Marine Diesel Engines and Fault Finding

Prerequisites

This lesson builds on the engine checks you learned at Day Skipper level (see the lesson on engine checks and maintenance). Before starting you should:

• Hold, or be working towards, the RYA Coastal Skipper / Yachtmaster Offshore shorebased knowledge, and have Day Skipper level practical experience of starting, running and shutting down a marine diesel.
• Be able to carry out the daily pre-start checks: oil level, coolant level, raw water strainer, belt tension, fuel and battery state.
• Understand basic electrics: volts, amps, the difference between a starter battery and a domestic battery bank.
• Be comfortable working in a cramped engine space with the engine stopped and cool, and know the safety rules for hot surfaces, rotating belts and fuel.

The reading behind this lesson is the RYA Diesel Engine Handbook (G25), which is the standard reference for skippers who must keep their own engine running at sea.

Learning Objectives

By the end of this lesson you will be able to:

• Describe the four-stroke diesel cycle and explain why a diesel needs no spark ignition.
• Trace the fuel system from tank to injectors and explain the purpose of each filter and pump.
• Bleed air from a fuel system correctly after running dry or changing a filter.
• Explain raw water and fresh water (indirect) cooling, and recognise the signs of cooling failure.
• Change a raw water pump impeller in a seaway and know how to find lost vane fragments.
• Describe the engine electrical system: starter motor, alternator, glow plugs or heater plugs, and battery isolation.
• Work through a systematic fault-finding sequence for the common symptoms: will not turn over, turns but will not start, loses power, overheats, smokes, low oil pressure.
• Compile a sensible spares and tool kit for a coastal or offshore passage.
• Apply a planned maintenance schedule and explain the effect of poor fuel and poor ventilation.

Why a skipper must understand the engine

The diesel engine is the one piece of equipment on a yacht that most crews treat as a black box until it stops. It will stop at the worst moment: entering a narrow harbour in a rising wind, motoring through a tidal gate, crossing a shipping lane in calm weather. The engine is also a safety system. It gets you off a lee shore, charges the batteries that power your radio, navigation lights and instruments, and allows you to reach harbour when the wind fails.

Most diesel failures are not mysterious. Industry experience, and the RYA's own, is that the great majority of breakdowns on cruising yachts come from a small set of causes:

• Fuel problems: contaminated fuel, water in fuel, blocked filters, air in the system.
• Cooling problems: a worn impeller, a blocked strainer or a closed seacock.
• Electrical problems: flat or poorly connected batteries, corroded terminals, loose alternator belts.
• Neglected servicing: oil, filters and belts not changed on time.

If you understand how the engine works, you can reason from the symptom back to the cause rather than guessing. The lesson is arranged in that order: first the principles, then each system, then a fault-finding method, then spares.

The four-stroke diesel cycle

Almost all yacht auxiliaries are four-stroke, water-cooled, direct-injection diesels with between one and six cylinders (three and four cylinders are the commonest), often rated 15 to 100 horsepower. The four strokes are repeated in every cylinder, in sequence, and the cycle takes two complete revolutions of the crankshaft.

1. Induction (intake) stroke. The piston moves down, the inlet valve opens and the cylinder fills with air only. No fuel is drawn in with the air, which is a key difference from a petrol engine.
2. Compression stroke. Both valves are closed. The piston moves up and compresses the air to roughly one-sixteenth to one-twentieth of its original volume (compression ratios of 16:1 to 22:1). Compressing air heats it: the air temperature rises to around 500 to 700 degrees Celsius.
3. Power (combustion) stroke. Just before the piston reaches the top, the injector sprays a finely atomised charge of diesel fuel at very high pressure (typically 150 to 300 bar or more) into the hot air. The fuel ignites by itself. The expanding gas drives the piston down, and this is the stroke that delivers power.
4. Exhaust stroke. The exhaust valve opens and the rising piston expels the burnt gas.

The essential point is that diesel ignition comes from the heat of compression, not from a spark. This has practical consequences:

• A diesel engine has no spark plugs, coil or distributor. Dampness in the ignition system, a common cause of petrol engine failure, is not a problem.
• A diesel needs a good seal in the cylinder to compress the air. Worn rings, burnt valves or a failed head gasket cause hard starting and white or blue smoke.
• A cold engine loses heat to the cylinder walls and may not reach ignition temperature. Small engines therefore have glow plugs (or heater plugs) in the combustion chamber or inlet manifold that are switched on for a few seconds before cranking.
• Fuel is the only thing that controls engine speed. The throttle lever does not restrict the air; it alters how much fuel is injected. This is why a diesel stops instantly if the fuel is cut off, and why the engine stop control cuts fuel at the injector pump.
• Diesel fuel burns hotter and more slowly, so the engine is heavy and strong and runs at lower revolutions than a petrol unit.

Typical figures to remember. Normal idle is about 700 to 900 rpm; cruising at about 70 to 80 percent of maximum rpm is kinder to the engine than flat-out running; normal coolant temperature is about 75 to 90 degrees Celsius; oil pressure when warm is typically 2 to 4 bar at cruising speed. Always check your own engine manual for the actual figures.

Why the engine needs air

Combustion needs plenty of clean air. A rule of thumb is that a diesel consumes roughly 100 cubic metres of air per hour for every 10 kilowatts of power at full load. If the engine compartment is badly ventilated, or the air filter is blocked, the engine will starve, smoke black and lose power. This is a common problem in boats where an extra locker or fitted cushion has been installed against the air intake.

The fuel system

Think of the fuel system as a chain. Every link must work, and the fuel must be clean and free of air and water.

Fuel path

The path from tank to cylinder is:

1. Fuel tank. Fitted with a filler, a breather vent, a gauge or sight tube, a drain at the lowest point, and a pick-up pipe set slightly above the bottom so that sludge and water settle below it. The shut-off valve on the supply line should be reachable from outside the engine space in case of fire.
2. Primary filter with water separator. The first line of defence, often a Racor-type element in a clear bowl, with a drain tap at the bottom. Water, being heavier than diesel, settles in the bowl where you can see it.
3. Lift pump (feed pump). A small diaphragm pump operated by a cam on the engine, with a manual priming lever so that you can pump fuel by hand to refill the system.
4. Secondary (engine-mounted) filter. A finer paper element, typically 2 to 10 microns, which protects the injection pump and injectors, whose internal clearances are only a few microns.
5. Injector pump. A precision pump that delivers fuel at high pressure, in the correct amount and at exactly the right moment, to each injector in turn. In a modern common-rail engine, a high-pressure rail feeds electronically controlled injectors.
6. Injectors. Spring-loaded nozzles that open at a set pressure and spray an atomised charge into the cylinder. The unused fuel leaks back to the tank through the leak-off (return) pipe.

The diagram below summarises the layout of a typical yacht diesel fuel system, including the bleeding procedure that applies after a filter change.

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Fuel quality problems

Contaminated fuel is the number one cause of auxiliary engine failure at sea. The main contaminants are:

• Water. Enters through condensation inside a partly empty tank (a good reason to keep tanks full when the boat is laid up), leaking deck fillers, or poor fuel from the pump. Water causes corrosion, damages injector pumps and feeds bacteria.
• Diesel bug. Microbial growth that lives at the water-fuel interface. It forms a black slimy sludge that blocks filters. Warm weather and long storage make it worse. Biocide treatments and regular tank draining help.
• Sediment and rust. Settles at the bottom of tanks and is stirred up by rough weather, which is why filters often block just when the sea is rough and you most need the engine.
• Wax. In very cold conditions, wax crystals can form in some fuels and block filters. Winter-grade fuel prevents this.

Practical measures: fill with fuel from busy, reputable suppliers; use a funnel with a fine filter when filling from cans; drain a little from the bottom of the tank and the water separator at regular intervals; carry spare filters and know how to change them; and keep the tank as full as is practical.

Air in the fuel system and bleeding

Diesel injectors require hydraulic pressure to work. Air is compressible, so once air gets into the high-pressure system the pump cannot build pressure and the engine will not start or will stop. Air gets in when:

• The tank runs dry.
• A filter is changed and not refilled.
• A pipe union is loose and sucks air in.
• A lift pump diaphragm has failed.

The cure is bleeding. The details differ from engine to engine, so read your manual, but the sequence for most engines is:

1. Check that there is fuel in the tank and open the tank shut-off valve.
2. Open the bleed screw on the primary filter, if fitted, and operate the manual lift pump lever until fuel flows without bubbles. Close the screw.
3. Move to the secondary filter. Open its bleed screw and pump until fuel emerges free of air. Close it.
4. Move to the injector pump. Open its bleed screw (some engines have a self-bleeding pump) and pump until clear fuel emerges. Close it.
5. If the engine still will not start, crack open the injector pipe nuts at the injectors, one at a time, about one turn. Turn the engine over with the starter, with the stop control in the run position, until bubble-free fuel appears at the nut. Then tighten the nut and move to the next.
6. Wipe away spilled fuel. Spilled diesel in the bilge is a pollution offence and a slip and fire risk.

The golden rule is to bleed in sequence from the tank towards the injectors, never skipping a point. Remember to use the manual priming lever. If the lift pump cam is at the top of its stroke, the lever will feel solid; turn the engine over a quarter-turn and try again.

A number of modern engines (many Volvo Penta and Yanmar units) have self-bleeding systems, but still need the lift pump primed after a long period of dryness. Some electronic engines have an electric priming pump.

Safety. Never use a starter motor for long periods when bleeding: run it for no more than 10 to 15 seconds, then rest for a minute to let the motor cool. Stay clear of the high-pressure fuel spray at injector nuts, which can penetrate skin. Do not smoke and keep naked flames away.

Fuel consumption planning

For passage planning, a cruising diesel typically burns about 0.25 to 0.3 litres of fuel per kilowatt hour at full load. On a rough basis a 30 horsepower auxiliary at a comfortable cruising speed burns around 2 to 3 litres per hour. Always plan on the real figure for your own boat, measured over several passages. A prudent skipper holds a reserve of at least 30 percent of the planned fuel use, and for an offshore passage motors only with fuel in hand to enter harbour against the tide.

The cooling system

Diesel engines turn only about a third of the fuel's energy into useful work; the rest goes out as heat in the exhaust and the cooling water. The cooling system must remove the heat from the cylinder block and head, or the engine will seize.

Direct (raw water) cooling

Simple older and small engines are cooled directly by seawater. Water is drawn through the hull via a seacock, passes through a strainer, is pumped by the raw water pump through passages in the engine block and head, and discharged with the exhaust.

The advantages are simplicity and cheapness. The disadvantages are serious: salt corrodes the passages, salts deposit inside the engine and the engine runs cooler than ideal, increasing wear. For this reason direct cooling is rare in modern yachts.

Indirect (fresh water) cooling

Nearly all modern yacht engines use a closed-circuit fresh water (coolant) system, rather like a car. The engine block is cooled by a mixture of fresh water and antifreeze (typically 50:50, which also protects against corrosion). A thermostat controls the temperature. The heat in the coolant is passed to seawater in a heat exchanger, and the seawater discharges to the exhaust.

There are therefore two separate circuits:

1. The fresh water circuit. Coolant is circulated by an engine-driven circulating pump through the block, the head, the thermostat housing and the heat exchanger. A header tank (expansion tank) with a pressure cap allows for expansion. The cap holds the system at about 0.5 to 1 bar, raising the boiling point.
2. The raw water circuit. Seawater passes in through the seacock, strainer, raw water pump and heat exchanger (and often an oil cooler and gearbox cooler), and then goes to the exhaust mixing elbow, where it cools and quietens the exhaust gas and leaves the boat through the transom.

The diagram below shows the raw water path through a typical indirectly cooled engine.

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Routine cooling system checks

Before every start check that:

• The coolant level in the header tank is correct (engine cold!). Never open a pressure cap on a hot engine: scalding steam and coolant can cause severe burns.
• The seacock is open. A very common cause of overheating is a seacock left closed after a haul-out or after a lay-up. Many skippers hang a tag on the ignition key as a reminder.
• The raw water strainer is clear of weed, plastic bags and jellyfish. Look through the glass bowl; clean it regularly in weedy areas.
• The drive belt driving the circulating pump and alternator has the right tension, normally about 10 to 12 millimetres of deflection under thumb pressure at mid-span.

After starting, always check that water is coming out with the exhaust within a few seconds. It is the only quick indication that the raw water pump is working. No water means: shut the engine down immediately, as running without cooling water will damage the impeller within a minute, can overheat the exhaust hose and may melt the exhaust.

Signs of cooling failure

• High temperature alarm or gauge reading.
• No water from the exhaust, or steam instead of water.
• Loss of coolant from the header tank, with overflow stains.
• Sweet smell of hot antifreeze, or a milky colour on the dipstick (coolant in the oil, which suggests a failed head gasket).
• Falling power, followed by seizure.

Changing a raw water pump impeller

The impeller is a rubber vaned rotor that sits inside the raw water pump. As it turns it sucks and pushes water through the pump by flexing its vanes against a cam. It is cooled and lubricated by the water. Rubber ages, and if the pump runs dry for even a few seconds the vanes overheat and tear. For this reason the impeller is the number one maintenance item on a yacht engine.

Changing an impeller is a skill that RYA examiners expect a Yachtmaster to be able to perform at sea, in under about 15 minutes, with spares on board. Replace the impeller annually, or every 200 hours, whichever comes first. If you buy a boat or have had a failure, change it at once and fit the pump cover plate.

The diagram below shows a good impeller compared with a worn one, with the replacement sequence.

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Procedure:

1. Stop the engine and let it cool. Close the raw water seacock (otherwise water floods in when you open the pump).
2. Remove the pump cover plate. Typically it has four or six small screws. Keep the screws in a tray or tin, because they love to fall into the bilge.
3. Extract the old impeller. Use an impeller puller tool or two screwdrivers, levering gently and evenly without scoring the housing. Mark the direction of the vanes before removal so that you can see how it was fitted.
4. Find every missing vane. If any vane is missing from the old impeller, it has gone downstream into the heat exchanger or oil cooler and will block it. Look in the housing, then in the hoses and exchanger inlet, until you have found all the fragments. A missing piece is a blockage waiting to happen.
5. Inspect the housing and the wear plate for scoring. A deeply scored wear plate needs replacing.
6. Lubricate the new impeller with a smear of silicone grease or washing-up liquid. Do not use mineral oil or petroleum grease: it can swell and rot the rubber.
7. Push the new impeller into the housing with a gentle twisting motion in the direction of rotation. The vanes must trail (point backwards relative to the direction of rotation) as they enter the pump body.
8. Fit a new cover plate gasket or O-ring. Replace the cover and tighten the screws evenly.
9. Reopen the seacock and check for leaks. Start the engine and verify that water flows from the exhaust immediately.

Keep the old impeller if it is undamaged apart from age: it may get you home if the new one fails, and it is worth keeping the vanes in the position they were in when removed, in a bag.

Other cooling system points

• Thermostat. Opens at a set temperature (often 71 to 82 degrees Celsius) to let coolant circulate through the heat exchanger. A thermostat stuck shut causes overheating; stuck open the engine will run cool and wear faster.
• Heat exchanger. Marine salts and debris can partly block the tubes, reducing cooling efficiency. It is normally cleaned and descaled during major servicing.
• Exhaust mixing elbow. Gradually silts up with carbon and salt, restricting the exhaust and causing back pressure and smoke. Inspect it annually.
• Anodes (zincs). The engine and heat exchanger carry sacrificial zinc anodes that protect against galvanic corrosion. Replace them when they are about half gone.
• Anti-siphon valve. If the engine sits below the waterline, a vented loop or anti-siphon valve in the cooling water hose prevents seawater being siphoned back into the engine through the exhaust when the boat is heeled. Siphoned water can fill a cylinder and cause hydraulic lock when the engine is next turned over. If the engine will not turn over by hand after the boat has been heeled on a long passage, suspect water in a cylinder. Do not use the starter in this case.

Lubrication

The oil system lubricates the bearings, piston walls and valve gear, carries heat away and holds combustion by-products in suspension. A gear pump in the sump draws oil through a strainer, through a full-flow filter and to the main bearings under pressure.

• Check the level on the dipstick, with the engine stopped and level, before each start. The level should be between the min and max marks.
• Use the oil grade stated in the manual, typically 15W-40 diesel engine oil with an API CF-4 or better specification. Do not use petrol engine oils.
• Change the oil and filter after the first 50 hours (running in) and then every 100 to 250 hours or at least once a year, ideally at the end of the season so that acidic combustion products are not left in the engine during winter.
• Low oil pressure warning means stop the engine immediately. The usual causes are low oil level, a failed oil pump, a blocked pick-up strainer, a faulty sender or worn bearings.
• Milky oil suggests water in the oil (head gasket failure, cracked head, or failed heat exchanger or oil cooler). Diluted oil, smelling of diesel, suggests a leaking lift pump diaphragm, or a leaking injector.

The gearbox has its own oil, checked and changed to the manufacturer's intervals. Many gearboxes of the hydraulic type use engine oil or automatic transmission fluid; some use special gear oil. Check the correct type before topping up.

The engine electrical system

The engine's electrical system has three jobs: start the engine, charge the batteries, and operate alarms and instruments.

Components

• Battery (starter battery). A 12 volt lead-acid battery (24 volts in some larger boats). Starter batteries are designed for short, heavy discharge. They should be kept separate from the domestic (house) battery so that running the cabin lights and fridge never leaves you unable to start. A diode or voltage-sensing split-charge relay allows both to be charged from the engine.
• Starter motor. An electric motor that engages the flywheel through a pinion. It may draw 200 to 400 amps for several seconds, so the cables must be heavy and the connections clean and tight.
• Solenoid. An electromagnetic switch that closes the heavy starter circuit when the key is turned. A click without cranking often points to a weak battery or a poor connection.
• Alternator. Driven by a belt from the crankshaft pulley, it generates charging current and powers the electrics when running. Output is regulated to about 13.8 to 14.4 volts. A voltmeter reading below 12.5 volts when the engine is running shows the alternator is not charging.
• Glow plugs. Electric heaters that warm the combustion chambers in cold weather. They are usually used for 10 to 20 seconds before starting.
• Stop solenoid or stop cable. Cuts the fuel to shut down the engine.
• Instrument panel and alarms. Oil pressure, water temperature and charge warning lights, and a buzzer.
• Battery isolator switch. Allows the battery to be isolated from the engine circuit. Leave it on while the engine is running: switching the main battery switch to OFF while the alternator is running can destroy the alternator diodes.

Basic electrical maintenance

• Keep battery terminals clean, tight and smeared with petroleum jelly.
• Check electrolyte level in non-sealed batteries and top up with distilled water.
• Check the alternator belt tension and condition regularly. A slipping belt squeals under load and reduces charging.
• Inspect the starter and alternator cables and connections for green corrosion.
• Check the charging voltage with the engine running; the reading should be about 13.8 to 14.4 volts.
• Check the earth (negative) lead to the engine block is secure.

Using the starter properly

If the engine does not start in the first 10 seconds of cranking, release the key and wait for 30 seconds or a minute. Repeated long cranking overheats the starter and drains the battery. Do not keep trying: look for the cause. If the engine is hard to start, check that the stop control has been fully reset, the fuel valve is open and the engine is not in gear. Always start with the gear lever in neutral. Many engines have a neutral safety interlock.

Exhaust, gearbox and propeller

• Exhaust colour is a diagnostic aid, discussed in the fault-finding section. Black smoke indicates too much fuel for the available air; white smoke is unburnt fuel or steam; blue is burning lubricating oil.
• Exhaust back pressure. A blocked silencer or restricted mixing elbow reduces power and increases smoke.
• Gearbox and drive. The gearbox gives forward, neutral and reverse. A sailing yacht with a folding or feathering propeller must be checked for pitch and fouling. Check the shaft and stern gland for leaks and alignment. A stern gland should drip a few drops per minute when running; do not overtighten it.
• Propeller fouling. Weed, a lobster pot line or a polythene bag around the propeller will stall the engine. Stop at once and cut the line away, or if in danger, call for help.

Daily and periodic maintenance

A planned maintenance routine prevents the majority of failures.

Keep an engine log of running hours, service dates, oil and filter changes, and every repair. It will help with fault diagnosis and with resale.

Systematic fault finding

Fault finding is a logical process. Rather than replacing parts at random, use the following method:

1. Stop and think. What exactly happened? Did the engine stop suddenly or die slowly? Did the revs fall, were there any alarms, smoke or odd noises?
2. Check the obvious first. Fuel on, stop control reset, gear in neutral, battery switch on, seacock open. Most faults are caused by something simple.
3. Work from the symptom to the system. Use the tables below. Test one thing at a time.
4. Change one thing at a time. Otherwise you will not know which fix worked.
5. Rest and think when it is not obvious. A cup of tea and a clear head are better than wrenching in panic.

The engine needs three things to run: air, fuel and compression (heat). Most starting faults are one of these.

Fault 1: Engine will not turn over (no cranking)

Fault 2: Engine turns over but will not start

Fault 3: Engine starts but stops or loses power

Fault 4: Overheating

Fault 5: Smoke

A small amount of white or blue smoke when starting from cold is normal and clears as the engine warms. Persistent black smoke under load is a sign that the engine is working too hard, often because of a fouled hull or propeller.

Fault 6: Oil pressure warning

If the red oil pressure light comes on or the gauge falls, stop the engine at once. Check the level. If the level is correct, suspect a faulty sender, a blocked pick-up strainer, or a failing oil pump. Do not run the engine again until the cause is found. Continuing to run with no oil pressure destroys the bearings within minutes.

Fault 7: Charging problems

• Charge warning light on while running: broken or slipping belt, loose connection, or failed alternator.
• Batteries repeatedly flat: faulty alternator, poor connections, aged battery, too much load, or an engine not run long enough.
• Voltmeter reading below 13 volts when running: charging is not happening.

If the alternator fails offshore, reduce the load, switch off non-essential equipment, and use the engine less for propulsion. A small portable generator or solar panel can help in an emergency.

Fault 8: Vibration and noise

• Strong vibration at certain revs: a fouled or damaged propeller, a bent shaft, a damaged cutless bearing or a failed engine mount.
• Knocking: low oil pressure, worn bearings, or air in the fuel.
• Squealing: slipping belt.
• Rattle from the stern gland or shaft: loose coupling, a missing split pin or a worn cutless bearing.

Emergency measures

If your engine fails in a critical situation, such as close to shore, think first about the safety of the vessel:

1. If under sail, sail away from danger.
2. If at anchor in a harbour, anchor and call for help.
3. Make a Pan-Pan call if a vessel could become in danger. Do not hesitate to ask for a tow before the situation becomes critical.
4. If a rope is round the propeller, stop engine, remove the key, assess the risk, and cut it away. In rough conditions call for help rather than diving over the side.

Never attempt to start a hot, seized engine repeatedly. If it has overheated, let it cool before you inspect it.

Spares and tools

A coastal skipper should carry spares and tools appropriate to the passage. For offshore cruising, the Yachtmaster standard calls for enough to cure the most likely faults. A sensible kit includes:

Keep the manual and the tools where they will be found easily. Practise using them when it is calm.

Worked Example

Scenario. You are skippering a 36 foot sloop with a three-cylinder, 30 horsepower fresh water cooled diesel. You are motoring in light winds across a tidal gate, 8 nautical miles from your destination. The engine temperature alarm sounds and you notice that the exhaust is no longer emitting water.

1. Safety first. You immediately reduce the throttle to idle, put the engine in neutral and then stop the engine. You check your position and the traffic around you. The tide is about to turn against you, so you decide to hoist sail and make way while you diagnose the fault. You tell the crew what is happening.
2. Do the obvious checks. The seacock is open (it is on the checklist you filled in earlier). You look at the raw water strainer through the glass bowl: it is clear, with water in it. You conclude that water is reaching the strainer, so the blockage is further on.
3. Reason it out. No water at the exhaust and a clear strainer suggest the pump is not moving water. The impeller is the most likely cause. You have not changed it for 14 months.
4. Wait for the engine to cool. You wait at least 15 minutes; the engine block is hot. You do not open the header tank cap.
5. Close the seacock and replace the impeller. With the pump cover off, you find a worn impeller with three missing vanes. You remove it and fit a new one, lubricating it with silicone grease. You then search for the three missing vanes, and find two in the pump housing and one jammed in the heat exchanger inlet hose. You remove all three.
6. Reassemble. You refit the cover plate with a new gasket, tighten the screws, and open the seacock. You check for leaks.
7. Restart. You check the coolant level in the header tank (it is slightly low, so you top it up when the system has cooled). You start the engine and immediately look at the exhaust: water is flowing normally. You monitor the temperature gauge for the next 15 minutes. It settles at 80 degrees Celsius.
8. Record and prevent. You log the failure, the cause and the repair in the engine log, add replacing the spare impellers to the shopping list, and resolve to change the impeller every year in future.

The key lessons: stop the engine before damage escalates, work systematically from the symptom, always find missing vane fragments, and keep spares.

Common Mistakes

• Running the engine with the seacock closed, usually after a lay-up. Prevent it with a tag on the key.
• Not checking the exhaust for water after starting.
• Opening a hot pressure cap. Wait until the engine is cool, then cover the cap with a thick cloth and open it slowly.
• Using petroleum grease on an impeller, which swells the rubber.
• Failing to retrieve missing impeller vanes.
• Overusing the starter motor. Give it a rest to cool and avoid flattening the battery.
• Bleeding in the wrong order, or not at all, and then wondering why the engine will not run.
• Leaving the tank nearly empty in rough weather, stirring up sediment and uncovering the pick-up.
• Mixing up the battery selector so the engine is started on the house bank that is already flat.
• Switching the main battery isolator off while the engine is running, which can destroy the alternator.
• Changing several things at once during fault finding, so you never learn the true cause.
• Not carrying the correct spares, particularly impellers, filters and a belt that actually fits.
• Ignoring early warning signs such as a squealing belt, an oil leak or a slightly high temperature reading.

Summary

• A diesel compresses air until it is hot enough to ignite fuel injected into it. It needs air, clean fuel, and compression. It has no spark, and throttle controls fuel only.
• The fuel system runs from tank, through a primary filter with water separator, lift pump and secondary filter to the injector pump and injectors. Air or water in the fuel is a major source of trouble; bleed in sequence from tank to injectors.
• Contaminated fuel causes more breakdowns than anything else. Keep tanks full and clean, drain the separator and carry spare filters.
• Most yachts use fresh water (indirect) cooling with a heat exchanger and a raw water circuit. After every start, check that water flows from the exhaust.
• The raw water impeller is the most common cooling failure. Change it annually, lubricate with silicone grease or washing-up liquid, and recover every missing vane.
• Check oil, coolant, seacock, strainer, belts and fuel before each start. Stop the engine immediately if the oil pressure warning shows.
• Never switch off the main battery switch while the alternator is running. Keep starter and house batteries separate.
• Fault finding is logical: check the obvious, work from the symptom to the system, change one thing at a time. Use the exhaust smoke colour as a clue.
• Carry the spares and tools that match the faults you are most likely to meet, including impellers, filters, belts, fuses, a multimeter and the manual.

Check Your Understanding

1. Why does a diesel engine not need spark plugs, and what do glow plugs do?

Answer: A diesel ignites its fuel by the heat produced when air is compressed to a high ratio (16:1 to 22:1 or more), which raises its temperature to well over 500 degrees Celsius. Fuel injected into that hot air ignites by itself. Glow plugs pre-heat the combustion chamber of a cold engine so that the compressed air reaches ignition temperature more easily on starting.

2. List the main components of a diesel fuel system in order, from tank to cylinder.

Answer: Tank, primary filter with water separator, lift pump, secondary fine filter, injector pump, injectors. Surplus fuel returns to the tank through a leak-off return pipe.

3. The engine will not start after you have run the tank dry and refilled it. What must you do?

Answer: Bleed the fuel system to remove air. Open the tank valve, then bleed in order from the tank towards the injectors: the primary filter, the secondary filter, and the injector pump, using the manual lift pump lever until air-free fuel emerges at each bleed screw. If necessary, crack the injector pipe nuts and turn the engine on the starter until bubble-free fuel appears, then tighten them. Use the starter in short bursts.

4. What is the difference between direct (raw water) and indirect (fresh water) cooling, and why is the latter normal on modern yachts?

Answer: In direct cooling, seawater flows through the engine block itself. In indirect cooling, a closed circuit of fresh water and antifreeze cools the engine, and this transfers its heat to seawater through a heat exchanger. Indirect cooling prevents salt corrosion and scale inside the engine and allows the engine to run at the correct, higher temperature under thermostat control, which reduces wear.

5. Immediately after starting the engine, what is the one check you must make, and what do you do if it fails?

Answer: Check that cooling water is coming out with the exhaust. If there is no water, stop the engine at once. Common causes are a closed seacock, a blocked strainer or a failed impeller. Running dry will quickly destroy the impeller and can overheat the exhaust system.

6. You are changing an impeller and discover that two vanes are missing from the old one. What should you do?

Answer: Locate and remove the missing vanes. They will have been pumped downstream into the pump housing, hoses, heat exchanger or oil cooler, where they can block cooling water. Search carefully before fitting the new impeller and restarting; if you cannot find them, inspect the heat exchanger inlet.

7. Why must you not use petroleum grease to lubricate a new impeller?

Answer: Mineral oil and petroleum grease can swell and perish the rubber. Use silicone grease, glycerine or a little washing-up liquid.

8. The engine cranks normally but does not start, and there is no smoke from the exhaust. Which three things might you check first?

Answer: Fuel supply: is there fuel in the tank, is the fuel valve open and has the stop control been fully pushed back to run? Then check for blocked filters or air in the system and bleed if necessary. Also confirm the glow plugs are working if the engine is cold.

9. What do black, white and blue exhaust smoke each indicate?

Answer: Black means too much fuel for the available air: blocked air filter, restricted exhaust, overload from a fouled propeller, or worn injectors. White means unburnt fuel or steam: cold engine, low compression, air in the fuel, or coolant entering the cylinders. Blue means lubricating oil is being burnt: worn rings or valve guides, or an overfilled sump.

10. The red oil pressure warning light comes on while you are motoring. What do you do?

Answer: Stop the engine immediately, because running without oil pressure destroys bearings within minutes. Check the oil level. If the level is correct, suspect a blocked pick-up, faulty sender or failing pump, and do not restart until the cause has been found, unless the safety of the vessel demands it.

11. Why is it dangerous to switch off the main battery isolator while the engine is running?

Answer: The alternator needs a battery to regulate its output. With the circuit open, the alternator can produce a high voltage surge that destroys its diodes and regulator, and may damage electronic instruments.

12. Name six items you would include in a diesel spares and tool kit for a coastal passage.

Answer: Any six of: spare raw water impellers and gaskets, primary and secondary fuel filters, oil filter and oil, spare alternator belt, jubilee clips and hose, spare fuses and bulbs, multimeter, impeller puller, spanners and sockets, funnel with fine filter, torch, silicone grease, engine manual, antifreeze/coolant, distilled water.