Elements of engine cool: raw-water pump and pressure cap

From making your way off a lee shore to charging batteries and taking a hot shower, your engine and, indirectly, its cooling system provide safety and convenience.

Here are two key elements of an engine’s cooling system.

Raw-water pump This pump does the bulk of the engine’s cooling. It draws raw water (this may be seawater, fresh water or whatever mixture of the two the boat is floating in) into the engine’s open or raw-water cooling circuit using a rubber impeller. The impeller and its weaknesses are well understood. It should, in my opinion – and contrary to many manufacturers’ claims – be replaced seasonally. It’s cheap insurance and the best way to avoid impeller-induced overheating.

A few of the less well known parts of the raw-water pump include the cam, wear and cover plate. The cam is a half-moon-shaped component that resides within the pump body. The impeller blades are, as they pass it, deformed by its eccentric or cam shape, which creates the pumping action. Most cams are replaceable. They typically are cast from a metal slightly less noble than the pump body itself, and as a result, in addition to wearing away by friction induced by the impeller blades, they corrode sacrificially.

The cover plate, which is removed to access and change the impeller, wears very much like the cam. Just a few thousandths of an inch of wear or recession on the inside cover of the plate can reduce impeller efficiency. Wear that can be perceived with the fingertips is too much and necessitates replacement (cover plates that do not have stamped writing on the external surface simply may be reversed).

The wear plate, when present, is located behind the impeller and as such is often overlooked during service intervals. This plate, just like the cover and the cam, suffers from abrasion as well as corrosion. Its replacement interval, along with those of the cam and cover plate, varies with application and water turbidity; however, it should be replaced whenever the cam or cover plates are changed, or when worn.

Finally, check the weep holes found on most raw-water pumps. Depending on the pump’s configuration and drive mechanism (some pumps are belt driven while others are driven off a gear in the engine); these may leak either raw water, oil or both in the event of a seal failure. Don’t ignore this warning sign: Seal wear and leaks never repair themselves. The drip is an early warning sign that the pump requires either a rebuild or replacement.

Pressure cap Closed marine cooling systems use coolant rather than raw water in a closed circuit that circulates through the engine’s cooling passages, and they typically are pressurized. Nearly all marine diesels (and car engines) use this system for engine cooling, the most important of which is improved overheat prevention. At sea level, water boils at 212° F; however, as the pressure of the atmosphere acting on that water increases, the boiling point rises. For every pound of pressure, the boiling point increases by 3° F.

A pressurized cooling system also improves heat transfer between the engine and the coolant (the correct term is coolant, not antifreeze) and reduces cavitation. Cavitation in a cooling system involves the production of vapor bubbles around water pump impellers and around the walls of wet cylinder engines (where piston external cylinder walls make contact with the coolant). The same phenomenon occurs on the backs of overpitched propeller blades. As each vapor bubble is created and then collapses, it removes a small amount of metal, causing a pit or crevice. Increasing the pressure on a cooling system makes vapor bubbles less likely.

The pressure cap is responsible for keeping this pressure at the predetermined value. It does this with a set of seals, a spring and a cam lock. The pressure that the cap maintains is a function of the spring tension. Thus, a cap with a spring tension of 15 lbs will raise the boiling point of water to 257� F, while using a 50% mixture of coolant and water will raise it more, to about 265� F.

Yet another function of the pressure cap occurs after the engine is turned off. When the coolant within the closed system cools, it contracts, creating a vacuum. Ordinarily, this vacuum causes hoses to collapse or draw in air; however, the second mission of the pressure cap prevents this. Built into the cap is a check valve that won’t allow pressure to escape the system, but it will allow liquid – coolant contained within the recovery or overflow bottle in this case – to be drawn into the closed cooling system. If this simple component works well, it releases pressure too soon or too late, the cooling system can be damaged.

Inspect your cap for damage to either of the two gaskets, one of which is directly under the cap (this gasket is sometimes made of thin brass and is prone to cracking) and the other is below the spring. The latter gasket, which is rubber, works the hardest, keeping anywhere between 5 and 15 lbs of pressure on the system and venting it to the overflow bottle when the pressure exceeds this level.

The filler neck of the cooling system, the fitting to which the cap is attached, also should be inspected carefully. If its uppermost surface is even slightly dented or scratched, the pressure cap gasket no longer will be able to make a pressure- and liquid-tight seal. Additionally, the “ears�VbCrLf of the cap must engage the cam properly. If either is damaged or worn, the cap may not be able to hold its rated pressure. The cam also has a safety stop, to prevent complete removal of the cap without a delay. This allows any pressure to be vented before the cap is removed. Never remove the pressure cap from an overheating or hot cooling system.

By Ocean Navigator