Diesel Engine Troubleshooting

Top tips to help keep your engine going...

Diesel EngineTop tips to help keep you going.

Modern marine diesel engines are pretty reliable things - most of the time. Feed them with diesel and air, cool them with seawater/coolant and add sufficient lubricant to keep the cogs turning and they will often give hundreds or thousands of trouble free hours.

However we all have our off-days and diesel engines are no exception. Knowing a few troubleshooting tips will go a long way to keeping you going.


When checking the engine, isolate the engine battery and remove the engine keys, so that others cannot start the engine whilst you have hands in the way. Consider using plastic/latex gloves.


Be fastidious about keeping the engine compartment clean. It makes spotting a potential leak or problem so much easier.


The crankshaft, usually the pulley wheel at the bottom of the engine, usually drives the belts. Belts transfer power to other pulley wheels on the engine and drive the alternator, to provide power to the batteries, and the water pump to circulate cooling water around the engine.

If the belt is too loose:

  • The alternator may be inefficient resulting in uncharged batteries.
  • The water-circulating pump may be inefficient resulting in the engine running hotter.
  • Regularly checking gauges such as the voltmeter and engine temperature will highlight both of these problems.


A slipping or loose belt is often visually indicated by black belt dust around the engine near the pulleys. There are two common types of belt; flat belts or ‘V’ shaped. Consult your owner’s manual about their accurate testing and adjustment, but a common rule of thumb to check adjustment is:

  • V Belts can be deflected by about 12mm and no more.
  • Flat belts should be able to twist through 90 degrees and no more.

A belt adjusted too tight can put excess pressure on the pulley wheel shaft and increase wear on the shaft bearings of the pulley wheels.


A properly functioning engine does not really consume much lubricating oil; it just uses the oil to lubricate and cool its moving internal parts. So if the correct level is checked it should stay roughly the same and only change over longer periods, unless there is a problem. Oil is kept inside the engine and separate from the water and fuel system by internal seals and gaskets.


The ways that oil escapes from the engine is by leaking through an engine seal or gasket. There are three visual ways to check for oil leaks:

1) Externally looking for leaks around the engine and engine tray
2) Oil leaking through a seal and mixing with the water cooling system.
3) Oil leaking into the combustion chamber, through the piston rings and coming.

The oil will burn in the combustion chamber and change to blue smoke coming out of the exhaust.

Oil levels need monitoring and too much or too little oil is bad for the engine. Looking at the condition of the oil is a little like reading tealeaves.

  • Green/brown oil is often clean oil in good condition.
  • Black oil is oil that has picked up carbons or soot, is starting to age and could possible need a change.
  • Grey or emulsified oil possibly has water mixing and may signify an internal leak. Contact an engineer.
  • Burnt smelling oil could be due to overheating.
  • Particles in the oil could be pieces of worn engine. Contact an engineer.


Many modern marine diesel engines have both seawater cooling and fresh water cooling. The seawater cools the fresh water/coolant that circulates around the engine.


Checking the seawater intake strainer is the main check on the seawater system apart from running the engine and ensuring that seawater is exiting through the exhaust (if you can see it). The exiting seawater water also quietens and cools the exhaust so if water is not passing through the exhaust, the exhaust note sounds hollow.

Times when seawater circulation can be a problem are:

  • On a sailing yacht at heel – if the water intake seacock is out of the water.
  • Outdrives and sail drives pick up their cooling water through the drive unit and these easily get blocked by polythene bags or lumps of seaweed in the water wrapping themselves around the drive. Often slowing the boat or reversing gently will clear the obstruction.
  • If the seawater strainer is blocked or the water circulating pump or impellor malfunctions.
  • If the overheat alarm sounds

A way of establishing whether the seawater or freshwater system is the problem when the overheat alarm sounds, is to:

  • Reduce the revs but keep the engine running
  • If water is normally visible exiting through the exhausts, check if it still is exiting. If it is, then the seawater system is probably working ok and the problem will probably be with the fresh water system.
  • Check the temperature gauges, a reduction in revs may reduce the temperature.
  • If not stop the engine and deal with the problem.


Want to know more about diesel engines, then check out the RYA DieselEngine Handbook, written and illustrated by Andrew Simpson, availablefrom the RYA Shop

Buy RYA Diesel Engine Handbook at the RYA Shop
Buy RYA Diesel Engine Handbook at the RYA Shop
  Buy RYA Diesel Engine Handbook (eBook) at the RYA Shop
Buy RYA Diesel Engine Handbook (eBook) at the RYA Shop