Change Filters on a Schedule
or Fit a Vacuum Gauge

Easy to be fooled into thinking that because a diesel filter bowl looks clean, it doesn't need to be replaced. That can be a dangerous and expensive mistake, potentially causing engine failure at a critical time (e.g. entering a harbour in adverse conditions when crud in the fuel tank may be stirred up) or damaging the injector pump or injectors:

• best practice is to change filter elements on a regular schedule, or fit a vacuum gauge to detect when the fuel pump is sucking harder
• do not trust to human eyesight – we cannot see bacteria with the naked eye
• use 10 and 2 micron elements in primary and secondary fuel filters respectively
• 1000 microns in one millimetre (o.001 mm) or 25,4000 microns in one inch

Fuel filters are designed to ensure that only scrupulously clean fuel reaches the injection pump and especially that any HUM (Hydrocarbon Utilizing Microorganisms) are removed.

HUM can thrive in the fuel/water interface. In ideal conditions, a single cell weighing 1/1,000,000 of a gram can grow to 10 kilograms within 12 hours.

Good diesel fuel filters are also designed to remove water in the fuel flowing to the injection pump, as well as sand, grit and dirt.