Scheduled checks are recommended by all manufacturers and they are usually hidden deep within some manual somewhere. Not many understand the going on in our engine rooms but there are some important checks which can save your time and money.


A neat tidy engine room, time to do some daily checks. Starring Vessel is Ocean Fauna a Cockwells Custom Launch.

Daily Checks

A great acronym is WOBBLE. Water Oil Batteries Belts Look Exhaust. Lets Go!

W. Water – most engines have two types of water in them. The first is the raw water intake. This is an intake from the sea which is feed through the heat exchanger, oil cooler, gear cooler and often aftercooler on a marine engine. It exits via the exhaust system providing cooling and quieting to the exhaust. It is imperative that this cooling water is able to flow uninterrupted. There is a sea cock to turn on, best left off when the boat is unattended and a filter to check called the raw water strainer. A few varieties are pictured. They can be blocked with barnacles, mud sand, jelly fish and general flotsam.

A typical sea chest with the lid removed

A typical sea chest with the lid removed

Secondary to this is the closed water cooling loop. It contains antifreeze which while inhibiting corrosion, increases cooling efficiency.

Problems with this system can be caused by extending the service interval of engines too far. Antifreeze will gel especially if left for extended periods of time. This will block the heat exchanger tubes. The stack (heat exchanger tubes) can be removed and cleaned and inspected on a yearly basis or if a problem is detected. The o rings on many need regular replacement observance of the maintenance schedule is critical.

Keel cooling can be found in numerous configurations often on trawlers and canal boats. It must be carefully inspected for leaks and monitored carefully for changing levels. Problems can be often caused by Inadequately sized cooling tanks. Beta marine have an excellent technical paper on this subject on their website.

Clear header tank, with long life red coolant at the correct level

Clear header tank, with long life red coolant at the correct level

Engine Oil.

Some misunderstandings exist around the amount of oil that is required in an engine.

The dipstick relates to the most important level in the engine. Do not overfill your engine with oil. It will cause increased crack case pressure and will burn off the oil, and may cause the engine to heavy breath making it inefficient and risk running away causing irreparable damage. A good check to perform on any engine is to check the filler, often located in the rocker cover or on larger engines in the sump, regularly for oil contamination. This will give early indication a problem.

To little is not as damaging as you might first think. The low level on the dipstick is not the bottom of the sump. Even if there is no oil on the dipstick, unless its all in the bilge you’ll probably still have enough oil for lubrication. However in modern engines especially turbocharged ones, the oil is used as a coolant as well as a lubricant, so you may find oil temperature creep up. This will cause components to fail prematurely usually the turbo, but will also cook the oil, i.e the oil will change chemically and become less effective.

So the message is top up but do not overfill.


Dipstick marks, it should be right between them

Oil type.

Older engines and non turbo charged yacht engines will require mineral diesel engine oil. Shell Oils RT4 X will in most cases be more than fit for purpose.

For More Modern turbocharged diesel engines RT4 X may well be suitable, for high pressure injection engines like Cummins QSB and QSC engines for example, require more advanced oils just as RT4 L. Indeed as engine design advances so does oil technology. Many engines, especially high output versions for marine propulsion, require very specific oils. Closely checking with oil specifications is of great importance as not all 15w 40 oil is the same. Indeed ambient temperatures may require you to use a different grade.

The correct oils is of paramount importance.


Two types of belts exist on many engines.

Auxiliary drive belts which can be either poly v or v belt types. Ref figure 1

Poly v belts do not need to be over tight, they grip extremely well. They should be changed at manufacturer specified intervals however if the writing isn’t on the belt anymore it’s a good indication that the belt is ready to be retired! They are often rigged with self tensioners which are great, just check that there is good tension on the belt which is about half a turn.

Poly V Belt fitted to a Yanmar 4JH3-DTE

Poly V Belt. Retrofitted to a Yanmar 4JH3-DTE

V belts are less good and more difficult to set up. Tension should be a deflection of about 10mm under moderate pressure. Over tension can cause damage to the alternator bearings, water pump and even crank shaft. Under tension can cause belt dust to form all over the place. Black in colour. Over tensioning is oven caused by the appearance of belt dust however often it is caused by either rusty or misaligned pulleys. Check the mounts of each component carefully for signs of deformation and do your best to keep those pulleys rust free.


Batteries are not maintenance free, despite what some manufacturers claim.

All batteries contain what is know as an electrolyte, Hydrochloric acid in the case of a lead acid battery.

This starts off simply as distilled water, which over the charging cycle becomes more acidic. They are therefore quite a dangerous item and should be treated as so. When inspecting a battery at the very least eye protection should be worn.

Wet flooded batteries

Can be topped up from time to time, usually monthly, through the caps situated on the top of the batteries. Pure distilled water is put into these.

You can also check the specific gravity of the batteries to a retain their Heath with a suitable hydrometer.

Specific Gravity of a battery should be

1320 charged
1260 normal
1180 discharged
1120 severely discharged (battery is unlikely to recover from this.)

Maintenance free batteries

Many different types fall under this section, including gel, sealed, Absorbed glass mat. The great advantage of these batteries is that they are far less likely to spill, though not impossible. Also due to a series of cavities in the top of the battery they are less likely to produce hydrogen from the charging cycle and therefore potentially safer.

However the ability to top up the battery is unavailable leading to the term maintenance free, a more accurate term would be once use only!


Looking around the engine bay give you your first indication of a problem. Check all sides of the engine for signs of leaks or corrosion and this is a god opportunity to throw some grease where needed and clean up with a rag before any dirt becomes ingrained. This will all make it easier to perform repairs in the future.

Look especially at the most critical thing of all your fuel filters. It is imperative that these are clean and free from diesel bug or water. You need to know how to change and bleed your Diesel engine.


The fuel system consists of a few basic parts.

At the tank there is a pick up pipe. Unlikely but this could become blocked causing fuel starvation.

The fuel will flow from here to the primary filters, which vary enormously in quality between manufacturers. I recommend the RACOR Turbine series for ease of use and performance in larger engined vessels. The spin-on series is great for smaller engines. They are initially expensive, however worth thier weight in gold. Always specify a filter in excess of your fuel flow requirement and remember the amount of diesel returned to tanks in modern direct injection common rail engines is typically 3 x that used per hour In old fashioned systems.

VOLVO D6 with top cover removed showing the Common Rail System. Work on the fuel system on these engines should only be attempted my a competent marine mechanic

VOLVO D6 with top cover removed showing the Common Rail System. Work on the fuel system on these engines should only be attempted my a competent marine mechanic

This places additional strain on fuel systems. I recommend changing the elements on these primary fuel filters at least every service but certainly annually. Any self respecting skipper of any skill level should be able to do this and I not endeavour to learn. A good diesel mechanic will be happy to show you this basic skill.

Filters will normally have a water trap, often clear so any contamination can be seen in the bottom. These clear filters do not comply with the boat safety scheme. You will need to fit a metal bowl in this case and it will need to be inspected more regularly for contamination.




Exhaust Coloration is a great help in determining engine Heath.

Engines in good Heath and when warm should be smokeless, typically with a small amount of black smoke under heavy acceleration and a small amount of white steam when running hard in cold weather.

There are four to look for. – contact a diesel mechanic for further diagnosis!

White Smoke

White Smoke is generally caused by over fuelling, this is an injection timing problem of some kind. This will cause no end of problems so get it sorted out quickly. Apart from anything you will lack power and burn loads of fuel!

Sweet smelling white smoke which looks more like steam could be coolant leaking into the exhaust system in some way. The coolant level may not drop depending on where the loss is occurring and could be mixing with sea water or even combustion gasses and material.

Black smoke, is caused by poor combustion. Two sources for this. Lack of air is the first. Check your fire flaps are in the correct position and any forces ventilation is working as it should. Always remember a four litre engine uses 4 litres of air in each cycle. At 3000 rpm that’s 12000 litres. What’s more, in a turbo charged engine the figure could double as the turbo compresses the combustion air!

Blockages could also exist in the inter cooler, or air filter so worth check those out.

Equally, the problem could be back pressure in the exhaust, so check that.

You could equally be overloading the engine. If you have a heavy load onboard or you have changed the propellor or indeed if the boat is heavily fouled then this could cause black smoke.

Blue or purple smoke.

This is caused by the burning of oil in the engine. Some when the engine is cold is normal however this should not be occurring when the engine is warm. This is often caused in yachts by the glazing of cylinders by the use of engines for charging, especially at lower rpms for long periods. Prevent this by good battery management and running in gear while charging. Good oils such as shell Rimula can help to prevent this also. Run your engine at all times at moderate load.

It can also be a symptom of worn piston rings and valve stem seals. If it’s smoking really badly then it’s time for an overhaul. Do it before it lets you down or interrupts your boating season!

I hope this guide helps you to understand the reasoning and basic checks to undertake on your marine diesel. Explanatory Photos and Videos to come shortly.