Boat engines that are cooled with self-contained coolant that is cooled, through heat exchangers, by sea water are commonly referred to as fresh water cooled. The internal coolant is generally fresh water that should be treated with additives, usually ethylene glycol, more commonly referred to as permanent anti-freeze. The usual anti-freeze must be cut 50% with water. In warmer climes the antifreeze is needed for its anti-corrosive additives and heat exchanging properties. Oddly enough, if antifreeze is used straight, the engine temperatures may constantly climb on a long run.
Engine and antifreeze manufacturers recommend periodic changes of the antifreeze, largely because the anti-corrosive additives lose their effectiveness over time. Recommendations are for changes to be made in three to five year intervals. If the coolant is ever observed to be discolored, a prompt change is required.
Recommendations are to use the newer antifreeze of the DexCool type.
When making the periodic change, one should start by testing the anti freeze properties of the existing coolant mix. If this test comes in around minus thirty degrees you won’t have to be too fussy about how much you drain off – if you are going to use the same type of antifreeze to refill. If the hydrometer test shows little protection or if you are changing to DexCool for the first time, you must work at getting as much of the old coolant out as possible. If the change is to DexCool for the first time, the old coolant should be thoroughly removed and the entire cooling system flushed with fresh water until no trace of the old antifreeze is evident. The cooling system includes the engine block, heat exchangers, water heater and anything to which the coolant is piped, including the overflow bottle. The rinsing water should then be drained thoroughly.
New antifreeze should then be mixed 50/50 with water and put into the cooling system. Fill at the fill cap all the way and then start the engine. The level of the coolant should drop and refilling will be required. Now comes the hard part. All the air that was in the empty system must be displaced with the new mix. Sometimes the system will self bleed when the engine is started but more often additional steps must be taken to dispel the air. Often bleeding the air from a water heater line will require loosening connections at the water heater, especially if the heater is at higher elevation than the engine, until air coming from the line stops and coolant starts. A trick that usually works is to fill the engine through the heater hose. This is done by putting the antifreeze mix in a bucket and then disconnecting one of the heater hoses at the engine and, using a small hand pump, pump the antifreeze from the bucket into the hose going to the heater and filling the engine when the mix returns from the water heater into the engine. When the coolant level comes up in the fill tank, reconnect the hose, losing as little fluid as possible.
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