By Jim Kerr

Diesels are coming of age. Faster electronics, high gasoline prices and the better efficiency of diesel engines have all combined to make these engines more attractive. Diesels are now much quieter, accelerate faster, start easier and don’t require tune-ups like gasoline engines do, but diesels do require special preventative maintenance and most of it is related to the fuel system, especially on common rail fuel systems.

A common rail diesel fuel system’s fuel supply is similar to a gasoline port injection system, with fuel supplied to all injectors at the same time from a common source, typically called a fuel rail. The biggest difference is the pressures used by the diesel systems. Gasoline fuel pressure to the injectors may go as high as 70 PSI on some systems. Modern common rail diesel injectors may operate up to approximately 27,000 PSI. As a comparison, older diesel injectors operated with only a few hundred PSI fuel pressure. The high pressure of these common rail diesel systems does create some challenges.

One of the challenges for the engineers is to make the injectors survive. There are commercial machines that use a fine spray of water to cut metal with less pressure than used in common rail injectors. Clean diesel fuel has enough lubricating qualities to flow through the injector without eroding any material. The problems begin when dirt or water is mixed with the fuel.

Dirt, mixed with the fuel, cuts metal inside the injectors and causes them to leak internally. The dirt isn’t very large. Fuel filters on some diesel systems are fine enough that they will remove dirt particles 3 microns or larger. A micron is one millionth of a metre, so you can see that the filters are very good. But if dirty fuel is used, then the filters plug quickly and larger particles are sucked past the filter. Injectors can be damaged. The fix? Replace fuel filters regularly and use fuel only from clean sources.

Even clean fuel will eventually plug these fine filters. Diesel fuels contain small particles called asphaltines that are trapped in the filter. Much of the injector problems in my part of the country is caused by drivers filling with fuel from dirty storage tanks or industrial diesel fuel used on work sites. Service stations have filters on their pumps to ensure drivers get clean fuel. Many bulk tanks don’t.

Water in the fuel can also damage the injectors internally by cutting metal passages. Normally, the water is separated from the fuel by a water separator in the fuel filter housing. Regular draining of the water separator is part of routine maintenance. Diesel engines such as GM’s Duramax or the Dodge Cummins use an engine-mounted pressure pump that sucks the fuel from the tank and through the fuel filter. As the filter starts to plug, water in the separator can be pulled through the filter and damages the injectors. Again, changing filters is an important part of maintenance.

Some diesel drivers may be allowing water through their fuel filters without realizing it. There are many fuel conditioners on the market designed to prevent bacteria growth in diesel fuel. Fuel conditioners may also claim to provide additional lubrication for moving injection parts. Unfortunately, many conditioners have an alcohol base. The alcohol mixes with the water in the fuel tank and carries it through the filter and into the injector, where it can damage the injector. If you wish to use a fuel conditioner on a modern diesel engine that operates with high fuel pressures, make sure it is not alcohol based.

Older diesel systems didn’t have these problems because the fuel pressures were so much lower. Water or microscopic dirt particles were not propelled with enough force to damage the injectors. The high pressures on today’s systems has helped improve starting, noise, emissions and economy, but also requires quality fuels and careful attention to maintenance.

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