Engine checks

A diesel needs only two things to run: clean air and clean fuel. That is why most of the checks concern air, fuel, cooling water and oil, and free ventilation, good compression and a free exhaust matter as much as a full tank. The battery is needed only to start the engine: once it is running, a typical diesel needs no electricity to keep going, except one fitted with a stop solenoid.

In the four steps of the figure the cycle always repeats the same way: the piston goes down and draws in air; then it rises and compresses it until it is very hot, and at the end of the stroke the injector sprays fuel at high pressure, which ignites by itself; the expanding gases push the piston down and, through the connecting rod, turn the crankshaft and the propeller; finally the piston rises again and pushes the gases out through the open exhaust valve. No spark is needed. Some four-stroke drawings show a spark plug: those are petrol engines, a diesel has none.

To learn the names, start with what you can see at the front and what turns: the drive belt, the alternator and the raw water pump. Then look up: at the top are the air inlet filter, the heat exchanger with the header tank cap and the exhaust injection elbow. On the side you find the dipstick, the oil filter and the fuel filter; behind is the marine gearbox, and the whole engine sits on flexible mounts. In the figure, find each part in its place. With the engine running, keep well clear of the belt and pulleys, and no scarves or loose clothing.

In direct (raw water) cooling there is a single open circuit: sea water enters through the seacock, passes through the engine (head, cylinders, manifold) and then leaves together with the exhaust gases, with no heat exchanger. It is simple, but the salt corrodes and the temperature is hard to keep steady.

The route, from the bottom of the boat inwards, is always the same: seacock, strainer, impeller pump, engine, exhaust injection elbow, outlet at the stern or through the topsides. The strainer comes before the pump precisely to protect the impeller. In the figure, follow the water in this order.

Indirect cooling has two circuits. Inside the engine a closed circuit of fresh water and antifreeze circulates, driven by its own circulation pump; the heat passes to the sea water through the heat exchanger, and the sea water never enters the block. The header tank takes up the expansion of the coolant. It is more complicated than direct cooling, but it keeps the temperature steadier and reduces internal corrosion.

In the figure the sea water side is: seacock, strainer, impeller pump, heat exchanger, injection into the exhaust; the fresh water side is: circulation pump, passages in the block, thermostat, heat exchanger, header tank. The thermostat is a valve: closed, it keeps the coolant circulating only in the block, so the engine warms up quickly; open, it also sends the coolant to the heat exchanger. If it sticks closed the heat does not reach the heat exchanger and the engine overheats.

The impeller is the rubber part, with flexible vanes, of the raw water pump. It turns with the engine and, as it passes the cam, the vanes bend: where they straighten they draw water in, where they bend they push it towards the outlet. The rubber is lubricated by the very water it pumps.

That is why, if the impeller runs dry (seacock closed, or blocked by, for example, a plastic bag), it overheats and the vanes crumble. After a failure it is not enough to replace it: find the cause and recover every fragment of the vanes before restarting, otherwise they stay in the circuit and block it. After every start check that water comes out of the exhaust: if it does not, do not leave the engine running.

Changing the impeller is a job done in order. First close the seacock, so that water does not come in while you open the pump; then remove the screws, the cover and the gasket, and put the gasket aside. Check in the manual how the impeller is fixed (screw, groove or key) and pull it out with a lever. Check the vanes: if a piece is missing, it must be found in the circuit before restarting. Fit the new impeller, replace the gasket and cover and only at the end reopen the seacock. The typical mistake is to start with the seacock still closed: the new impeller runs dry and is ruined at once. At start-up check the water at the exhaust.

The siphon break (vented loop) sits at the highest point of the sea water circuit and does its job when the engine is stopped. With the seacock open, the siphon could fill the exhaust with water, and the water could reach the cylinders through the exhaust valves. The air that enters through the siphon break breaks the siphon. When the engine is running, the pressure of the pump keeps the valve closed. In the figure, look for the highest point of the circuit. If you see water coming out of it while running, there is a problem: the valve is dirty or faulty.

The fuel leaves the tank and follows this route: shut-off valve, water-separating pre-filter (with the drain tap at the bottom), lift pump, fine filter, injection pump, injectors. What is not burnt returns to the tank through the return line. Water and dirt are the enemies of the injection pump and the injectors: that is why the pre-filter, which traps water and dirt, is the first to suffer and must be checked often.

Keep the tank as full as you can: there is less condensation and less froth and dirt stirred up by pitching. When you plan a passage, allow at least 20% fuel in reserve. In the figure, follow the line from the tank to the injectors and find the return pipe.

Bleeding removes the air from the fuel system, and it is needed after changing a filter or if air has got in. Follow the line from the tank and check that the valve is open and that there is fuel. Then open the bleed screw on the fine filter by about two turns and work the lever of the lift pump until the fuel comes out without bubbles; close the screw. If the system has been completely emptied, also bleed the stretch up to the injection pump, loosening the banjo bolt by two turns, and then the injectors. When you fit a new cartridge, smear a little oil on its rubber seal.

The oil holds carbon, acids and metals in suspension: that is why oil and filter are changed regularly, and an oil leak while running causes serious damage. The belt is checked on its longest free stretch, where it should deflect by about half an inch (12-13 mm): if it is too tight it wears the bearings of the alternator and the pump, if it is too slack it slips, does not charge the battery and does not drive the fresh water pump, and the engine overheats.

In the electrical system the contacts must be clean, dry and free of corrosion. Do not switch off the main battery isolating switch until the engine has stopped, because the alternator can be damaged.