* Loss of coolant: The most immediate problem is the coolant will evaporate quickly. This will lead to overheating.
* Overheating: Lack of pressure in the cooling system prevents the coolant from reaching its optimal temperature range. This causes the engine to overheat, potentially leading to significant and expensive damage, including warped cylinder heads, cracked engine blocks, and blown head gaskets.
* Pressure buildup: Although it seems counterintuitive, the lack of a cap can also lead to *pressure* problems. While a cap *retains* pressure, it also *releases* it when necessary to prevent dangerous pressure buildup. Without the cap's pressure release valve, pressure could build up and damage hoses or other components of the cooling system.
In short, do not drive your Ford Ranger without a radiator cap. Replace the cap as soon as possible. Driving with a faulty cooling system is extremely risky.
