A Pre-filter should be there to do exactly what its name says. Filter the fuel ‘pre’ the main filter. Why? If the fuel is in a poor state (contaminated with gunk/algae and other matter) it will have some of that removed so that the MAIN filter is left to worry about filtering the remaining stuff.
Will the Primary (PRE) Fuel Filter effect restrict the fuel flow?
No, the only time this might happen is if and when the filter becomes blocked. Most modern Diesel filter systems come with flow restriction indicators that would alert you if the fuel flow was restricted. On average we would expect the Pre-filter to be changed every 30,000-40,000km.
Water in Fuel and Algae
Diesel and water separate very easily. If you have ever heard of ‘algae’ growing in Diesel fuel its actually growing in the water. When you have algae you have a large problem of a growing mass of blobs that can cover sensors and make them inoperable. Installing a water sensor alone will not guarantee protection from water contamination. Pre-filtering the fuel removes all these large messy bits and pieces so that ‘treated/filtered’ fuel is allowed to flow on to the main filter where its sensors and alarms can operate more successfully.
These are installed BEFORE the main filter on ALL applications