If your car is like just about every other MKI MR2 I've seen, the coolant reservoir cap is crumbling and has at least one hole in it. This can be a serious problem, since the holes normally occur in the molded tube that is integral to the cap - this allows the coolant system to pull air back in as it cools instead of coolant and can lead to overheating in severe instances.

This is an OEM part, made out of a more pliable rubber than the original clear plastic caps. Includes the cap, hose going into the reservoir and both hoses that attach to the top of the cap with clamps.