How to Keep Your Yacht's Diesel Engine Purring

The Complete Guide to Diesel Engine Care:
65+ Years of Tricks I've Learned the Hard Way

Look, I've been working on marine diesels for over 65 years, and I can tell you this: your engine is either your best friend or your worst enemy when you're out cruising. I've fixed engines in rolling anchorages, diagnosed problems by flashlight in the middle of the night, and learned a whole bunch of tricks that make life aboard so much easier.

So let me share how I take care of my diesel. These aren't just textbook procedures—this is what actually works when you're living on a boat and need to keep that engine humming.

Diesel Engine Calculus: My Mental Trick to Engine Problems

The most important mechanic’s tool you can have aboard to fix any engine problem is your mental attitude; the sure knowledge you can fix it.

Lots of cruisers completely freeze when their engine acts up. The first and only thought that enters their head is: "I need to find a diesel mechanic." And sure, that's fine IF the engine failure happens when you are in a port where there are diesel mechanics and IF you can afford a good diesel mechanic and IF you can find one who won't take advantage of you.

It happens, I've seen mechanics tell people they need to pull the engine, completely disassemble it, and replace a laundry list of expensive parts—when the actual problem was something simple. Not all mechanics are dishonest, but when you're a stressed-out cruiser in a foreign port and someone in coveralls starts talking about worst-case scenarios, it's easy to get taken for a ride. It’s money in their pocket to come up with the most expensive solution they can talk you into.

I've maintained my own diesel engines for the past 65 years, and I'm not some kind of mechanical genius. I just learned one mental trick long ago that changed everything for me.

My "Mental Defreeze" Technique

Do you know how calculus works? Don't worry, this isn't a math lesson. In calculus, when you're trying to figure out the formula for a curve, you divide it into very small segments—tiny little pieces that are easy to understand. You solve each little piece, and suddenly you've solved the whole curve.

That's exactly how I approach engine problems. I break down every failure into the smallest possible steps

Let's say your transmission won't go into reverse. That sounds scary and expensive, right? I start by dividing the problem into tiny segments – what could be causing this?

Step 1: Is the gear shift lever on the transmission actually moving when I shift in the cockpit?

• If yes, the problem is in the transmission

• If no, the problem is in the control cable or linkage

Step 2: Is this an external problem (something I can see—cable, linkage, adjustment) or internal (inside the transmission case)?

Step 3: What does my engine repair manual say about this? (I always download the repair manual when I get a new engine—if you don't have the shop manual for your engine, find it online NOW). In fact I have a whole folder on my computer and the backup USB hard drive called Manuals and it has the repair manuals and parts manuals for every bit of gear on the Moira. Vital!

Step 4: Has anyone else had this problem? Sometimes I’ll go online to a diesel mechanic page on social media or, even better, a page specific to my engine make and model. Someone somewhere has dealt with this exact issue.

Step 5: If I still don't know, I ask an AI to help me diagnose it. Today's AI tools are absolutely fabulous at helping narrow down problems, recommend parts you might need and where to find them. The new AIs can walk you through diagnostic steps and explain what different symptoms mean. Hint: A chat on engine repair can go on long enough you’ll run out of “free time.” Plus AIs can be wrong and it always pays to ask the same questions with two or three AIs and see how their answers compare. The best way to do this is an amazing collection of almost all the AI’s called Merlin. For $5 a year you have unlimited access to a treasure trove of AI models and you can switch from one to the other in one click. Check out Merlin right now.

One Bolt at a Time

Once I've got some idea what the problem might be, here's what I do: I get my wrenches, open the engine room doors, and ask myself one simple question:

"Can I undo that bolt right there?"

The answer is always: Yes, sure I can.

So I undo it. Then I look at the next bolt and ask the same question. One bolt at a time. One step at a time.

Maybe I need to remove the transmission. Sounds huge, right? But broken down into its calculus-like segments :

• Can I disconnect the shift cable? Yes.

• Can I disconnect the prop shaft coupling? Yes.

• Can I remove these mounting bolts? Yes.

• Can I figure out how to lift it out? Well, let me look... maybe with a come-along attached here... yes, I can.

See what I mean? The whole job is just a series of small steps, each one totally manageable. You don't need to see the whole staircase to take the first step.

You're More Capable Than You Think

I've fixed engines in rolling anchorages, replaced injectors at sea, rebuilt raw water pumps and done transmission work in remote anchorages where the nearest mechanic was 40 miles away. Not because I'm a qualified diesel mechanic; because I refused to freeze up and I took it one small step at a time.

The engine doesn't care whether you have a diesel mechanic's certificate or not. It just needs the right bolt turned, the right part replaced, the right adjustment made. If you can follow instructions and turn a wrench, you can fix your engine.

Resources That Help Me "Defreeze"

1. My Engine Manual - This is my bible. It has diagrams, torque specs, troubleshooting charts, and exploded views of every component. Download yours (often free from the manufacturer's website) and actually look through it before you have a problem.

2. Online Forums - There are forums for Beta, Yanmar, Volvo Penta, Perkins, Westerbeke, Cumins—every engine out there. Sailors who've had your exact problem and figured it out. Don't be shy about asking questions.

3. YouTube - Someone has probably done a video on exactly what you're trying to fix. Watching someone else do it first makes it way less intimidating.

4. AI Assistants - I'm not kidding—these have become incredibly useful. Describe your symptoms, your engine model, what you've checked so far, and ask for a diagnostic tree. The AI can help you think through what to test next and interpret what your findings mean. Give it a try.

5. Other Cruisers - The cruising community is amazingly helpful. Get on the VHF or walk the docks and ask if anyone knows your type of engine. You'll often find someone who's been through the exact same problem.

When to Actually Call a Mechanic

I'm not saying never use a mechanic. There are times when professional help makes sense:

• You've diagnosed the problem and it requires specialized tools you don't have

• It's a major internal engine rebuild that's beyond your skill level and needs to have the engine removed from the yacht to do the repair.

• Time pressure—you need to move the boat and can't spend days learning how to fix it

• You want confirmation of your diagnosis before ordering expensive parts

But even then, knowing what's wrong means the mechanic can't snow you. You can have an informed conversation about what actually needs to be done.

Before selecting a mechanic in a foreign port ask other yachts who they recommend.

The Confidence That Comes From DIY

Here's what I've gained from 65 years of fixing my own engines: confidence. Deep, unshakeable confidence that whatever goes wrong, I can figure it out. That confidence makes cruising so much more enjoyable because I'm not constantly worried about breakdowns or repair bills.

And honestly? Once you've diagnosed and fixed a problem yourself, you know that engine intimately. You understand how it works in a way you never would if someone else did it for you. That knowledge compounds over time.

So the next time something goes wrong—and it will, because we're talking about mechanical systems in a harsh marine environment—don't freeze. Don't immediately reach for your wallet and start looking for a mechanic.

Take a breath. Get out your manual. Break the problem down into tiny pieces. Ask yourself: "Can I undo that bolt right there?"

The answer is yes. And that's where you start.

Why I'm Obsessive About Diesel Maintenance

Here's the thing about diesel engines: they're incredibly forgiving if you treat them right, but they'll punish you for neglect at the worst possible time. I learned this the hard way in a remote New Caledonia anchorage when I found red Fleetguard coolant pooling under my engine. It was leaking from a hole electrolyzed into the aluminum thermostat housing. Why? A broken O-ring in the heat exchanger had let seawater into the coolant, creating a corrosive soup that made the housing fizzle away against the iron block. Never again.

The secret isn't complicated—it's about having the right tools, knowing your engine intimately, and staying ahead of problems. Let me walk you through my setup.

The Tools I Keep Aboard (And Actually Use)

Disclosure: Some links in this post are affiliate links. If you buy through them, I may earn a small commission at no extra cost to you.

Engine rooms are often (always?) difficult to work in and poorly lit. One of my most used tools is a head-mounted LED light so I can clearly see every detail in every crevice and have both hands free. Essential.

I carry complete sets of both metric and imperial(SAE) wrenches—open-end, combination, and ratcheting. Yeah, it's a lot, but here's why: modern marine diesels are mostly metric, but every alternator, pump, and accessory seems to use a random mix of both. Nothing's more frustrating than being one wrench short when you're troubleshooting.

My ratcheting wrenches get the most use. In the cramped quarters of an engine room where you've got maybe 15 degrees of swing, they're absolute lifesavers. I've got combination sets that give me both the ratcheting box end and open end on the same tool.

For socket sets, I run 1/4", 3/8", and 1/2" drive in both metric and imperial (SAE), with a collection of extensions. That 3/8" drive set is my workhorse—covers about 80% of what I need. But those long sockets? Those are critical. Injectors and glow plugs sit deep in the head, and you need sockets long enough to clear everything. I've got specialized injector sockets (27mm for my engine).

Most bolts and fittings on a diesel need to be tightened to specific torque pressures. The best socket/torque wrench set is the KARRYTON 35pcTorque Wrench Set

For changing the oil filter strap wrenches are fine until you've got a filter that's been on there through two seasons in the tropics and won't budge—then you need real leverage.

That;s when I get out my Two Way Universal Adjustable Oil Filter Wrench 
I use the big socket-style one that fits my 3/8" drive.

My Diagnostic Tools: The Real Problem Solvers

Digital Multimeter - This little device has diagnosed more problems for me than anything else in my toolkit. I use it to check battery voltage, verify my alternator's charging, test sensors, and track down electrical gremlins. Get a good one, like the KAIWEETS KM200PDigital Multimeter —the marine environment destroys the cheap ones. 

But here's a trick I bet you haven't heard: I use my multimeter to check for electrolysis in my cooling system. When the engine is cold I remove the heat exchanger pressure cap and put one probe in the coolant inside the heat exchanger and touch the other to ground. If I'm reading more than about 0.3 volts, I've got electrolysis happening.

Digital Infrared Thermometer - This one's a game-changer that most people don't think about. I use my Etekcity Infrared Thermometer Laser TemperatureGun to check coolant temperatures going into and coming out of the heat exchanger. 

Here's what I'm looking for: When the engine is at normal temperature and under load there should be a decent temperature drop (usually 15-20°F) as coolant flows through the heat exchanger. If that temperature difference is getting smaller, it tells me the tube stack is scaling up and losing efficiency. I catch this way before I have overheating problems, and it tells me when it's time to descale or service the heat exchanger. No guesswork—just point and shoot.

Compression Tester - I test compression annually and anytime performance drops with my Compression and Leakage TestKit.

It tells me the truth about what's happening inside those cylinders. Low compression on one cylinder? I know I've got ring or valve problems before I'm broken down.

Hand Tools That Earn Their Keep

I've got complete Allen key sets—both metric and imperial, in L-shapes and T-handles. Marine diesels use Allen bolts everywhere, and those T-handles give you the leverage to break loose stubborn fasteners.

Screwdrivers: I keep a full range of flathead and Phillips in different sizes and lengths. Stubby ones for tight spaces, long-reach for getting back behind accessories. Quality matters here—I don't buy cheap screwdrivers anymore because they just chew up screw heads and make you miserable.

I religiously use my 35pc Torque Wrench Set  with 1/4, 3/8, 1/2-Inch drives and special sockets and even allen key sockets for anything with a spec—injector hold-downs, head bolts, oil pan bolts. Diesel engines need proper torque, and I've seen too many people crack heads or strip threads by guessing.

Compounds and Consumables I Always Stock

RTV Gasket Maker - The high-temp stuff Permatex 82180 Ultra Black Maximum OilResistance RTV Silicone Gasket Maker,  has gotten me out of jams when I didn't have the right paper gasket or when I needed a seal that would actually hold. I keep several tubes aboard. I also use the special sealant for seals exposed to coolant Permatex 22071 Water Pump gasket maker

Anti-Seize Compound - I put Loctite 38650 Copper Anti-Seize Lubricant on every threaded fastener that might need to come off later. Exhaust components, injectors, glow plugs—anything exposed to heat or salt gets anti-seized. Trust me, you'll thank yourself when you need to remove that fastener years later and it actually comes out.

Hose Clamps - All-stainless Non-Slip Hose Clamp Assortment  is a 90 clamp kit of the non-perforated clamps that are really strong and reliable. No more clamping and then having the perforated notch snap.

O-Rings - A good assortment kit is cheap insurance. Get a quality nitrile O-Ringset with 826 Pcs Universal SAE and Metric O-Rings Kit. Everything from fuel system components to raw water pumps uses O-rings, and they're always deteriorating.

My Maintenance Tricks You Won't Find in the Manual

Oil Changes: My 12V Gear Pump Setup

Okay, this is one of my favorite modifications. About 30 years ago, I got tired of fighting with those terrible manual oil change pumps. You know the ones—you pump for 20 minutes, your arm gets tired, and you're still not done. Most marine engines have impossible-to-reach sumps, so you can't drain from the bottom.

So I installed a small 12-volt gear pump with fuel/oil-certified hose and a valve that connects right to my oil drain plug. Now when it's time for an oil change, I run the engine to warm the oil, shut it down, and connect my pump hose to the drain valve. I put the exit hose into an empty oil container, open the valve, flip the switch, and that pump sucks out all the hot oil in seconds. No effort. No mess. Just be careful to turn it off before the container overflows.

Then I change the filter with my socket wrench, and here's the really slick part: I put the pump's exit hose into a fresh container of oil, flip the switch the other direction, and it pumps the new oil directly into the engine. Takes maybe 30 seconds to refill. Check the dipstick, top it off if needed, and I'm done.

The whole oil change—draining, filter change, and refilling—takes me about 10 minutes. Compare that to wrestling with a manual pump for half an hour. Here’s one you can install to make your oil changes a pleasure instead of a chore. SEAFLO 12V Self Priming Impeller Gear Pump 

Coolant Changes: My Douche Bottle Trick

Changing coolant in a crowded engine room is a nightmare when you're trying to pour from gallon jugs into the heat exchanger or expansion tank. Coolant goes everywhere except where you want it.

So I use a douche container—the kind with a solid bucket and a long hose like this Reusable Enema Kit

Completely drain to old coolant from the coolant drain, use a radiator cleaner according to the instructions and then rinse it thoroughly with desalinated or rain water (never tap water with chlorine). When it comes out clean you are ready to fill the cooling system with new OAT coolant

Hang the douche container well above the engine and hold the hose up parallel with the container and pour in a little coolant so it fills the hose. Then snap the hose shut-off closed and attach the hose to the coolant drain (you may need to make an adaptor hose if it does not fit your coolant drain fitting). 

Pour the coolant into the douche container and open the snap hose shut-off. It fills the engine system from the bottom up, a reverse flow, and it is much faster and easier than trying to pour it into the heat exchanger and wait for it to dribble down against the entrapped air. And you can actually see what you're doing. Game changer.

Heat Exchanger Freshwater Flush: My Secret Weapon

This one's probably my best trick for long-term engine health. After every run, I flush my heat exchanger with fresh desalinated water from our tanks. Here's my setup: I've plumbed our SEAFLO 33 freshwater pump, a through a valve to push about 2 liters of fresh water through the raw water side of the cooling system after I shut down. The pump I use does it in 10 seconds and the heat exchanger has just one liber of salt water to flush out.

Why do this? Two reasons. First, it drastically reduces calcification and scale buildup in the heat exchanger tubes. That scale reduces cooling efficiency and eventually requires major descaling or replacement. Second, it reduces electrolysis corrosion by flushing out the seawater that would otherwise sit in there.

I've been running this setup for years, and when I service my heat exchanger, it's always remarkably clean inside. Meanwhile, I see other cruisers dealing with scaled-up heat exchangers after just a couple of seasons. Those 2 liters of fresh water saves me eating up zinc anodes and from time consuming rescaling and repair jobs.

How I Actually Maintain My Engine

My Daily Routine

Before I start the engine, I do a quick walk-around that takes maybe two minutes:

• Check oil level on the dipstick

• Check the coolant level in the heat exchanger

• Scan for any leaks—oil, fuel, or coolant puddles

• Check the drive belt tension by pressing on it – it should deflect about ½” with moderate pressure.

• Verify the seawater strainer is clean and full

• Quick look in the bilge

While running, I'm watching:

• Engine temperature with my gauges and occasionally with my infrared thermometer

• Oil pressure (should jump up right away at startup)

• Water flowing from the exhaust (steady stream means raw water pump is happy)

• Listening for anything that sounds different

• Looking at my instruments every few minutes

After shut down 10 second fresh water flush.

Regular Maintenance—What I Actually Do

Every 200 Hours: This is my bread and butter. I change the oil and filter religiously—it's the single most important thing you can do for a diesel. My 12V pump makes this so easy that I actually look forward to it (okay, maybe not look forward, but I don't dread it).

I check and clean the air filter, inspect all hoses and clamps for deterioration, verify belt condition and tension, and wipe down the engine room. A clean engine shows me problems immediately.

White Catch All super trick: When I installed the enegine I put a white fiberglass container under the entire engine, above the bilge. It has been a God-send. If the engine has a coolant, raw water or oil leak I can see it right away under the engine. I can also see if the belts are starting to come apart by the black dust that appears on the white fiberglass.

Every 400 Hours: Fuel filter changes are critical. I change both primary and secondary filters and keep multiple spare sets aboard. I also service my raw water pump impeller during this interval—even if it looks okay, because impeller failure usually happens at the worst time.

I check my heat exchanger zincs, and this is where my fresh water flush system really shows its value. When I open things up, everything looks great.

Annually: Change the Air Filter. Full cooling system service—change the zinc anode, flush with descaling solution if needed. Check the antifreeze coolant and replace if necessary.

Every 800 Hours: Check the valve clearance.

Fuel System: Where I Don't Compromise

Fuel issues cause 90% of diesel engine problems, so this is where I'm most careful.

I use a funnel with a filter to be sure no debris gets into the tanks.

I have an excellent water/sediment pre filter with a transparent bottom so I can immediately see if the fuel is contaminated. iFJFR12H (R12T Upgrade) Fuel Water Separator Marine Complete Combo 

The secondary filter is mounted on the engine itself.

I religiously use a fuel additive, specifically Liqui Moly Marine Super Diesel Additive that keeps the fuel tanks and fuel pump and injectors clean.

Cooling System: Two Systems to Monitor

My diesel has two cooling circuits: the raw water system that pumps seawater through a heat exchanger, and the closed freshwater system that actually cools the engine.

Raw Water Side: I replace my impeller every year and carry three spares (two impeller kits and a spare raw water pump). I use only quality impellers because cheap ones disintegrate and can clog up the whole cooling system.

My seawater strainer gets checked daily and cleaned whenever needed—in estuaries, that might be every other day. And my fresh water flush system after each run keeps everything much cleaner than it would otherwise be.

Freshwater Side: I use quality OAT marine antifreeze with corrosion inhibitors, and I test the concentration and pH annually. Old coolant becomes acidic and eats your engine from the inside—literally corroding cylinder liners, heads, and the heat exchanger from within.

Here's what I look for: Fresh coolant should have a pH between 8.5 and 10.5—slightly alkaline. When the pH drops below 8.0, that coolant is turning acidic and needs to be replaced immediately. By the time you're down to 7.0 (neutral), you've already got corrosion happening inside your engine.

The best way to measure this is with Phoenix Systems 8001-B Coolant Test Strips —they're specifically designed for antifreeze/coolant and give you a reading in seconds. I keep a pack in my spares kit and test annually, or anytime I'm concerned about the cooling system. Just dip the strip in the coolant when it’s cold, wait about 30 seconds, and compare the color to the chart on the package. Super simple and the Phoenix pack has 100 strips so you won’t ever run out unless you start testing coolant for your friend’s engines.

Don't use regular pH strips meant for pools or aquariums—they're not accurate in glycol-based coolant. Get the ones specifically labeled for engine coolant or antifreeze.

When I flush and replace coolant, I do it thoroughly—no shortcuts. I drain the system completely, run fresh water through it to flush out old coolant and any corrosion products, then refill from the bottom up using my special douche bag filler attached to the drain valve and I use a proper 50/50 mix of quality marine antifreeze and distilled water. That 50/50 mixture gives you the best corrosion protection and freeze protection down to about -34°F, plus it keeps the pH stable longer.

Spares I Actually Carry

Beyond tools, I stock:

• 2 primary and 2 secondary fuel filters.

• 3-4 oil filters

• 3 raw water pump impellers plus a spare raw water pump.

• Complete set of drive belts

• Injector sealing washers and O-rings

• Hose clamps in every conceivable size

• Spare thermostat

• High-temperature hose in common sizes

• Various lengths of fuel line

What I've Learned About Running Diesels

Diesel engines need to work. Don't baby them by running at idle or low RPM constantly. I load my engine properly—typically 50-80% of rated RPM—and periodically I run it hard to clear carbon deposits and keep injectors clean.

I keep a detailed log of everything: maintenance performed, parts replaced, oil consumption, running hours, any unusual observations. This history has helped me troubleshoot problems and predict when things need attention.

The Big Picture

I've seen cruisers spend a fortune on fancy electronics and chart plotters while running their engines into the ground from neglect. Don't be that person. Your diesel engine is one of the most reliable pieces of equipment on your boat—when you take care of it.

The tricks I've shared—the 12V oil pump, the fresh water flush, using the infrared thermometer and voltmeter for diagnostics—these aren't complicated or expensive. They just make life easier and keep your engine healthier longer.

Learn your engine. Download the service manual and actually read it. Join online forums for your engine model. Practice maintenance at the dock so you can do it in rough conditions when necessary.

Most importantly, stick to a schedule. It's easy to skip an oil change when you're exploring some beautiful place, but those hours add up fast. Set reminders based on engine hours, and follow through.

My diesel has taken me thousands of miles, and I trust it completely because I know it intimately and take care of it consistently. That peace of mind is priceless when you're crossing an ocean or entering a tricky harbor.

Keep your oil clean, your fuel cleaner, and your cooling system flowing. 

Do that, and your diesel will run forever.

What's the best diesel maintenance trick you've learned on the water? Drop a comment below—we all learn from each other out here.