Instead of a radiator, a marinized diesel engine has a secondary cooling system that draws seawater in through the hull, circulates it in a 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.
ImpellerThe raw water pump uses an impeller, made of rubbery material formed into vanes, which is driven by a belt off the crankshaft and lubricated by the seawater it pumps. Proper belt tension and the condition of the impeller are both critical to prevent overheating. Generally housed behind a screwed-on faceplate, the impeller should be removed often and inspected for bent or cracked vanes. Carry a spare on board. If you notice that the impeller has a vane broken off, try to find it downstream from the pump in the hoses or heat exchanger, as it will likely cause trouble later if left there.
open its intake seacock, for example, or if it somehow loses prime, can damage the impeller within a few seconds. If it contains a copper
Engine
zincsIf it contains a copper coil, as most do, the heat exchanger is likely to have a zinc zincs to protect from electrolysis. Check the zinc regularly. Most engine zincs unscrew like a bolt and look much like a gray crayon. If the zinc is merely tarnished or lightly pitted, polish it up. If half or more of the zinc is worn away, replace it.
while
crankingIf the engine will not start immed-Caution iately, or before cranking it for more than 15 seconds, close the raw water intake to prevent seawater from being drawn in by the starter at a time when there is no engine exhaust to expel it. Seawater drawn in during cranking will eventually end up in the cylinders if the engine fails to start. If someone cannot re- open the seacock the instant that the engine starts, shut it back down. Running the engine without raw water input can damage the impeller in seconds.
OverheatingWhen coolant temperature rises steadily during the year, it is time to remove and rebuild the heat exchanger and, possibly, the oil cooler. When the engine overheats suddenly, shut it down and wait until it cools. Removing the filler cap from a hot engine is dangerous and adding coolant to a hot engine can crack the block. After overheating, perform first the easy tests that can be done on a hot engine.
1. Is ample water coming out the exhaust port? If not: Is debris or weed blocking the raw water intake? Is the strainer blocked? Is occurs, after fixing the problem you will probably have to replace the impeller.
2. 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?
3. 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.
4. 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.
CoolantOther, 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.
Engine coolant, or antifreeze, contains ethylene glycol or, less often, propylene glycol. It has additives that protect against foaming, corrosion, and deposits. Some brands are better than others and a marine diesel needs the heavy duty, long life kind. Concentrated coolant must be mixed with an equal amount of distilled water engine passages, causing overheating. Premixed coolant has the water already added and is much more convenient (at twice the cost). Besides lowering the freezing point, coolant also raises the boiling point. A 50-50 mix of coolant and water freezes at -34°F (and boils at 265°F with a 15-psi cap installed), if it is ethylene glycol, or only down to -24°F if propylene glycol. The amount of water can be reduced to 40%, but no less, in cold climates.
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.