Understanding Water Traps and Pre-filters in Diesel Engines

Most modern fuel injection equipment is made to very close tolerances. In practice, this means that for solid particulate contaminants in diesel not to damage injection pumps and/or fuel injectors, the fuel must either contain particles that are smaller than 1- 4 microns to pass through clearances, or, the fuel filter must be able to remove all particles that have the potential to damage and/or clog injection equipment.

Most OEM diesel fuel filters that are fitted to new diesel vehicles are effective in removing objects and/or contaminants in fuel that fall into the 2 – 5 micron range. However, most OEM diesel fuel filters fall into the 80% or so efficiency range, which means that most OEM fuel filters only remove 80% or so of solid contaminants in diesel fuel that are bigger than 2 - 5 microns in diameter.

So, unless an OEM fuel filter is 100% efficient, most OEM diesel fuel filters will allow some contaminants that are smaller than 2- 5 microns in diameter to pass into the injection pump to scratch, mar, or otherwise damage surfaces and seals, or to enter injectors to damage moving parts and/or clog minute injection orifices. This type of damage to injection equipment can be hugely expensive to repair, so clearly, prevention of the problem is always better than curing it, this is where pre-filters and water traps come in.

Adding a second or additional layer of fuel filtration is always a good idea, but there's lot of confusion about how to do it the right way. For instance, some sources advocate adding a second 2 – 5 micron after the factory filter, while others state that adding a secondary 30-micron fuel filter and water trap close to the fuel tank is the best way to go, so who is right?  There are several issues to consider here, so let us look at each in turn, starting with fuel pressure sensors.

All common rail diesel injection systems are fitted with pressure sensors whose function it is to monitor the condition of the fuel filter. As a fuel filter clogs up, it becomes progressively more difficult for fuel to pass through it; thus, by monitoring the pressure differential across a fuel filter, the ECU can calculate the extent to which a fuel filter is blocked. When the pressure differential reaches a predefined threshold, the ECU will conclude that the filter is clogged, and it will illuminate a warning light as a result.

In practice, adding a second filter after the primary factory filter could create an artificial “restriction” that the ECU might interpret as a clogged fuel filter. While adding a secondary fine filter before the primary factory filter might have the effect of doubling the effective surface area of the filter medium, the lift pump in the fuel tank will have to pump twice as hard to pass the fuel through two filters. Thus, on vehicles that have fuel lift pumps, the ECU might also interpret the added pumping load as a clogged fuel filter, and illuminate a warning light as a result.  

Australian diesel has a lot of water in it and although OEM fuel filters usually incorporate a water trap, adding a second water trap is a better idea than fitting a second mechanical filter. This is because water in diesel, whether dissolved or in its free state, arguably causes more damage to modern injection equipment than any other contaminant. In the first place, it causes rust and corrosion in metal components, and secondly, it offers a perfect breeding ground for a wide variety of microorganisms. If left untreated, some bacterial colonies can double in size every few days, which translates into blocked fuel lines, clogged filters, and very likely, clogged injection pumps and injectors as well.

Therefore, removing as much water as possible from the fuel before it reaches the primary filter and downstream injection equipment makes sense. Moreover, high quality water traps do not actually filter out and entrap water droplets; instead, the water collects on the filter medium until larger droplets form that then drop down into a collection bowl where the collected water can be easily drained off, which brings us to the issue of microorganisims in diesel.

Microorganisms do not actually live in diesel; they live in the interface between the diesel and water molecules. This makes it very difficult to remove colonies of bacterial or algae, and although there are many effective chemical controls available, adding this kind of remedy to diesel fuel is not recommended.
The problem is that while these agents do kill microorganisms, they do not remove the remains from the fuel. In practice, the dead organisms sink to the bottom of the tank, and when the next colony is killed off by a chemical agent, this colony also sinks to the bottom of the tank. Eventually, you could end up with a several-inch thick layer of sludge in the fuel tank that can only be removed by removing the tank and having it cleaned out. In this instance, prevention is better than a cure! Prevention involves fitting an additional high quality water trap to the fuel system to remove the water in the fuel that microorganisms depend on to live.

Based on the above, it starts to become clear that while adding secondary mechanical fuel filters may have some benefits, these are limited. In fact, only using quality fuel filters that are designed for Australian conditions, i.e. having efficiencies greater than 80%, and replacing that fuel filter with every oil change will almost always produce better mechanical fuel filtration than adding a secondary fuel filter with a lower efficiency will.  

Secondary or additional water traps on the other hand, greatly reduce the load on the primary filter, but bear in mind that to get the best results, the secondary water trap must be checked and drained at least several times per week, if not daily.