In an effort to meet federal emission standards that have become more and more stringent, ultra-low sulfur diesel fuel was introduced in 2006. While ultra-low sulfur diesel improves air quality by reducing emissions, it also causes fuel to become hygroscopic. This means fuel has the ability to attract and hold water molecules from the surrounding environment and its ability to do this increases as its temperature increases. Adding to this problem is the functionality of engines manufactured after 2010, which return large volumes
of high temperature fuel back into the fuel tank.Regardless of the quality of fuel delivered to your fuel tank, or the amount of additive in that fuel, this combination of hygroscopic fuel and high temperature fuel, ensures water will enter vehicle fuel tanks. Additionally, operating conditions such as where a unit runs and parks, idle times, total up times and ambient temperature fluctuations can amplify this increase in water content.

Options for Water Removal

The three methods to manage the water content in your fuel include emulsification, demulsification and absorption.

Emulsification is typically accomplished with additives that allow a greater concentration of water to remain suspended in fuel. Levels as high as 3600 parts per million (ppm) are possible, though the ASTM specification for emulsified water is under 500 ppm and some engine manufacturers have specification numbers as low as 200 ppm. These water particles are suspended in the fuel, allowing them to pass through the injectors and exit via the exhaust.

Demulsification, also called stripping, causes water to drop out of the fuel. It’s achieved with water separating filters that strip water from fuel and enable it to be drained. This type of water removal is highly effective, however it may require additional preventative maintenance than what is typical.

Absorption of water takes place when the media within the filter absorbs the water passing through it. This method is effective for absorbing free water, but it is not particularly effective at removing emulsified water. Additionally, filters can become waterlogged, leading to a blockage in fuel flow, an increase in water content and cold flow problems.
Recognizing the need for emulsified water management most OEM’s have focused on demulsification offering primary water separating filters on newer engines,
however they are not standard on all engines.

Take Action

Select a preventative maintenance method that works for your operation, include a fuel monitoring schedule if needed, and stick to it. When proper protocols and measures are in place, water content can be controlled and equipment will run effectively and cost efficiently.

When using an additive, be aware that the water content of by-pass fuel can continue to increase unchecked. When water content is high enough and the temperature low enough, emulsified water will drop out of the fuel as free water. Since there is no control on emulsified water content other than free water fall out, field or lab testing should be done to ensure levels do not climb above engine manufacturer’s specifications. (This kind of testing can also be used to confirm the quality of fuel upon delivery.)

When using a water separating filter, only use filters that meet engine specifications and ensure the correct filter media is being used. Water separating filters must have a synthetic layer designed to strip water from fuel. Follow manufacturers’ preventive maintenance schedules and be ready to drain more frequently during cold weather seasons. Install heaters as needed to improve operability. If there is no water at the bottom primary water separating filter, it is not operating properly and needs to be serviced.

For more information about how to keep water out of your fuel, use the links below or call Osco Customer Service at
(877) 672-6645 or email us at

Water risks:

Water management techniques:

Primary water stripping filters:

Emulsification and demulsification of water in fuel:

Water test kits: