Risk posed by diesel ‘bugs’ in standby generators


Operators may find that the fuel in diesel standby generators that have been standing unused for long periods of time is no longer fit for purpose. Gavin Donoghue, fluid analysis manager at Finning, outlines the risks that this can present to mission critical systems, as well as discussing practical steps that can be taken to help solve this issue.

One way to mitigate any potential risk of a shortage in electricity supply is to ensure a reliable, backup power supply is in place, typically through the use of standby diesel generator sets. In this day and age, it is extremely rare for a mission critical application to not have a standby generator installed.

By nature, standby generators will operate occasionally, which means that the fuel required to run those sets could have been stored for many weeks, months or even years.

As a result, because such systems are commissioned on a standby basis and will only come online in the event of an emergency, they are not in regular use and may not be subject to the same stringent maintenance regimes as other capital plant. This should be a key concern for operators, as failure to maintain modern standby generators effectively can result in components failing to operate correctly when they finally do need to be used.

Diagnosis: diesel bugs

During this time diesel fuel can become contaminated, which can cause serious damage on startup to the generator’s high pressure fuel system – potentially affecting the engine’s performance and, at worst, its ability to start up in the event of a power failure.

Diesel is an organic fuel, providing an ideal environment for fungi, yeast and bacteria (ie diesel ‘bugs’) to develop. Dissolved water allows for germination, while the carbon present acts as food. Finally, the oxygen, sulphur and other trace elements in the fuel enable this bacteria to grow.

It is important to note that diesel bugs and micro-organisms found in tanks do not grow in the fuel itself but are found in the water that may exist in the tank through condensation or contamination. If water is present in the system it does not necessarily mean that diesel bugs are guaranteed but this water should be removed to prevent germination.

Diesel fuel could also potentially have been contaminated by dirt ingress or rust, which can lead to a range of problems including filter blockages and the premature wear of fuel injectors or pumps. Should a backup generator fail to start because of these reasons, the financial repercussions could be significant.

An increasing problem in recent years has been the rise of low-sulphur diesel fuels due to EU fuel directives. The reduction of sulphur-related emissions is, naturally, to be encouraged. However, sulphur acts as a natural pesticide, and so reducing sulphur content in diesel increases the opportunity for bacterial growth. Indeed, bio-diesel has created a highly attractive environment for diesel bugs and, once water is present – even in the smallest quantities – this problem develops even further.

Curing diesel bugs

For environments where water has been detected but there is little evidence of bacterial development, a water soaker can be a cost-effective solution, which involves a desiccant to absorb any freestanding water. To help restrict any water from returning, efforts should be made to identify the cause of water entry and address this as a priority.


The old adage ‘prevention is better than cure’ is very relevant for the issue of diesel bugs and there are a number of steps that can be taken to avoid the need for fuel polishing or other remedial action.

First, ensure fuel is purchased from a reliable source. Understanding your supply chain is important to ensure you have the confidence that the source fuel has been stored and transported effectively. Laboratory analysis will help verify the fuel is fit for purpose.

Secondly, make sure fuel systems and tanks are well maintained and work to OEM guidelines. Particular attention should be given to fuel lines and tank caps to prevent water ingress. Where possible, regularly drain standing water from the bottom of storage tank or source tank.

Next, introduce a periodic fuel-sampling regime as part of your preventative maintenance programme. Laboratory fuel analysis details the suitability of the fuel as well as its condition. This practice identifies the presence of water, bacteria and contaminates that can block filters and results in fuel system issues.

Benefits of fuel analysis

It is clear that modern diesel backup generators demand the highest quality fuel. Many organisations try to ensure this by implementing a scheduled maintenance programme.

However, this rigid approach to servicing fails to offer a predictive means of identifying any potential maintenance issues, anticipating any problems that could reveal themselves in the future.

Alternatively, implementing a comprehensive fluid analysis plan – covering not only diesel for standby generators but also monitoring essential fluids for a whole host of critical equipment, including sampling oil and coolant – can realise a significant range of benefits, while also providing visibility of vital asset performance and component health.

A clear advantage is reduced maintenance costs. Some companies may be tempted to avoid the risk of diesel bugs by regularly polishing the fuel or introducing additives but this can result in unnecessary expense. Servicing equipment when required, rather than adhering to a rigid maintenance schedule, can considerably reduce total cost of ownership.

As well as the obvious benefit of minimising risk and the consequences of a standby generator failing to come online, equipment life is also optimised and any repair and maintenance required can be scheduled for a date and time convenient to the operator, ensuring any unscheduled downtime is avoided.

Only changing fuel or indeed oil when it actually needs replacing ensures the value of the fluid is optimised, plus also minimises the cost of disposal.

Finning tests more than 225,000 fluid samples every year at its state-of-the-art laboratory in Leeds. The team provides expert advice on which fluids can and should be analysed and make practical suggestions based on their findings.

Tips for preventing diesel bugs 

To prevent the issue of diesel bugs, Finning has prepared the following top tips:

• Purchase fuel from reliable sources

• Ensure systems are well-maintained, within OEM guidelines to minimise water ingress

• Use OEM grade fuel filters

• Introduce a periodic fuel sampling regime as part of your engine service schedule

• Keep fuel tanks as full as possible to reduce the possibility of water condensation

• Where possible, regularly drain standing water from the bottom of the tank

• Store fuel in a cool place



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