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October 10th 2018. By Alex Smith.

Engine maintenance

We look at the basic essentials of marine engine maintenance to help keep things running smoothly afloat.

Sophisticated though modern marine engines might be, there are still plenty of simple jobs you can do to keep your engine in good working order.

Top marine engine maintenance tips

  1. Buy reliable fuel and keep tabs on the build up of water and particulates.
  2. Keep the oils clean and topped up.
  3. Ensure your engine has access to lots of fresh, cool air.
  4. Clean and flush your engine after every use and monitor the efficacy of your pump impeller.
  5. Use marine grease on any moving parts and a corrosion inhibitor on electrical connections.
  6. Keep batteries topped up and alternator belts properly tensioned.
  7. Keep your props free of any fouling and protect your efficiency and your gearbox by repairing any dings.
  8. Try to use your boat often and all year round.
  9. Precisely observe all stipulated filter and fluid requirements.
  10. Get your engine serviced at an authorised centre in accordance with manufacturer schedules.

There are plenty of simple jobs you can do to keep your engine in good working order.

Outboard engines

When you remove the cowling of a modern outboard engine, the unerring cleanliness of the space often makes it look like there’s no longer anything for the DIY fan to get his teeth into – and with increasing demands for intuitive docking solutions, refined operation, exhaustive engine data and all kinds of user-specific settings, the computerisation of engines is a trend that’s unlikely to slow. And yet, from simple post-trip routines to problem-spotting and regular maintenance, there’s still a great deal you can do, even on the most modern outboard motor…

For instance, after every day out, whether you go boating in salt or fresh water, you need to flush clean water through your engine to remove any corrosive or obstructive deposits – and not just for a cursory 20 seconds but for a good few minutes. A set of ‘earmuff’ style seals are usually necessary for older engines but newer models tend to have dedicated mounts for exactly this job.

While you’re flushing out your engine, you should also check that water is spouting freely from your tell-tale. If the flow is weak, turn the engine off and pop a bit of wire up there to dislodge any debris. If that doesn’t rectify the issue, you may need a new water pump impeller, which is a part of the system that can often become clogged with gunk. It is, in any case, a good idea to replace it at least every couple of seasons and it’s a quick and affordable job that you can do with nothing but a screwdriver and a pair of pliers, simply by removing the cover plate on the impeller housing, giving it a clean and replacing like for like.

You should also take the cowling off and have a general look around for anything that seems out of the ordinary. Check for any leaks, as the presence of pooling water, fuel or lubricant can be an indication of an issue requiring the assistance of a professional marine mechanic. You should also check that your oils are clean and you should check for corrosion on any electrical connectors. It makes good sense to spray these with a lanolin-based corrosion inhibitor and to lubricate any moving parts with marine grease before wiping everything down and replacing the cowling.

You should also periodically remove and examine your propeller for any fouling, as if you leave a bit of fishing line in there, it can chew away at your seals and cause problems inside your gearbox. It’s very easy to do simply by removing the pin and loosening the nut with the prop levered against a block of wood on the underside of the cavitation plate. While you’re in there, it’s again worth cleaning it out and applying some fresh marine grease. And if there are any minor dings or burrs around the edges of your prop blades, you can often rub these smooth with nothing but a pair of hand files. If you find anything more significant, then you should do yourself (and your gearbox) a favour and take it to your local prop shop for a service.

As for more routine maintenance, changing the spark plugs is another simple, affordable and rewarding job that helps maintain optimum performance and fuel economy. Do them one at a time to avoid any confusion, make sure you don’t over-tighten them and, to keep your life as easy as possible, you can even add some anti-seizing lubricant to the threads before you put the new plugs in. Replacing the wires at the same time also makes sense – and while once a season might seem like overkill, it’s good practice and helps avoid the prospect of a misfire. The job involves simply popping off the plug wires, removing the plugs with a wrench and reversing the process with the new ones. Just make sure the wires are replaced in exactly the same pattern as you found them.

You should also pay regular attention to your fuel. Check the fuel line, paying particular attention to any junctions and fittings. And even the lines themselves can corrode as they get older, so make sure there are no leaks. Also check for water and impurities in your separator and drain any you find. And if your boat does limited hours or is likely to be sitting unused for a month or more, it makes sense to prevent any potentially problematic breakdown in your fuel by using a stabilising additive.

Clean and flush your engine after every use and monitor the efficacy of your pump impeller.

Marine diesel engines

Marine diesel engines are wonderfully simplistic and durable pieces of equipment. They need very little from you to return decades of trouble-free service. Beyond keeping your oil clean, your filters fresh and your service book stamped, all a marine diesel engine really needs is good fuel and a ready supply of clean air – but both are entirely vital because, in addition to generating power, the fuel in a diesel engine has to lubricate the moving parts and dissipate excess heat. If it contains particulates, its capacity to do that can be critically compromised; and if it contains liquid water, which is then introduced to the combustion chambers, the results can be catastrophic.

Plainly then, in addition to buying your fuel from a highly regarded supplier, you should make a point of checking your fuel-water separators on a regular basis and you should change all fuel filters precisely in accordance with the maker’s stipulations. It’s also worth dipping the hose from a fuel extraction pump into the bottom of your diesel tank and sucking out the stuff at the bottom to see what you’re looking at. Water is heavier than diesel so that’s where it will collect – and if left sitting for long periods, condensation from the inside of your tank or from a poorly stored fuel supply can collect at the bottom, kick-starting the growth of diesel bug, a microbial slime that can form at the layer between your water and your fuel. If you get rid of the water, you can often get rid of the problem but you still need to use a fuel stabiliser when your fuel is likely to be unused for long periods. And while you’re treating your fuel to a much needed pampering, it’s also worth drying out your bilge, cleaning it up and painting it white to help highlight any fluid leaks as and when they occur.

The other major issue as regards keeping your diesel engine in top condition is air. In terms of a diesel engine’s ratio of air to fuel, it can be four or five times higher than that of a petrol engine, which means you need to ensure it has access to lots of air – and we’re not talking about the hot, sweaty, fetid air of a poorly ventilated engine room, sucked desperately through a clogged filter. We’re talking about the cool, clean, oxygen-rich air of the outside world. And even then, passing hundreds of cubic feet of air through your engine with every gallon of diesel means enormous potential for the ingestion of dirt, salt and other contaminants, each looking to gouge at your cylinder linings. Plainly then a diesel engine’s air filtration system is vitally important. Even a partial blockage will cause your engine to burn more fuel and to lose power, while a seriously clogged unit can stop your engine altogether – and if you have a turbocharger, insufficient air can damage your oil seals, causing black smoke and overheating. So check the filter and if it looks dirty then, whether it’s due for replacement or not, slot a new one in its place.

Important provisos

The best way to keep a marine engine in tip-top condition is to use it regularly all year round. However, if that’s not possible, you still need to pay regular attention to your fuel, coolant, lubricants, filters and batteries in order to ensure your engine is ready for use when it’s called upon for a day out. Just be aware that, however much maintenance work you choose to take upon yourself, it remains vital that you follow prescribed engine servicing schedules, both to fulfil the terms of your warranty and to preserve your boat’s market value when the time comes to sell. A professional service at an authorised service centre every 100 hours or once a year tends to be a fairly standard arrangement, but make sure you follow the stipulations laid down in your manual.



Alex Smith is an ex-Naval officer, with extensive experience as a marine journalist, boat tester and magazine editor. Having raced as a Pilot in the National Thundercat Series and as a Navigator in the inaugural Red Sea RIB Rally, he has now settled in the West Country, where he lives and works as a specialist marine writer and photographer from his narrowboat in Bath.