Many cases of diesel engine failure (see Diesel engine maintenance tips) are a result of a relatively small number of faults so it’s often possible to get the motor running again within a few minutes. However, when diagnosing problems it’s important to work in a systematic fashion and avoid the temptation to jump to conclusions.

Bleeding fuel diesel engine

Bleeding the fuel in a diesel engine with the primary bleed screw (left). Bleeding the fuel at the injector pump (right).

With a fuel problem, for instance, it’s important to start by being sure there’s enough fuel in the tank – it’s very easy to assume the problem is elsewhere. Many gauges are inaccurately calibrated, so checking with a dipstick or sight gauge is more accurate. Yachts with shallow tanks squeezed under aft cabin berths should ideally keep them at least one-third full to avoid problems when heeled while motor sailing.

Other problems that may also result in similar initial symptoms – typically a loss of power – include a hull that’s badly fouled by marine growth, a propeller of the wrong size or pitch and plastic sheeting or netting around the propeller.

Checking the fuel tank level with a dipstick

Checking the fuel tank level with a dipstick.

Many larger and newer diesel engines have integrated diagnostic systems, which should always be checked first in the event of a problem. However, smaller or older units are more likely to have no warning systems beyond the lights/buzzers for oil pressure, overheating and battery charging. On the plus side, their simplicity makes it relatively easy to trace and rectify faults.

Diesel engine fuel and air problems

These can be a cause of failure to start, loss of power, or the engine stopping when under way. A basic diesel engine needs only a constant supply of clean fuel, plus air to provide oxygen for combustion. The amount of air needed is easy to under estimate – around one cubic metre per minute is required for every 10hp of engine power. Defective engine compartment ventilation, or clogged air filters, can therefore result in a loss of power.

Diesel engine air filter

The air filter on a diesel engine.

To ensure no particles of dirt – even microscopic ones – can reach the precision parts of the fuel injector pump, there should be a pair of filters in the system. Even if changed at the engine manufacturer’s recommended service intervals these can become blocked by debris that collects in the fuel tank. This often happens as a result of a passage in rough seas that agitate sediment that normally sits on the bottom of the tank. A second possibility is bacteria that live in any water in the tank and feed on the diesel, which appears to be a bigger problem with modern fuels that have a component of biofuel.

A drop of power over a period of a few minutes is a sign of partially blocked fuel filters. While the engine may continue to run at slow speeds, eventually the filter is likely to become further clogged until the point at which complete engine failure becomes inevitable is reached. Air in the fuel also causes similar problems – this can enter the fuel system through two mechanisms, running out of fuel, or leaks on the suction side of the fuel pump, which will draw air into the fuel lines.

Any air in the system needs to be vented (bled) before the engine will operate properly. Many modern engines are described as self bleeding, but a degree of intervention is often needed. Loosen the primary bleed screw – this may be on top of engine mounted fuel filter or at the injector pump – and pump the manual lever on the fuel lift pump 6-10 times. If a mixture of air and fuel bubbles out around the bleed screw continue pumping until no further air bubbles are seen and only fuel emerges, then re-tighten the bleed screw.

If the engine doesn’t now start, it may benefit from being bled in the same manner further along the fuel system at the injector pump and in extreme cases by loosening the nut holding the fuel pipe at the injectors themselves. If it still won’t start, this points to blocked fuel filters. Changing the filters and bleeding air out of the system will get you under way once more, but won’t solve contamination in the tanks – take the first available opportunity to clean them.

Primary diesel engine filter

The primary fuel filter (left) and a dirty filter (right).

The fuel delivered to the engine must also be free of water. Under the primary fuel filter, between the diesel tank and the engine, there should be a water separator bowl – water here will show as an opaque layer below the fuel. It can be drained by loosening the plastic thumbscrew at the bottom of the bowl and allowing the water to drip into a container held underneath.

Starting problems

If the diesel engine won’t start, but turns over quickly on the starter motor, then a fuel problem – as above – is the most likely cause. However, in cold weather it’s worth checking the glow plugs: these are pre-heaters fitted to most diesel engines to warm the air in the intake to improve cold starting. It’s a simple matter to check whether they warm up using a damp cloth held in a gloved hand. If they don’t heat up, first check the wiring; if this is okay, replace the plug.

(Left) Checking the open circuit battery voltage - a well-charged battery will show 12.6 or more. (Right) Cranking test - a minimum 9V is needed to start a diesel engine, 10V is ideal

If the starter motor fails to turn the engine quickly the most likely faults are a problem with the battery, or in the wiring between the battery and the starter motor. These can be checked with a cheap electrical multimeter. Start by measuring the voltage at the battery when no load is being drawn from it. A well-charged battery will show 12.6V or more, while 12.2V represents a 50 per cent discharged battery. If the voltage is less than 12.0V, the battery will need recharging before it will start the engine.

If the battery passes this test, the next stage is to check the voltage while cranking the engine for 5-10 seconds. Around 9V is the minimum cranking voltage needed to start a diesel engine, although for optimum reliability 10V is ideal. A lower figure indicates a battery that's unable to deliver enough charge to start the engine. If the initial battery voltage showed a relatively poor level of charge, then the battery should be recharged. However, if the initial charge was good, but the cranking voltage low, this suggests an old battery that will no longer hold adequate charge and needs to be replaced.

Basic diesel engine wiring checks

If the battery passes the voltage tests, loose, dirty or corroded contacts are likely to be reducing the power that reaches the starter, with the energy being lost as heat. Terminals or connections that are hot after cranking the engine for 5-10 seconds are a clear sign of this problem. It’s possible for these to become very hot, so don’t burn yourself: touch the terminals with a slightly damp cloth held in a gloved hand – the cloth will hiss and steam if the contact is too hot to touch safely. Dismantle any suspect connections, clean with medium to fine emery paper and reassemble.

For information about other engine problems see Diesel engine overheating and charging repair and Diesel engine maintenance tips. For handy tips when at sea, take a look at Fixing your marine engine at sea.

Written by: Rupert Holmes
Rupert Holmes has more than 70,000 miles of offshore cruising and racing experience, in waters ranging from the North Sea to the Southern Ocean and Cape Horn. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular new boat and gear tests. He currently sails around 5,000 miles per year and in the past couple of seasons has cruised from the UK to the Azores, as well as winning his class in the 2014 two-handed Round Britain and Ireland Race. He also owns two yachts, one based in the Mediterranean and the other in the UK.