which the pressure increases with the vehicle's engine speed, and without a power steering cap, the power steering fluid leaks and the steering wheel becomes more and more difficult to turn. Most vehicles can still be driven without a power steering cap, but it is dangerous since the power steering fluid will leak on other parts, possibly causing accidents or additional damage.
Since the power steering system operates under pressure, it does no good to replace a missing cap with a makeshift lid, as it is likely to simply blow off when the engine cranks. The power steering pump is activated by an accessory drive belt that runs to a pulley that spins with the crankshaft. The most straightforward way to disable the power steering pump and system is to remove the power steering accessory belt, which usually involves removing other belts for access.
A power steering system leaking from a missing cap can foul the ignition wiring and distributor, air conditioning compressor, various electronics and especially the timing belt and other drive belts including the belt that drives the water pump. If a vehicle has leaked power steering fluid due to a missing cap, its owner should clean the bottom of the hood and engine compartment carefully and replace the power steering cap and lost power steering fluid before starting the vehicle again.