With the engine running at cruising revs, and taking care not to catch hair, scarf, fingers etc in any moving parts, examine the engine with a torch for water leaks, fuel leaks, oil leaks and excessive vibration.
Water leaks
Water leaks are often easier to trace when the engine is running at operating temperature. On engines with indirect cooling a leak between the freshwater coolant in the sealed system and the raw water from outside – which can, of course, be either fresh or salt – will generally result in water overflowing from the filler cap overflow pipe.
This is caused by raw water contaminating the freshwater system. You’ll generally find that the leak is in the heat exchanger or the connections to it and will need to be cured when the engine is stopped. The freshwater system should then be flushed and refilled with the recommended water/antifreeze mixture to stop any corrosion in the water ways.
Fuel leaks
Check the entire fuel system for the smallest trace of leaks. These can only be cured properly with the engine stopped.
A clean engine makes troubleshooting that much easier. Water or oil leaks are then easily spotted and can be rectified. Flaking paint on an old engine could mean localised overheating.
Lubricating oil leaks
There are very few engines that have no oil leaks and a couple of drops are no more than a nuisance. If the leak is more serious, however, keep tabs on it, top up the level as required and solve the problem as soon as possible.
When replacing gaskets, plugs or seals always make sure that the area is clean and use the manufacturer’s replacement parts. Rubber seals can be manufactured in many types of material, the commonest being nitrile for mineral oils, neoprene for water. To avoid pollution, dispose of oily bilge water as you would used lubricating oil – in a proper disposal facility in a marina or garage.
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.
Excessive vibration
This can be caused by a number of factors, including the engine not firing on one cylinder, engine-to-propeller shaft misalignment and loose mountings. Stop the engine before attempting to cure the problem.
