(Diesel bleeding, leak finding, maintenance and more)
There is a lot to know when it comes to understanding your marine diesel fuel system, from fuel bleeding and servicing to tracing fuel leaks and treating bad fuel, it can be a lot to take in especially if you are a first-time diesel owner.
In this article, I will be talking about all things diesel fuel, to give you a complete understanding of all the aspects that contribute to a diesel fuel system.
- What is compression ignition?
- Why bleeding up a diesel is so important
- Common rail vs non-common rail
- Forced induction
- Types of bleeding systems
- How to bleed up a diesel
- Bleeding a hard to bleed system
- Leak finding
- Checking fuel quality
- Primary and secondary filters and why they are important
- Where to fill up?
- Talking tanks
- Fuel system maintenance
- What is fuel polishing?
When you think of a diesel engine what are the first things that come to mind? Is it industrial engines in diggers, tractors, stationary engines like generators? Or maybe buses and trucks?
And what benefits or differences come to mind when thinking of these engines, maybe increased torque, robustness or long service life?
When it comes to diesel engines application is everything, most modern diesel engines you see in boats, big and small these days started out being used in commercial applications like busses and tractors or machinery then converted to marine applications by adding seawater cooling and remote mounted oil and fuel filter assemblies.
There are a lot of benefits to using a diesel engine for marine like –
- Reduced risk of fire or explosion compared to petrol
- Robust engine construction
- large amount of torque
- Long service life
- Fuel efficient
What is compression ignition?
Compression ignition is the process of diesel fuel being compressed to the point that it spontaneously combusts; this is partly due to the compression and partly the heat created by the high compression that creates the ignition of the fuel.
Some diesel engines have other means to help facilitate combustion like glow plugs and intake heating.
A diesel engine can ignite and run without these but these make the process much more efficient for some models.
Why bleeding up a diesel is so
Important!
A diesel fuel system relies heavily on fuel pressure to run so simply put, any air that enters the system can and will cause an engine to stop.
Diesel fuel systems are what is known as circulatory systems meaning that they have return lines back to the tank.
The diesel fuel will go from tank to primary filter to secondary filter then enter the injection pump where it is sent to the injectors this is where some fuel goes to combustion and the rest gets circulated back to the tank via a return line.
Also because a diesel engine requires a feed and a return line any blockage in the return system will also cause issues and engine stalling.
Common rail vs non-common rail
Basically, the difference between a common rail engine and a direct-injected engine is the common rail engine has a high-pressure fuel rail providing fuel to all the injectors.
While the direct-injected counterpart has a fuel line per injector, going straight from the injection pump to the injectors.
Common rail diesel engines have been around for a while now and the benefits compared to the older direct-injected ones is considerable.
By providing fuel to the injectors at super-high pressures this increases fuel efficiency by facilitating a more controlled, more efficient burn within the combustion chamber.
All this comes down to better fuel delivery, control and fuel economy.
Forced induction
You’re probably wondering why I’m talking about forced induction when this article is about diesel fuel systems, well my reasoning is based around the fact that air and fuel are directly related to each other when talking about combustion, you can’t have one without the other.
Forced induction allows a diesel engine to suck in more air into its cylinders than it could on its own, enabling more fuel to be added creating more power.
Most diesel engines you will see in boats will have some form of forced induction like turbos, superchargers and even combinations of both.
By adding forced induction to these engines you have a much more fuel-efficient and more powerful engine.
Types of bleeding systems
Bleed screws
Most of your older diesel engines can have bleed screws and this is simply a screw that gets
lever style diesel fuel primer
loosened on top of your secondary fuel filter then as you turn over the engine having the screw undone will allow fuel to fill the filter and overflow out the bleed screw indicating the filter is full and the engine is ready for starting.
Manual pumps
There are a few different manual pumps around for priming, like button pumps and plunger pumps, these will still need bleed screws or nipples to be undone to bleed fuel through.
Lever pumps
This style pump is something you will see on older diesel engines and is simply a lever that you pump until the filter is full, these will still need bleed screws or nipples to be undone to bleed fuel through.
Auto pumps
These are common on generators and consist of an electric fuel pump feeding the system, these make priming very easy and don’t require bleeding via screws or nipples.
How to bleed up a diesel
- Make sure your fuel taps are on
- Find your bleed screw or nipple and undo
- Pump your primer button or lever (use a rag or old hose and container to catch fuel)
- Once you have a steady stream (no bubbles) close the bleed screw or nipple
- Pump primer button or lever some more (primer should become firm)
- Check for leaks
- Start engine
- If engine stalls start again from step one
Bleeding a hard to bleed system
Unfortunately having a hard to bleed system is very common and a lot of factors can contribute to this like tank design, fuel line runs, types of filters used and their positions or even bad or failing engine components like low-pressure fuel pumps in an auto system.
If you do find yourself with a hard to bleed system, your first step is identifying and understanding what kind of diesel engine you are looking at and the bleeding system it uses.
Let’s just say you have a common rail engine with a button primer and bleed nipple on the fuel secondary filter, you have opened the nipple and have been pumping for a long time now and no fuel is coming out.
The most important thing to remember when working with common rail systems is never to loosen injectors or rail lines because this is where the extremely high fuel pressures live and this high-pressure diesel can cause you serious injury or death. Extreme I know, just don’t touch anything past the injection pump on these engines, if your unsure contact a qualified mechanic.
My first move would be to attach a manual suction pump to the bleed nipple if you are lucky enough to have one and try to suck the fuel through, you can try assisting the sucker by pumping the priming button as well.
If this fails then you will have to start at the beginning of the fuel system and work your way forward.
I would take the outline off the fuel primary filter and suck fuel from there first this will fill the primary filter, then reattach the outline and move to the fuel secondary filter in this case it has a nipple if you know you have fuel at the primary, you should be able to now suck fuel through to the secondary once this is done then the engine should start.
If you were working on an older direct-injected engine you can work your way from primary filters all the way up to the secondary filter then you can start loosening off injector lines and cranking the engine to get the fuel the rest of the way to the injectors.
Leak finding
Any leak in a diesel fuel system is a fire risk and can stop an engine from running, so being proactive and actively keeping an eye out for any fuel leaks when you are doing your general pre boating checks is very important.
When hunting down any leaks I always start at the bottom then work my way up.
Look in bilge under engine
The bottom of the bilge is usually the lowest point in the engine bay and if you have any significant leaks this is the spot to find them.
Look around engine
Run your eye around your engine starting from the bottom then working your way up, you are more likely to spot a leak lower down on the engine where the leak has a chance to pool in and around grooves, mounts brackets etc… making it easier to spot.
Look around primary filters and tank fittings
Because we are checking for fuel leaks it makes sense to check where the fuel comes from and returns to, have a good look around fuel tank fittings, valves and primary filters
Identifying what’s leaking
Before you start honing in on where the leak is coming from you need to confirm that you are looking at a fuel leak.
So the best way to do this is by using sight, touch and smell.
Looking at the fluid, It should have a translucent yellow / golden hue, using your touch rub some of the fluid between your fingers it should feel oily and lastly have a smell, does it smell like diesel?
Leak tracing
OK now you should be somewhat confident that you have identified you have a fuel leak and how bad it is by the fluid volume in the bilge, you also have felt and smelt the fluid confirming your suspicions.
Now it’s time to follow the trail left behind by the leaking fuel, feel around fuel filter bowls, around fuel lines etc.. and look closely around your engine for any run marks or wet patches, slowly working your way up until you find something suspicious.
Common leak points
These common leak points are from my experience and should be considered a starting point only, what you find may be totally different.
What I’ve found in the past is fuel filter drain bungs leaking, fuel lines cut or pierced by broken brackets or hose clamps, failed fuel line swaging, loose solid fuel line fittings, failed copper washers under banjo fittings, cracked fuel delivery and return rails.
Checking fuel quality
Checking your fuel quality is done by either inspecting any fuel filter clear bowls or by draining the contents out of filters, by using drain bungs on the filter or removing the filter and pouring it out into a clean container.
Fuel quality is determined by a few factors.
water and debris in diesel fuel
Colour
Fuel should look a translucent yellow/gold colour.
All diesel fuel will look slightly different, it can take on a greenish hue, look dark or almost fluorescent and still be perfectly fine.
Foreign bodies
By this I mean is there anything floating around in the fuel? This can be bacteria, fungi, sludge or any other debris.
One of which is diesel bug, this is a common term for bacteria and fungi that likes to grow inside diesel fuel tanks.
Other foreign bodies include sludge and other larger debris, these can be anything from the linings of fuel lines returning to the tank to manufacturing debris in the tank.
Fuel primary and secondary filters do a very good job of catching a large amount of this stuff.
Water content
Any water in your fuel is bad news, water is heavier than fuel so it will sink to the bottom of your tank and also the bottom of your fuel filter bowls where you can drain the water away.
It is up to you to determine if the amount of water found in your fuel filters is enough to warrant a full tank flush or not.
A good way to work this out is how often do your filters need draining and how much water are you getting out of them?
If you are draining only a small amount of water out every now and then, then I would say a tank flush is not necessary but if you are draining filters weekly with large quantities of water coming through then yes go get your tank drained and cleaned immediately.
Primary and secondary filters and why they are important
diesel fuel primary filters
You will find most diesel fuel systems will run primary and secondary filters and this is
because diesel engines are dependent on fuel pressure, quality and delivery.
As mentioned earlier a diesel engine will not run without a constant steady stream of diesel running through all fuel lines, delivery or return.
So if you have a problem with fuel delivery because of the fuel quality you won’t get the required pressure for your engine to run efficiently.
So basically your primary and secondary fuel filters catch all the stuff you don’t want running through your engines, they are your first line of defence against bad fuel.
Where to fill up?
Everyone has heard the stories at some point in their boating life of how bad marina fuel is and to avoid marinas at all costs.
But what if I was to tell you that is not the whole truth? Yes, marina fuels can absolutely be bad especially in remote areas where they get minimal traffic and low turnover of fuel.
But have you considered that your own fuel tank can be your own worst enemy?
I will be talking more in-depth about this in the next section of this article, but for now, where do you go if you want to fill your tank.
This all depends on vessel size and where the boat lives and/or operates.
If the boat lives near a popular marina with a large turnover of boats coming and going, chances are the fuel will be reasonably ok.
If you have a trailer boat, well you are spoilt for choice and can make your own decision on where to purchase your fuel.
It’s only really remote areas where fuel turnover is very low you need to avoid if possible.
Talking tanks
Have you considered that your own fuel tank can be your own worst enemy?
Think about this for a moment, what do you think happens down there?
Well I can tell you it’s a humid environment down there and heat and moisture creates bacteria, fungus and algae, add to this any swarf or other manufacturing leftovers floating around inside your tank and you have a recipe for disaster.
That’s where your primary and secondary fuel filters come in, these filters do a great job at catching water and debris.
But filtration is only part of the story when it comes to tanks. There are many other aspects that need to be considered when trying to improve fuel health.
If you’re finding that you are constantly getting excessive amounts of water in your fuel you may have to consider you have a possible breach in your tank system somewhere.
The most common spots to start your search are from my experience, failed fuel filler caps o-rings and badly positioned tank breathers and fillers, such as flush deck mounted breathers and fillers where water can sit over these openings allowing water ingress.
Another common tank problem is corrosion causing large amounts of rusty sludge to build up in the tank.
Apart from catching water and debris with filters another way you can improve your fuel health is by doing a full tank clean, this entails draining the tank and having the internals of the tank cleaned out, this kind of procedure should be considered either as part of a periodic service schedule for older boats or preventative maintenance for boats after heavy rain if large amounts of water are found in the fuel system.
Fuel system maintenance
A lot of the stuff I’m going to cover here, about maintenance I have already covered in previous sections but I will list here again because I can’t stress enough how important having a properly working fuel system is with no leaks and good fuel is for a diesel engine or any engine for that matter.
- Checking for leaks
- Checking fuel quality
- Using fuel conditioners and additives
- Regular fuel system servicing
Checking for leaks
As mentioned earlier in this article, have a good look around your bilge under engines under filters and around tanks and fittings if you find anything leaking get it rectified as soon as possible.
Checking fuel quality
This is simply a visual check of fuel bowls or draining some fuel out of a filter to check the condition of the fuel.
Using fuel conditioners and additives
These are products that can prolong the life of your fuel by helping to break down bacteria, fungi and also help remove water from your fuel.
These are great for adding to a tank when the boat is going to get stored for long periods of time, some brands can still be effective for up to 2 years after treatment.
As mentioned earlier fuel can easily get all sorts of contaminants like water, diesel bug and sludge, so I would suggest using these fuel stabilizers regularly to keep these contaminants at bay.
Regular fuel system servicing
Annual servicing is a must for any boat and your fuel system should be no exception. By keeping up with regular fuel system servicing, you are ensuring your engines run as they should.
What is fuel polishing?
Lastly, you may have heard the term fuel polishing but are unsure what that actually is, well fuel polishing is the process of fuel being circulated through a specialized filtration system that will remove microbial contaminants, water, sludge and any other debris that make its way into your fuel, then cycles the clean fuel back into your tank.
These systems are designed to help increase engine filter longevity and avoid the necessity of draining tanks and cleaning them out, potentially wasting anywhere from hundreds to thousands of litres of fuel in doing so.
Conclusion
As you can see there is a lot to take in when it comes to understanding your marine diesel fuel system.
From understanding how diesel combustion works to how to bleed a diesel system to looking after fuel filtration and conditioning to inspecting for possible leaks and other issues.
As mentioned at the start of this article marine diesel engines have evolved from their humble beginnings as industrial and commercial workhorses to being used in boats big and small.
This transition should come as no surprise when you consider the demanding nature of boating and the marine environment, diesel engines do make sense, they are big heavy units built with longevity and robustness in mind, perfect for any sea voyage you should choose to take them on.
Keeping your vessel in top shape doesn’t have to be difficult. Explore The Everyday Captain for more, actionable advice, step-by-step guides, and helpful resources designed to keep you safe, save you money, and make boating more enjoyable. Whether you’re tackling a repair, improving maintenance, or just learning the basics, The Everyday Captain has you covered.
