5.7 Mercruiser Cooling Theory

The Mercruiser 5.7 L engine has two cooling systems, one a seawater system and the other a closed coolant system like that of an automobile. The seawater system absorbs the heat that the closed cooling system picks up from the engine. The heat picked up from the closed cooling system is then pumped over the side through the seawater discharge. The key to its operation is the raw-water inlet location.

  1. Raw Water vs. Ethylene Glycol

    • The relationship between the raw water -- or seawater -- system and the closed cooling system is similar to the relationship between an automobile's closed cooling system and the radiator. As the coolant is pumped through the radiator by the seawater pump -- not the coolant pump for the closed cooling system, but a separate pump -- the radiator acts as a heat exchanger. The heated coolant passes its heat to the radiator. The radiator then disperses its heat into the air.

      The same exchange happens with the raw water cooling system in the Mercruiser 5.7 L marine engine, only instead of shedding heat to a radiator, the 5.7 hands its heat off to the raw water cooling system.

    Location, Location, Location

    • The raw water -- or seawater -- cooling system, has specific requirements for the raw water inlet. The seawater cooling system’s capacity is 20 quarts. This means the inlet must be large enough to allow the pump to pick up 30 gallons of water per minute. The inlet must also be large enough to provide for a positive head at the seawater pickup pump when underway.

      The water pickup must be as close to the raw water inlet as possible. It has to be located in a place that allows an uninterrupted, solid stream of water to flow past when the boat is under way, meaning it must be at a location where, even when the boat is up on plane, it’s still underwater in an area relatively undisturbed by the movement of water across the surface of the hull.

    Interdependence

    • The closed cooling system is wholly dependent on the seawater system for the transfer of its burden of heat to the outside environment. The closed cooling system is normally filled with 20 quarts of a proprietary ethylene glycol antifreeze. In the heat exchanger -- similar in appearance to a radiator -- the seawater flows through the tubes while the coolant flows around the tubes.

    Coolant Recovery

    • The coolant recovery system pulls the coolant back into a reservoir from the 1/2-pint coolant recovery bottle as the engine cools. A full coolant recovery bottle indicates that the coolant system is fully charged, a happy condition. Should the level in the coolant bottle fall, there is a vacuum leak at one of two points in the system. If you note this condition, check the hose that leaves the reservoir or check the gasket under the recovery bottle filler cap.