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Water in Diesel ...The good, the Bad and the Ugly

Here we look at how you get water, it's consequences and how (and how not) to remove it..

Well, unfortunately there is no good.... only bad and ugly.

So how does water get in your fuel tank? Water comes from the supplier, insecure tank fittings and the atmosphere in the form of condensation and absorption. Let's look at these four ingress points.

The Supplier

Fuel changes hands, and tanks, as much as 7 times before you receive it. After refining it is stored, transported, and transferred each time the fuel is traded. Be that in tanks, on ships, barges, or road tankers, to finally being received into your tank. At each point the fuel picks up contaminates and water. The debris is easily be filtered out and the water can be removed by via the drain cock, only it almost never is, assuming it has a drain cock?

Insecure Tanks Fittings

Although self-explanatory we hear a lot of stories of water in tanks that, after investigation, have come from loose filler caps, perished or missing gaskets, broken tank breather pipes, sounding pipes etc.


As the tank heats during the day and cools at night the fuel expands and contracts expelling air and drawing it back into the tank, this moisture laden air readily condenses on the tank walls and falls to the bottom as free water. It is insidious, a very small amount every single day, worse on some days better on others but non the less it very slowly builds up on the bottom of the tank.


Modern diesel is extremely hygroscopic due to the Fatty Acid Methyl Ester (FAME) introduced in response to the renewable fuels directive. This Biodiesel is 30 times more hygroscopic than standard diesel meaning it readily absorbs water from the atmosphere.

For Condensation and Absorption stopping the moisture in the first place can be helped with a desiccant breather

What is so harmful about the water?

Water in the bottom of a diesel tank is by far the single greatest contaminant in a fuel tank and is the precursor to 85% of diesel problems.

It forms a habitat for Diesel Bug.

Without water diesel bug cannot thrive. No water= No diesel bug. This diesel bug under ideal conditions can evolve into anaerobic bacteria and that can eat through the 1 inch steel plate on a ship, I've seen it in the ship yard!

It reduces the lubricity in the fuel.

Modern Diesel already lacks lubricity as a result of removing the sulphur. Saturated water in diesel will seriously effect the lubricating qualities of the fuel. This is one of the main causes for fuel injector failure. See video below

It accelerates the degradation of the fuel.

Free water on the bottom of the tank creates a mild acidic layer at the interface between the water and the fuel above it. This causes accelerated degradation of the diesel and promotes reaction with FAME producing salts of carboxylate acid, also known as soap.

it increases risk of resins.

These are super hard very small particles that can pass through filters wrecking fuel pumps.

It can rust the tanks and fuel systems

Imagine saturated water in diesel lying in contact with fuel injectors after the engine is stopped.

It can blow the tips of fuel injectors from superheated steam

OK, this is a bit extreme and rare, but it has happened!

It can lead to the formation of Sticky Diesel

See our blog Sticky Diesel

Take a peak of life inside your fuel tank with this video (complete with baffles)

Following video shows what actually happens in your fuel tank if/when you have free water (ie water lying on the bottom of the tank) picked up by the fuel suction.

This watery mix readily combines as it is agitated though the fuel pump returning as saturated diesel.

Incidentally your inline fuel filter does not always stop it. This is the reason we have not used them on the Diesel Dipper purifier attachment.

......So what can be done?

.................Remove the water !!

Be wary of fuel additives that are specifically designed to absorb water into the body of the fuel, whereas this was fine up to the late 1990’s it is probably the worst thing you can do in modern diesel. 

Generally enzyme based, these additives are designed to absorb the free water off the bottom of the tank. No water = no diesel bug, great you would think!

Not necessarily because meanwhile, our modern diesel, being very hygroscopic from the FAME is absorbing moisture from the air at the top of the tank.

The result is the diesel, is under attack from above and below, can become highly saturated.

Our own Diesel Fuel Complete (DFC) contains a demulsifier to promote separation of water from the diesel where it can lie on the bottom and be removed mechanically by draining etc.

Use the drain plug on the tank at least once a week. Or let the Diesel Dipper do it for you, whilst you're relaxing at sea and the contents of the bottom of the tank get picked up, as they are formed and deposited in the Diesel Dipper BEFORE they can get sucked into your engine fuel suction.

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