2015 Mercedes-Benz ML250 Fuel Filter Water Separator Replacement Cost

Diesel and marine engines use specialized fuel filters that not only catch debris from the fuel tank, but they trap water that has been mixed into the fuel - or emulsified. Since diesel fuel is much less volatile than gasoline, small amounts of water in diesel fuel will not be burned off inside the engine's combustion chamber, but it will actually cause the diesel to run poorly. This filter and separator unit solves that issue. Also, water in the diesel engine can cause fracturing of metallic components, excessive wear of tightly clearanced parts, and corrosion, for example.

When fuel is pumped out of the fuel tank, it moves through the main supply line toward the diesel engine. Before the fuel can reach the high pressure fuel pump it is intercepted by the fuel filter and water separator. The water flows into the filter with the fuel, but the heavy water is retained in the bottom of the filter housing, while the lighter fuel will be passed on through the main supply line. The water in the filter must be drained periodically for some types of filter housings, but many require the replacement of the fuel and water separator each time the diesel engine is serviced.

When the fuel the engine is getting is contaminated with water, the diesel engine will begin to perform sluggishly, it may misfire, or it may not start and run. This is because too much water built up inside the fuel and water separator will cause the water to run into the engine with the fuel. The high pressure fuel pump will pressurize the fuel and water, and the injectors will spray it into the combustion chambers. Once in the combustion chambers, the diesel fuel will ignite, but the water will not. The water may evaporate into steam, but otherwise it is a hinderance to the normal operation of the engine.

Since water in the fuel over a period of time can lead to mechanical damage to the engine, it should be taken seriously and corrected right away. Also, driveability and normal operation of the vehicle or equipment may lack to the point of obsolescence. For these reasons, the fuel and water separator should be replaced on time, every time, and if the vehicle provides a warning, it should be serviced immediately.

Depending on the vehicle, manufacturer, and year of manufacture, the timeline will vary, but not to a great extent. The fuel filter and water separator should be replaced and drained when recommended by the owner's manual, and any time the water separator becomes full of water. This may be once every time the oil is changed, every six months, or even monthly, depending on use and grade of fuel. A bad tank of fuel can cause the fuel and water separator to clog and fill with water in just a few miles.

When the engine control module (ECM) has detected the presence of water in the fuel, the water separator will be emptied if possible and replaced if not. Otherwise, no diagnosis is necessary when determining when to replace the fuel filter and water separator.

Replacement of the fuel filter and water separator combination canister is as simple as changing an oil filter. It is done by unscrewing the filter from the filter housing using a filter wrench, cleaning the sealing surface, and installing the new filter. Remember, this filter is in a low pressure location of the fuel system, so diesel will not overpower the seal for the filter. If the water separator can be drained, there is typically a small drain to release the water from the filter housing.

We recommend sticking exactly to the manufacturer specification for maintaining the fuel system. Diesel engines are extremely dependant on high grade diesel fuels, and contamination will cause premature failure at some rate. Providing additional maintenance is at the discretion of the owner, and will cause no harm to the vehicle.

Though diesel fuel has a low flash point, it is still flammable. Water drained from the water separator should be treated as and disposed of as fuel. Though it is water based, it is still a volatile liquid. Also, avoid dumping this water into drains or garbage as proper disposal methods are always available.

This is a maintenance item that can be handled by anyone with a filter wrench and a bottle. In some cases one or two basic tools may be required. It is important that the vehicle be parked on a flat surface, especially if the water separator drain plug is located underneath the vehicle, and the wheels should be blocked with chocks and the parking brake. It is extremely important to contain the water from the fuel and water separator, thus preventing it from being poured into the soil or evaporating into the atmosphere.