Even though the individual processes that make a diesel engine run are basically simple, there are so many of them, and so many individual components involved, that trying to trace a particular fault can seem an almost impossible task. It can be done, though!
The first stage is to establish exactly what the symptoms are, and how they developed. Was the engine OK the last time you used it, but playing up now? Have you done anything in between which may have caused the fault to develop? Did it develop suddenly, or gradually?
Armed with this information, the dauntinglooking fault-finding lists that follow can be whittled down to a more manageable size.
Suppose, for instance, that we’re faced with an engine overheating. The fault-finding list offers a wide range of possibilities, ranging from a faulty gauge to wrong valve timing. Working through all of them could easily take a professional mechanic a full day or more, so it’s important to establish what is likely, as opposed to what is possible.
Assuming you haven’t done anything to change the valve timing or fuel pump timing, it’s most unlikely to be either of those. Suppose, however, that you shut the seacock in order to check the raw-water filter, and the engine overheated quickly the next time you started it. In that case, it’s a fair bet that you may have forgotten to open the seacock again!
Having narrowed down the possibilities, it’s then time to work through them in a logical order. Try to avoid making random changes or adjustments to your engine: don’t do anything without a good reason, and always remember (or write down) what you’ve done so that you can undo it if it doesn’t solve the problem or makes it worse.
Most important of all, though, remember that common, simple problems, such as a blocked fuel filter or leaky pipe, are much more likely than more obscure or complicated ones such as a sheared fuel pump drive.
Starting problems
THE STARTER WON’T TURN THE ENGINE FAST ENOUGH TO START ... OR AT ALL:
- Battery charge low
- Electrical connections loose, dirty or defective
- Engine in gear
- Oil of wrong grade
- Starter faulty
- Starting procedure incorrect
THE ENGINE TURNS OVER BUT WILL NOT START:
- Air in fuel system
- Air filter blocked
- Cold start aid (if fitted) faulty
- Compression poor
- Engine in gear
- Fuel contaminated by water (or ice)
- Fuel filter blocked
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel of wrong grade or quality
- Fuel pipes blocked or leaking
- Fuel tank empty (or nearly so)
- Fuel tap shut
- Fuel ‘waxing’ (caused by using ‘summer’ fuel in low temperatures)
- Injection pump drive broken
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Starting procedure incorrect
- Stop control faulty
THE ENGINE IS HARD TO START:
- Air in fuel system
- Air filter blocked
- Cold start aid (if fitted) faulty
- Compression poor (see page 83)
- Exhaust pipe blocked
- Fuel contaminated by water (or ice)
- Fuel filter blocked
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel tank vent blocked
- Fuel of wrong grade or quality
- Fuel pipes blocked or leaking
- Fuel tap shut
- Fuel ‘waxing’ (caused by using ‘summer’ fuel in low temperatures)
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Starting procedure incorrect
- Stop control faulty
Problems shown up by the gauges
THE ENGINE OVERHEATS:
- Air filter blocked
- Cold start aid (if fitted) faulty
- Cooling water low
- Cylinder head gasket faulty
- Exhaust pipe blocked
- Gauge faulty
- Heat exchanger blocked
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Raw-water filter blocked
- Raw-water pump faulty (eg impeller worn or broken)
- Raw-water system blocked or seacock shut
- Thermostat faulty
- Valve timing incorrect
- Valves leaking
- Vee-belt on water pump broken, loose or greasy
THE OIL PRESSURE IS UNUSUALLY LOW:
- Bearings worn
- Gauge faulty
- Oil cooler clogged
- Oil filter clogged
- Oil level low
- Oil of wrong grade
- Oil pressure valve faulty
- Oil pump faulty
THE OIL PRESSURE IS UNUSUALLY HIGH:
- Gauge faulty
- Oil of wrong grade
- Oil pressure valve faulty
Smoke
THE ENGINE PRODUCES BLACK OR GREY EXHAUST SMOKE:
- Aftercooler clogged
- Air filter blocked
- Cold start aid (if fitted) faulty
- Compression poor
- Excessive load caused by dirty hull or too much weight on board
- Excessive power losses due to internal friction in engine or transmission
- Exhaust pipe blocked
- Fuel of wrong grade or quality
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Inlet manifold leaking
- Leak-off pipe blocked
- Propeller damaged or wrong size
- Thermostat faulty
- Turbocharger faulty
- Ventilation to engine room inadequate
THE ENGINE PRODUCES BLUE OR WHITE EXHAUST SMOKE:
- Air filter blocked
- Cold start aid (if fitted) faulty
- Crankcase breathers clogged
- Compression poor (see page 83)
- Oil level too high
- Oil of wrong grade
- Thermostat faulty
- Turbocharger oil seals worn
Unusual noises or behaviour
THE ENGINE MISFIRES:
- Air filter blocked
- Air in fuel system
- Cold start aid (if fitted) faulty
- Compression poor (see page 83)
- Fuel filter blocked
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel pipes blocked or leaking
- Injection pump faulty
- Injection pump timing wrong
- Injector pipe(s) distorted or broken
- Injector pipe(s) leaking
- Injector(s) faulty
- Overheating
ENGINE IS UNUSUALLY NOISY, WITH A HAMMERING OR KNOCKING SOUND:
- Bearings worn
- Cold start aid (if fitted) faulty
- Engine mountings loose or broken
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel of wrong grade or quality
- Injector(s) faulty
- Oil level low
- Overheating
- Piston rings or cylinder bores worn
- Shaft coupling loose
- Valve clearances incorrect
- Valve spring broken
- Valve timing incorrect
- Valves sticking
ENGINE RUNS UNEVENLY, ‘HUNTS’ OR SURGES:
- Air filter blocked
- Air in fuel system
- Cold start aid (if fitted) faulty
- Control system faulty
- Compression poor
- Fuel filter blocked
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel pipes blocked or leaking
- Fuel tank empty (or nearly so)
- Fuel tank vent blocked
- Injection pump faulty
- Injector pipe(s) distorted or broken
- Injector(s) faulty
- Stop control faulty
THE ENGINE VIBRATES EXCESSIVELY:
- Air filter blocked
- Control system faulty
- Compression poor
- Engine and shaft out of alignment
- Engine mountings loose or broken
- Fuel of wrong grade or quality
- Injection pump faulty
- Injector pipe(s) distorted or broken
- Injector(s) faulty
- Propeller damaged or wrong size
- Propeller or shaft bent, broken or unbalanced
- Shaft coupling loose
- Shaft misaligned
THE ENGINE LACKS POWER:
- Air in fuel system
- Air filter blocked
- Control system faulty
- Compression poor (see page 83)
- Excessive load caused by dirty hull or too much weight on board
- Excessive power losses due to internal friction in engine or transmission
- Exhaust pipe blocked
- Fuel filter blocked
- Fuel lift pump faulty (or sucking air through leak on suction side)
- Fuel of wrong grade or quality
- Fuel pipes blocked or leaking
- Fuel tank vent blocked
- Fuel ‘waxing’ (caused by using ‘summer’ fuel in low temperatures)
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Inlet manifold leaking
- Overheating
- Propeller damaged or wrong size
- Thermostat faulty
- Turbocharger faulty
- Ventilation to engine compartment inadequate
THE ENGINE USES MORE FUEL THAN USUAL:
- Air filter blocked
- Cold start aid (if fitted) faulty
- Control system faulty
- Compression poor (see below)
- Excessive load caused by dirty hull or too much weight on board
- Excessive power losses due to internal friction in engine or transmission
- Exhaust pipe blocked
- Fuel of wrong grade or quality
- Injection pump faulty
- Injection pump timing wrong
- Injector(s) faulty
- Inlet manifold leaking
- Leak-off pipe blocked
- Thermostat faulty
- Turbocharger faulty
Compression poor
- Cylinder head gasket faulty
- Piston rings or cylinder bores worn
- Valve clearances incorrect
- Valve guides in cylinder head worn
- Valve spring broken
- Valve timing incorrect
- Valves leaking
- Valves sticking
References
Adlard Coles Book of Diesel Engines
