Diesel Engine Troubleshooting

Whether the engine is mated to its drive shaft by a flexible coupling or connected directly to the propeller shaft flange, it must be carefully aligned. The engine output must run both square and concentric with the mating propeller shaft flange.

Although shafts, couplings and flanges are machined to precise tolerances, perfect alignment is often difficult to achieve. Nevertheless, significant misalignment will result in excessive wear of the bearings. If the drive flange is not square the propeller shaft will vibrate, causing rapid wear of the bearings. Bent shafts and flanges should be repaired ashore.

Engine mounts isolate vibration from the hull and allow the engine position to be adjusted for accurate alignment. The top nut is loosened and the lower nut turned to raise or lower the engine.

Alignment of the engine is carried out by adjusting the engine mounting bolts for position and height, to achieve close contact on all parts of the connecting flanges. You can test for close contact by trying to insert a feeler gauge between the flanges. The flanges must also be aligned, up and down and sideways.

Perfect alignment between engine and shaft is difficult to achieve, but excessive misalignment will reduce bearing life.

Various types of shaft seal are available, some water lubricated. This Vetus system will prevent water leakage into the hull and does not rely on a messy, grease-filled stuffing box.

Tightening the flange bolts on this old-fashioned stuffing box compresses a fibre wadding and, together with a gland greaser, will prevent most leaks around the shaft. Overtightening will damage the shaft.

Adjustments are made by moving the engine up or down using the lower nuts on the extended threaded rods on top of the engine mounts. When the engine is finally aligned, tighten the top nuts and use a lock washer.

Saildrive units are self-aligning, but the mountings still need to be properly set up to spread the loads correctly over the hull.

The alternator constantly senses the output voltage and when a preset voltage is reached – usually 14.2 volts – will progressively shut itself down to supply just enough current to maintain that voltage.

It is possible to buy devices which sense battery condition and allow the alternator to push through a higher voltage, thereby ensuring faster charging. These are useful when the batteries are used a lot, but not the engine. Consult a specialist when considering this type of equipment.

Some alternators can be offloaded electrically at start up to reduce the load on the starter motor. The lubricating oil pressure switch is usually the agent for bringing the alternator onto load. When the oil pressure rises, the warning light goes off, simultaneously switching in the alternator.

These generate electric power and are driven by the engine, usually through a V-belt or multi V-belt. The alternator normally runs faster than the engine, the speed ratio being determined by the diameters of the respective pulleys.

The drive belt should be neither too tight nor too slack. It should be possible to move the belt at its longest part by plus or minus 12mm (l/2in). A belt that is too tight will shorten the life of the bearings in the alternator, while a slack belt will provide insufficient friction to transmit the power and the belt will slip, giving reduced speed. You can usually adjust the tension by rotating the alternator about its primary fixing point and locking it in position using the bolt in the slotted adjustment arm.

The alternator supplies alternating current (AC) which is converted into direct current (DC) by its own rectifiers and used primarily to charge the batteries. The batteries themselves, since they are accumulators, smooth the alternator output.

The starting system comprises two components, the motor itself and its solenoid-operated electrical relay.

The relay is necessary because the heavy electrical currents delivered to the starter motor by the battery are too large for the key-type starter switch to carry. When the key is moved to its spring-loaded start position, the relay solenoid closes the large contacts, connecting the batteries directly to the motor.

Simple electrical system
1 Battery. 2 Alternator. 3 Regulator (usually integral to alternator). 4 Starter motor. 5 Starter relay. 6 Preheater/starter switch. 7 Fuse. 8 Fast fuse to protect alternator. 9 Warning lamp. 10 Ammeter. 11 Starting aid. 12 Voltmeter.

As the motor starts to turn, the drive pinion on the output shaft is thrown outwards towards the ring gear on the engine flywheel. The pinion and ring gear have shaped teeth which allow the pinion to engage easily. The output shaft of the starter motor picks up speed and turns the engine.

As soon as the engine fires, the start switch is allowed to return to the run position. The ring gear accelerates, the pinion is thrown out of contact with the ring gear by a spring and the starter motor stops.

If the engine is particularly quiet, you may not be aware it is running, so watch the instrumentation. Never engage the starter while the engine is running as the gears may be damaged beyond repair.

Another type of starter motor has the pinion engaging with the ring gear before the electrical connection is made. Again it is thrown out of contact when the engine picks up and when the starter switch is returned to the run position.

The electrical resistance of starter motors is extremely small and it is impossible to check accurately with a multimeter if the resistance is correct or not. If there is an electrical problem, the motor needs to be taken to a specialist.

Engine lubricating oils are designed to be mixable with oils of a different standard. Engines cover a wide range of service conditions from low powered, lightly loaded applications to heavily loaded applications using high powered turbo engines. The specification of lubricating oils reflects the service conditions, for example the more taxing the service the higher the specification of oil required.

Naturally aspirated engines, particularly those operating in lightly loaded applications, may need a lower grade oil for running in, to bed in the bores and avoid bore polishing which can result in high lubricating oil consumption. Consult the manufacturer for recommendations when running in a new or reconditioned engine.

Where a high sulphur content fuel is used, the lubricating oil should be changed more frequently.

Oil companies can provide an oil analysis service which can determine wear rates and combustion efficiency.

In the writer’s experience it is difficult to buy anything but high grade car diesel lubricating oil at automobile filling stations and oil suppliers are always in large towns miles away from the coast or any marinas. If a long trip is planned take at least enough oil and spare filters for as many changes as you expect to make and avoid any anxiety caused by exceeding the recommended change interval or using oil that you are unsure of.

Waste oil and the oily water found in the bilges should never be disposed of in the sea or inland waterways. As well as damaging the environment, this is illegal and could result in a heavy fine. Most marinas offer disposal facilities, and these should be used.

Oil absorbing mats are now available which can be laid under the engine or in the bilge.

These will absorb oil/fuel and can be disposed of correctly ashore.

1. Run the engine up to operating temperature and stop.
2. Drain the engine oil through the sump plug into a container, or pump oil out through the dipstick tube using the hand pump if fitted. If you use a separate pump, push the tube down until you feel it touch the bottom of the sump.
3. Remove the old oil filter, fill the new one with engine oil and fit as tight as you can by hand, first smearing some oil on the rubber sealing ring.
4. Fill the engine with clean oil to the correct dipstick level.
5. Run engine, stop, check the oil level again, and retighten the filter.
IMPORTANT. Waste or spilled oil should always be disposed of in designated receptacles ashore, never in the sea, river or canal.

Gearbox
Some engines share their lubricating oil with the gearbox, in others the two systems are separate. In the clutch-type gearbox the normal lubrication recommended is Automatic Transmission Fluid, or ATF.

In all cases refer either to the engine manufacturer or the gearbox manufacturer for the fluid recommended.

Heat is generated in the gearbox through the end loading imposed by the propeller shaft on the bearings, and where high powers are being transmitted the oil needs to be cooled. This is usually achieved with a heat exchanger cooled by water from the raw water engine cooling system.

Always use clean gearbox oil, as there are no filters incorporated in the system. Fill to the recommended level shown on the dipstick or the fill plug. Be careful, also, not to lose the plastic fill pipe often supplied with the plastic oil container; it will invariably disappear into the gearbox. A funnel is the answer.

Changing the oil (clockwise from top left). 1. Using the built-in pump to change the oil, typically about a gallon for a 30hp engine. 2. A strap wrench makes easy work of removing the oil filter. Alternatively, and messier, spear the filter with a screw-driver and twist. 3. A plastic bag will catch most of the leaks. A smear of oil will help bed the new filter. 4 and 5. After filling and running the engine, recheck oil level and top up if necessary. Dispose of oil and filter responsibly.

The gearbox will have a separate dipstick and may well share the same grade of oil as the engine. Gearbox oil is not filtered during use. It not only lubricates the gears but cools the box, often along with a water jacket.

Keep the oil level between the marks on the dipstick. The colour of the lubricating oil in the system says much for the engine’s health and can be analysed for evidence of internal wear. Diesel lubricating oil becomes black through the action of its additives. Grey means water is present.

The oil used for lubrication is a sophisticated product, and some diesels require a higher specification oil than that used for petrol engines. The reasons for this are:
1. Working pressures are higher in diesel engines.
2. Running temperatures can be higher.
3. Chemical conditions imposed by the combustion process are more severe.

In an oil developed for diesel engines the basic oil content is about 75 per cent, with additives making up the balance. These include:
1. Detergents to keep the inside of the engine clean.
2. Anti-wear additives to extend the life of wearing parts, particularly the valve gear.
3. Oxidation inhibitors which reduce the rate at which the oil base deteriorates when used.
4. Anti foam agents.
5. Viscosity index improvers to keep the oil fluid at low temperatures and thick at high temperatures.
6. Alkaline additives to counteract acidic compounds formed by the diesel combustion process.
7. Anti rust agents for engine protection.

There are various systems of classifying lubricating oils for viscosity. The common ones are
SAE (US Society of Automotive Engineers). A typical specification would be either SAE
10W30 for low temperature applications or SAE 15W40 for summer temperatures.

The standard method of specifying the quality of diesel engine lubricating oils is the API (American Petroleum Institute) reference CA to CE, API CE being the highest quality. The SA to SG range is for petrol engines.

European quality standards are defined by the CCMC (Comité des Constructeurs d’Automobiles du Marché Commun), a typical European engine oil being D4 or D5, or for passenger cars PD2. D5 is for Super High Performance Diesels.

Four-stroke lubrication system
1 Oil strainer. 2 Oil pump. 3 Pressure release valve. 4 Oil cooler. 5 Oil filter. 6 Oil gallery. 7 Oil feed to main bearing. 8 Oil feed through crankshaft to big end bearing. 9 Oil feed through connecting rod to small end bush and gudgeon pin. 10 Oil feed to valve gear. 11 Dipstick. 12 Sump pump. 13 Sump drain plug.

Lubricating oil acts as a barrier between moving parts, reducing wear. It is normally distributed around the engine by means of a pump before being returned to the sump. In some slow running engines lubrication is by ’splash’. With this system, as the name implies, oil is thrown to the bearings, bores etc as the engine turns over.

In a pumped system the oil normally flows to the bearings etc through holes either cast or drilled into the engine casing.

The lubricating oil level in the engine should always be kept within the limits marked on the dipstick.

Pumps
Constant displacement pumps are continuously driven from the engine, their output depending on engine speed. The internal parts of the pump are machined to fine tolerances and use very close fitting parts. Any scoring or wear will harm their efficiency. This will become apparent if the pressure generated at low speed is insufficient to operate the pressure switch or fails to meet the manufacturer’s recommended minimum pressure.

Suction strainer
Fitted primarily to protect the pump from sucking in stray nuts and bolts, the strainer is sited low down in the sump and connected to the pump by a steel pipe.

Relief valve
Oil pressure in an engine is determined by the resistance to flow, and limited by a factory-set relief valve. This is a spring-loaded plunger which lifts to allow oil to bypass the system and flow directly from the outlet of the pump to the inlet or to the sump when the setting is reached. If debris jams this relief valve open, oil will bypass continuously and the working pressure will not be reached.

Oil filter
The filter ensures that the oil reaching the moving parts and bearings is clean. The quality of filtration is determined by the engine manufacturer.

Filter cartridges may incorporate reverse flow non-return valves and a bypass in case the filter becomes clogged, set to operate at a specific pressure. Only manufacturer’s recommended spares should be used.

The non-return valve stops oil draining out of the bearings back into the sump when the engine is stopped, and thereby ensures that pressure builds up promptly when the engine is started.

Lubricating oil washes the engine parts and collects debris. Most of this will settle in the sump but fine particles will remain suspended in the oil and pass through the pump to be collected by the filter.

The filter element should be changed whenever the lubricating oil is changed. All engines in current production use spin-on filter cartridges, which can be removed easily with a chain or strap wrench.

It may be possible to undo a cartridge by hand: if a two-handed grip is possible wrap a piece of emery paper around the cartridge to provide a better grip. Another solution is to fit a large hose clip around it and tap round with a hammer.

Fill the new cartridge with fresh oil. Then, making sure that the rubber sealing element is in place, screw on until the sealing ring is compressed, and tighten by hand using the emery paper to provide a better grip.

Before restarting the engine, turn it over by hand or on the starter motor with the fuel pump engine stop valve closed to circulate the new oil around the engine and fill the filter and passages. Check and tighten the filter after running the engine.

Leaks in raw water systems can cause extensive engine corrosion so they should be cured as soon as possible.

Rubber pipes deteriorate with age and lose both flexibility and elasticity. Temporary repairs to hoses can be carried out with insulation tape and a hose clip.

Heavy duty hose clamps are available in larger sizes and provide an even pressure around the hose. Include in any maintenance programme replacement of hoses and clamps after five or six years.

Most direct-cooled engines have a sacrificial zinc anode to minimise corrosion, fitted somewhere inside the engine block. This should be checked annually and renewed if more than 50% eroded.

Leaks
If a leak is detected between the raw water and freshwater systems, it is most likely to be in the heat exchanger. The seals between the two systems may need to be renewed, tightened etc. This is a water system, so jointing compound or gaskets can be used with no problems.

A leak will be apparent if water drips from the freshwater overflow pipe when the engine is running. This pipe is usually fitted to the side of the filler cap on the header tank, and exits under the engine. The pipe can be removed from the cap to check for any leaks.