The risk of driving a Volkswagen car with a coolant tank cap problem is that it can allow coolant to evaporate from the tank. This can cause the engine to overheat more quickly than normal. This can cause serious damage to the engine, such as cracked cylinder heads, blown head gaskets, or other expensive repair costs. If the engine were to seize or stall while driving, it could cause a dangerous situation. Additionally, the coolant can leak onto other parts of the vehicle, causing potential corrosion and other damage.