DIESEL ENGINE PROBLEMS: FUEL, AIR, STARTING, WIRING
Many cases of engine failure 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.
(Left) Bleeding the fuel with the primary bleed screw (Right) bleeding the fuel at the injector pump
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. Many larger and newer 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.
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 meter 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.
(Checking the fuel tank level with a dipstick)
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.
The air filter
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...