Closed system is sealed and pressurized, while the open system is vented to an overflow bottle. The pressure allows the engine to run a bit hotter, which in turn provides more efficiency/power. It can be a temperamental system though. The pressure bottle is known to sprout leaks after 20 years of service. Also, the 87-90 Jeeps aren't the only vehicles to use a pressurized system. My neighbor's Caddy Deville uses a similar set-up.