Cooling System

Summarize

Instead of a radiator, a marinized diesel engine has a secondary cooling system that draws seawater in through the hull, circulates it in a heat exchanger where it takes up heat from the coolant, and discharges it through the engine exhaust hose. Two critical parts of this seawater cooling system are the seawater strainer, which traps any debris in the water, and the seawater pump.

The strainer is often installed below the water line to keep the entire run of hose from the input thru-hull to the pump always primed. It is usually transparent, so that trapped debris can be seen, and it has a cover that can be removed for cleaning out the debris. Close the seacock, when cleaning out the strainer, unless you work quickly.

It is good practice to remove the impeller if the engine will not be run for more than a month or two, taking care that the engine must not be started until after the impeller has been reinstalled. This prevents those vanes that are bent against the side of the pump from becoming deformed over time. Running a seawater pump dry, by neglecting to open its intake seacock, for example, or if it somehow loses prime, can damage the impeller within a few seconds.

• Is ample water coming out the exhaust port? If not: Is debris or weed blocking the raw water intake? Is the strainer blocked? Is there air in the intake line? If any of these occurs, after fixing the problem you will probably have to replace the impeller.
• Is there coolant in the overflow reservoir? If so, the coolant level is probably adequate, but not necessarily. Is the v-belt that drives the raw water pump broken or slipping? Is the engine low on oil? Does the oil seem thin or contaminated?
• Is the engine really too hot or is the temperature sender or gauge broken? Use an infrared temperature gun to check the temperature at the thermostat housing.
• Inspect the impeller. If it looks OK, wait for the engine to cool, verify that there is enough coolant and if so, replace the thermostat. Never run the engine without a thermostat installed because either it will fail to warm up or, if the engine has a coolant bypass circuit, it will overheat.
• Other, less common causes of overheating are restricted exhaust injection elbow or manifold, worn raw water pump and overloading the engine. Adequate engine room ventilation improves combustion and also helps with cooling. Try running with the engine room door open.

When the engine is running, seawater flows into the raw water pump, then through the heat exchanger, then through the exhaust elbow where it is injected into the exhaust, and then through the waterlift muffler to the exhaust outlet.

When the engine stops, and the pump is no longer turning, if both ends of the cooling-water circuit are below the water level, gravity can create a siphon that keeps seawater flowing. The water then backfills the muffler, rises up the exhaust hose and backs into the exhaust manifold where it can enter an open exhaust valve and into a cylinder. The result can be hydrolock, bent connecting rods and a ruined engine.

Usually located just before the exhaust elbow, the vented loop must rise above the highest waterline level when heeled: 12 to 18 inches above the water level at dock. While the engine is running, pump pressure closes an air valve at the top of the loop and water flows normally. When the engine stops, the valve opens, air enters and the siphon is broken. The most common design has a duckbill valve at the top. It is compact, inexpensive and has no overboard discharge. It does, however, need periodic cleaning to prevent it from sticking open or closed.

An open vent with no valve has a small hose at the top that emits a tiny stream of water (called the pee stream) while the engine runs and lets air in when the engine stops. This design is lower maintenance, almost impossible to clog and easy to inspect. It does, on the other hand, require dealing with the pee stream.

The duckbill or air valve at the top of a vented loop should be inspected at least annually. Common problems are salt buildup, calcium deposits, insects, corrosion and rubber hardening. A blocked vent is the same as having no vent at all. As an alternative to cleaning the valve it might be best to carry a spare.

An antisiphon loop—or two—might also be needed where a marine head is below the waterline when heeled, to prevent seawater from siphoning into the bowl and flooding the boat. The intake side can siphon if a landlubber leaves the head in the wet bowl setting. The output side can siphon, and in a most unpleasant way, if the holding tank discharge thru-hull is left open.

Coolant is poisonous and must never go overboard, on the ground or in the drinking water system. It is usually yellow or green, sometimes red or orange, whereas the potable antifreeze used in drinking water lines is always bright pink. Do not mix different brands of coolant, regardless of color, without flushing the cooling system.

Diesel engines need the special additives in what is sometimes called a heavy duty coolant. The engine manu­fac­turer often suggests a particular brand. The least expensive kind is probably not what you want. Be careful not to pour coolant into the oil filler cap, nor vice versa.

Change the coolant yearly by draining it completely from every drain port that the engine has (bottom of block, reservoir tank, heat exchanger, etc) and from the overflow tank. Add new coolant, run the engine until it warms up to operating temperature, stop it, let it cool completely and only then check the coolant level.

Opinions vary on whether it is worthwhile or even advisable to flush the cooling system when changing the coolant. On some engines, the thermostat must be removed before freshly added coolant can drain into the lower block. Failing to remove the thermostat and add coolant below it will cause these engines to overheat.