Treating diesel engines is not too different to treating patients, says Gilbert Park


An idea struck me when I ordered an oil analysis for Merlot, my lockdown project boat. Routine health checks are part of medicine so why not for my engine?

Merlot used to be the Poole Harbour workboat and is probably 30 to 40 years old. I was told by the previous owner that she was re-engined in 2008 with an inboard Yanmar 3GM30 diesel engine. The service history was a little scant so I decided to do an oil analysis. This showed a lot of iron (perhaps crankshaft wear) and silicon (dirt).

Being inquisitive and remembering my earlier days as a doctor I thought I would listen to the crankshaft and bought a mechanic’s stethoscope for a fiver off eBay. I listened and it was fine. The Oil Lab, which did the test, recommended an oil change and retesting in six months; just like we used to do with blood tests when I was a doctor.

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The important part of any health check is to record values and findings so that if something goes wrong you know what the normal value was when all was well. It’s usually trends that are important in detecting problems, not absolute values. Remember, also, that as engines wear things change and some of the changes are expected, so a range may be given for values. The health check can be done at the same time as an engine service (it is not intended to replace it) or on a more regular basis.

The other thing you do as a medic is to think of systems. So as I serviced my engine I remembered what I had been taught and done over the years. The three common things that will stop a diesel engine from running are lack of cooling, poor fuel and no air.

1. Present history

The old air filter (black) next to the new filter (grey). This may explain the silicon in the oil analysis, as the filter appears not to have been changed for many years.

The first part of any health check is to ask if there are any current problems – the ‘present history’? In Merlot’s case it was the oil analysis and was the crankshaft in danger? I spoke to Luke, the very helpful Yanmar agent in Emsworth, and he told me not to worry, just change the oil. Listening to the engine revealed no lower end knocks so all seemed well. As for the silicon, I changed the air cleaner filter noting that the difference between the old and new was striking.

2. Past history

Gilbert records engine oil and temperature in his logbook

Next, was to investigate the past history. The service history was sparse! The oil was a supermarket own brand and the oil filter an aftermarket variant. The gearbox oil and engine fuel filter were six years old. I tried to get the history of the reconditioned engine (fitted in 2008) but unfortunately the company who did the reconditioning didn’t have records that far back.

It’s worth looking back through your logbook to see how much oil and water you have had to put in over the last year. Is it increasing or static? If it’s increasing is there a reason, such as has the engine having been run for more hours than previous years? If not then looking for a cause becomes important during the rest of the checks.

3. Family history

The final part of taking medical history is family history. Are there any known faults you should look out for in the family of your engines? For this you can search the internet, or, in my case ask Luke. He told me that the exhaust elbow on my engine is regarded by Yanmar as a service item. They recommend that it should be changed every two years at a cost of over £200! He showed me one, but the one on my boat was different and appeared to address some of the corrosion problems caused by the injection of salt water near a weld.

4. Examination

Onto the examination. Look, listen and feel was always the order of the examination.


First start by looking at the engine without starting it. Use a very bright inspection torch. Not only does this help you see leaks, etc. in the depths, it will help you to concentrate where you are looking so you are less likely to miss things. Are there any stains caused by leaking water, oil or exhaust? Are there dangling wires that need reattachment, cracked hoses, missing hose clamps and the like? Is there water in the fuel filter housing?

Then, after the routine prestart tests, start the engine. If you had seen stains before is there any active leaking going on? Look at the exhaust. Is there water coming out, what colour (if any) is the exhaust (see box)?


Before starting the engine make sure you are safe (see box). When the engine is running listen to it – are there any unusual noises? If so, increase the rpm and see if the noise changes. Some may disappear, some may get worse. This is where the mechanic’s stethoscope comes in useful. You can put the point on where you think the noise is coming from to pinpoint the bearing or whatever. It’s good practice to listen to the rest of the engine so you know what normal is. However, before using the stethoscope on a moving engine please read the safety panel and be careful if you become engrossed in what you are hearing.


Feeling comes down to mostly checking the hoses – try gently squeezing them to see if they are still flexible and supple. If they are hard and inflexible then they may crack with the engine vibrations in the future and you may want to consider changing them as a preventative measure. The same with belts. Are they cracked or loose?

5. Tests

Newer engines and displays enable a screenshot to be taken of the various engine parameters and are an easy way to store a lot of data. The file name usually has a date on it as well.

So that’s the history and examination done. Now onto the tests. With all tests it’s important to record the conditions under which they’re done as well as the results themselves. Were the recordings made at tickover or at wide open throttle (WOT)? Was the engine hot or cold?

On any boat I take out under power, once the engine has warmed up and I’m clear of the harbour I always do a WOT test. If the engine is going to break then near the harbour is the place for it to happen, not far out to sea. The same is true for the annual health check.

If the boat has an engine that is interfaced to your multifunction display you can take a screenshot, print it out and compare later results with it. If you do find something different a screenshot will be invaluable, easy and straightforward to compare. Modern engines have many variables you can record. Normal values can usually be obtained from the handbook, or the manufacturer’s website or the agent.

What, if like this Merlot, you don’t have that level of sophistication?


Gilbert took several pictures of the engine, such as this one, and noted the measurement points. Another option is to print off the photo and mark the measurement points with a pen. Each time he measures these, it’s a simple matter to record a new column of temperatures, along with date, engine hours and other comments.

Temperature is a good place to start. It may be that your engine has a temperature gauge, like mine, with its sensor on one of the hoses that you can record a value for. Alternatively, it may have an overheat/lack of water sensor built into its control panel. An infra-red remote thermometer with a built in laser spot where the measurement is being taken is invaluable. For an annual health check you might like to take several readings from known spots so you can compare results.

This simple, inexpensive device will tell you the temperature that the coolant will freeze and boil at by counting how many of the five coloured balls are floating. In this case it is three which translate to -23ºC and 105ºC. Should be enough!

Fresh water coolant

The other thing to check is the fresh water coolant. Not only do you need to make sure there is enough, but also if the amount of antifreeze is sufficient for the winter. An inexpensive solution can be found on various websites that have different coloured, floating balls and the ones that float tell you the temperature protection you have.

Oil pressure

Next is oil pressure. Merlot doesn’t have an oil pressure gauge. So instead I record the time it takes for the oil pressure alarm to go off when the engine is cold and warm. If it starts to take longer then it may be that something is going wrong and further investigation is needed. Remember that oil pressure will be dependent on temperature and the grade of oil you have put in.

Battery voltage

A low battery voltage with the engine running will tell you if you have a problem with the charging system. Again this may be obtained from a gauge or NMEA data, but if you don’t have these but do have a simple chartplotter or depth finder you may find concealed in a sub-menu ‘voltage’, useful for the health check. If you have none of these then it’s a case of connecting a multi meter to the batteries when the engine is running to check the voltage, or wait until the system fails and the red light on the panel comes on!

This is the oil analysis for the Yanmar engine in Merlot that started off the whole train of thought about annual health checks for engines. Click to enlarge

Oil analysis

The final test is the expensive one and the one I started with – oil analysis. Like a blood test, an oil analysis will tell you what you can’t see. Not only does it look at wear of various parts of the engine, it will also tell you the state of the oil and if water or fuel is leaking into the engine. You can also see if the oil needs changing or not, saving money and the environment if it doesn’t. Like all complex tests, however – just because something is abnormal doesn’t mean there is a problem. Usually the test comes back with an interpretation, but without detailed knowledge it can be misleading. Speak to the oil lab or an agent if you don’t understand something.

6. Keep a record

Once you have all the results of the history, examination and tests, remember to keep a copy on board. This could be a paper record or on a tablet or smartphone. If something does go wrong at least you’ll have the normal results with you. You can also use this record for the next health check to see what, if anything, has changed.

Merlot’s Yanmar has had her health check and engine service. Now it’s time to relax when I am out in Chichester Harbour knowing I’ve done my best to maintain this part of her. If you hear her purring as you pass us give us a wave.

In many boats there may be an engine compartment. Damaging noise and fumes are a risk if you are in that compartment while the engine is running and there is a leak from the exhaust system. Deaths have been reported from carbon monoxide poisoning, especially with petrol engines. Most compartments will have fans that can be used to ventilate the area while you are in it. Alternatively, many have parts of the floor or a hatch that can be opened to ensure good ventilation.

Safety when examining engines

  • If you have an old engine some of the moving parts such as belts and crankshafts may be exposed. Take great care that you don’t have loose clothing, hair, jewelry and the like that can get caught and injure you.
  • If you have a large engine or are working in a confined space protect your hearing with ear defenders.
  • If there is a risk of “bits” coming off the engine such as loose particles of paint or rust use eye protection.
  • Remember to wear gloves. There are a variety of nasty compounds around your engine. It also means it’s much easier to clean your hands at the end of the day. Even doctors wear gloves when examining patients!

Make sure you are in a well-ventilated area, especially if there is any risk of inhaling exhaust fumes. There is a risk of carbon monoxide poisoning. Furthermore, diesel engine exhaust is a cause of lung and bladder cancer. Although the studies reporting this looked at traffic exhaust they recommended that everyone should reduce their exposure to this carcinogen (find out more at the American Cancer Society).

Diesel smoke

A proper running engine should produce no smoke.

White smoke

This may indicate that there is unburnt diesel in the exhaust. Often this is from a fault with the injectors, but there are other causes as well.

Black smoke

The cause of this is incomplete burning of the fuel, perhaps from a clogged air cleaner or several other causes.

Blue smoke

This is the rarest colour of smoke and is caused by oil being burnt. Overfilling with oil is one cause but like the other colours there are many other causes.

About Merlot

Merlot (previously Rumpas) is a Romany 21 made in Poole in the 1970/80s. She used to belong to the harbourmaster, and has a towing post fitted just behind the engine box. In about 2008 she was re-engined with a Yanmar 3GM engine and gearbox. Gilbert bought her as a lockdown project and has enjoyed all the challenges she has posed

About the author

Gilbert has been sailing in various craft for much of his life. For the past few years it has been motorboats. Having done the RYA Yachtmaster Ocean course and gone straight into the first lockdown he decided it was time to slow down and travel at 6 knots most of the time.

This allowed him to take sights and also make tea and bake bread underway instead of having to sit in the helm seat and hang on. Much more relaxing!

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