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How Does a Fuel Water Separator Work?

How Does a Fuel Water Separator Work?

A fuel water separator tucked away in the side compartment of a machine

How Does a Fuel Water Separator Work?

Reading time: 5 min

Diesel fuel can hold two types of water: water in solution or free water. Water in solution is found in all diesel fuels. It is comprised of low levels of water that can be dissolved in the fuel. Free water can be dangerous in fuel because it is water that is not a dissolved component. Free water can cause corrosion, micro-bacterial growth and frozen filters.

Fuel that is contaminated with free water will damage your engine, reduce your machine's lifespan and can result in downtime if these issues are left untreated, which can cost thousands of dollars in the long run. You should utilize your fuel water separators to avoid dealing with free water damaging your machine.


Fuel water separators provide an extra layer of protection for the engine against water and other contaminants. The separators have a small filtering device that removes water and other debris from the fuel before it reaches the engine. Not all heavy equipment fuel filters feature fuel water separators, and this varies by manufacturer. 

A fuel water separator differs from a standard fuel filter with its water filtration. A standard fuel filter will remove waste particles, such as dust or debris, from your system. A fuel water separator removes free water and small debris from your fuel system. The fuel water separator is installed before your machine's OEM fuel filter and acts as an extra defense for your engine. Fuel water separators can range from $14 to $419, depending on the make and model of your machine. 

All diesel fuel contains some water in its solution. This water is normal as long as it does not exceed saturation limits. Saturation limits can vary depending on temperature and your machine's diesel/biodiesel ratio. This range could vary from 50 ppm to 1800 ppm. As long as the water in your fuel stays under its saturation limit, it will stay dissolved instead of becoming free water. 

How does a fuel water separator work?

Free water that enters the fuel water separator falls to the bottom of the separator due to its density. Then, any fuel filtering through the device is pushed to the second phase of the filter. During the second phase, the fuel rises through the separator unit, removing water still in the fuel through its repellent surface. Then, any remaining water particles will become large beads and fall into the water trap. The fuel will then move onto further refinement as it passes through to the fuel filter.


There are several ways fuel can become contaminated: during delivery, leakage into your tank, condensation and operator error. 


If your machine is being delivered, there is potential for water to enter the fuel. This could happen through improper storage or weather conditions during the delivery. 


Leakage from rain or power washing can contaminate your machine and fuel. 


Condensation can enter your fuel tank from temperatures rising and falling. The climate of your job site also impacts your machine; humid air can cause condensation. 

Operator Error

Proper machine storage is important to prevent water from entering your fuel. Water can enter your fuel through unprotected vents. Fill ports and seals if they aren’t properly secured. 


Recognizing Fuel in your Water

Fuel that contains free water will be hazy. Water in your fuel can cause black or white exhaust smoke. Another sign of water in your fuel is the engine is cutting out, particularly during your machine's acceleration. You may also hear a knocking or sputtering sound coming from the engine. 

Free water accumulating in fuel has several negative outcomes for your machine. 


Diesel fuel can start to gel at around 15°F, and water freezes at 32°F. If water is in your fuel filter and temperatures reach the water's freezing point, your filters will end up frozen. Frozen filters will cause your fuel to stop flowing, and the engine will stop working. This will then result in downtime for your machine. 

Micro-bacteria Growth

Bacteria and microbe growth will occur over time if free water is left to cultivate in your fuel. Microbes and bacteria breed in water, feeding on the hydrocarbons in your fuel. Microbial breeding also creates the formation of acids which can cause your engine to rust and corrode. The microbes and bacteria will result in slime, which can clog your filters. Clogged filters will cause your fuel to stop flowing, and the engine will stop working. This will then result in downtime for your machine. 


Corrosion is the process of metal wearing away. This process occurs when water and metal come into contact. An example of corrosion is rusted iron, but corrosion can occur with many common materials. Corrosion can spread throughout your machine and can lead to the gradual deterioration of engine components

Water will spread throughout your machine if it gets the chance, and if you don't frequently drain your fuel water separator, the outcomes mentioned above can become detrimental. Your hoses and gaskets will start to degrade, which will then lead to their early failure. Your cylinders and pistons will also begin to warp, further damaging the engine. Corrosion will then spread throughout your machine. All of this will then lead to engine and machine failure. 

If your machine fails, this will then lead to downtime. Your machine going down can cost thousands of dollars as you wait for it to be repaired and, depending on the damage water has caused, may require you to get a new machine. All of this can be avoided with the proper maintenance of your fuel water filter. 


Your environment greatly impacts the lifespan of your fuel water separator and all your filters. While your machine’s manual provides recommended service intervals, you may need to replace your filters more frequently depending on your job site environment, weather conditions and climate. 

Replacing your fuel water separator

Most fuel water separators need to be changed at 1000 hours. The replacement frequency also depends on your job site's environment and how often you use your machine; if it sits for a long time, you may need to replace it sooner. You will replace your fuel water separator similar to replacing oil and fuel filters

  1. Turn your machine off. Most manufacturers recommend disconnecting your machine's battery before beginning any maintenance.
  2. Locate your fuel water separator.
  3. Remove your previous fuel water separator. You can do this by using a filter wrench until loose and unscrewing the separator with your hand the rest of the way.
  4. Drain the used fuel into a pan, then properly dispose of the fuel and water. 
  5. Take your new fuel water separator and a small amount of fuel and rub it along the sealant to lubricate it.
  6. Prime the separator before screwing it back into place. 
  7. Screw your separator until hand-tight, then secure it with a fuel wrench.
  8. Reconnect your battery and turn your machine back on. 
Draining your fuel water separator 

How often you drain your fuel water separator depends on your job site's environment and machine use. Some machines have warning lights on the dashboard that alert you if water is in your fuel. 

  1. Turn your machine off. Most manufacturers recommend disconnecting your machine's battery before beginning any maintenance.
  2. Locate your fuel/solenoid shut-off valve. Be sure this is off so there is no chance any clean fuel will escape. 
  3. Locate your fuel water separator. 
  4. Unscrew the drainage valve and drain the water straight into a pan. 
  5. Screw the valve back on.

Final thoughts

Fuel water separators are vital in protecting your machine against the harmful effects of water infiltrating your fuel. Without proper maintenance, your machines may become damaged beyond repair, increasing downtime effects. 

Check out the EquipmentShare Shop’s selection of fuel water separators or other filter elements to find the right one for your machine. Still can’t find the part you’re looking for? Reach out to one of our parts experts to get personalized assistance.

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