A truck that struggles to start, loses power on a delivery run, or repeatedly suffers fuel filter problems can quickly become an unexpected expense. Water in the fuel tank is one possible cause, and identifying it early can help prevent a manageable issue from developing into a more serious repair.
As a fleet manager, your challenge is knowing what to investigate first. Is the problem confined to one vehicle, or could other trucks have been affected by the same fuel supply? And if water is present, what needs to happen before the vehicle can safely return to work?
At TankSafe, we've spent more than 20 years developing fuel protection technology for the haulage and transport industry. We understand the importance of protecting fuel, controlling avoidable costs and keeping vehicles available when they're needed, and we’re here to help.
Let’s break down the causes and risks of water in your fuel tanks, and explore how to recognise it, deal with it safely, and reduce the risk of the same problem happening again.
How water in the fuel tank affects your vehicle’s fuel system
Contaminated fuel is one of the biggest issues your fleet can face. We’ve touched on this in a previous blog on fuel degradation and storage risks in diesel fleets, but it’s worth looking more closely at what happens when water gets into diesel and the symptoms to look out for.
Water is heavier than diesel, so any free water tends to settle at the bottom of the tank when the fuel is left undisturbed. Smaller droplets can remain suspended throughout the fuel, depending on the conditions and level of contamination.
As the vehicle moves, the fuel shifts around inside the tank, disturbing any settled water and allowing it to mix temporarily with the diesel. When fuel is drawn towards the engine, any contamination carried with it passes through the fuel lines and towards the filtration system.
Many diesel vehicles have a water separator designed to remove free water before the fuel reaches sensitive components. As the fuel passes through the separator, water droplets are collected and gather in a chamber for draining, depending on the system design. However, a separator’s effectiveness depends on its design and condition, and it may not remove all the water present, particularly when very small droplets remain dispersed throughout the fuel.
Common symptoms of water in fuel tanks include:
Difficulty starting the truck – the engine may take longer than usual to start or struggle to start at all.
Rough running or hesitation – the engine may run unevenly, hesitate during acceleration or feel less smooth to drive.
Loss of power – the affected vehicle may struggle to accelerate or maintain power under load if contaminated fuel is affecting engine performance.
Engine stalling – in more serious cases, contamination can interfere with engine operation and cause the engine to cut out.
A water-in-fuel warning – if your vehicle has a water sensor, a dashboard warning may indicate that water has collected in the fuel filter or separator and needs attention.
Recurring fuel filter problems – repeated filter blockages or restrictions may indicate a fuel-quality issue, particularly if other symptoms occur at the same time or several vehicles are affected.
These symptoms don’t confirm water contamination on their own. A blocked filter, air entering the fuel system, injector problems or an electrical fault can cause similar issues, so it’s important to establish the cause before replacing components.
Establishing how water has got into a diesel fuel tank
Suspecting there’s water in the fuel tank of one of your trucks is one thing; working out how it got there is key to preventing the problem from happening again. Contamination can be difficult to trace, particularly if it develops over time or goes unnoticed until a vehicle starts showing signs of trouble. Taking the time to understand the circumstances can help you get to the root of the problem and avoid unnecessary disruption to your fleet.
Investigate the fuel supply
If problems start shortly after refuelling, it’s worth checking the fuel supply. Water can enter diesel during storage or handling, so contamination may originate in a delivery or depot tank rather than the vehicle itself.
If several trucks develop similar symptoms after using the same bulk tank, investigate the shared supply before treating each case as a separate fault. Review delivery records, identify which vehicles have used the tank and arrange appropriate fuel sampling. If contamination is suspected, stop using the supply until it has been assessed.
This doesn’t automatically mean the supplier is responsible. Contamination could have entered earlier in the supply chain or accumulated in the depot’s storage tank. The aim is to establish the likely source before deciding what action to take.
Check the vehicle for water ingress
If the problem appears to be limited to one vehicle, start by checking the filler cap, seals and filler neck for damage or signs of poor sealing. These areas can allow rainwater or wash water to enter the tank if they’re not in good condition.
Again, the timing can help narrow down the cause. If the symptoms started after heavy rain or vehicle washing, pay particular attention to the filler area. If the issue continues despite using fuel from a supply that has been checked for contamination, the vehicle’s tank and associated components may need a closer inspection.
If these checks don’t reveal an obvious cause, look beyond the vehicle itself. Condensation can contribute to water accumulating in fuel storage tanks over time, so review your depot tank’s inspection and maintenance history, particularly if contamination has occurred before.
What water contamination means for your fleet
Water contamination can lead to vehicle downtime, unexpected repair bills and disruption to planned work. The impact depends on how much water is present, whether it reaches sensitive fuel system components and how quickly the problem is identified.
If one of your trucks develops fuel-system problems during a delivery run, it may need to stop for inspection or be recovered to a workshop. Loads may need transferring, routes rearranged, and customers kept informed while alternative transport is arranged. If water damages components like the high-pressure fuel pump or injectors, repairs can be more costly, and the vehicle may be off the road for longer.
Even when serious damage is avoided, identifying the source of contamination can take time. Fuel sampling, tank inspections and diagnostic checks all require resources. Vehicles may also need to be taken out of service while checks are carried out and the extent of the problem is assessed. If several vehicles are affected, these costs and delays can quickly add up, placing further pressure on workshop capacity and day-to-day operations.
Practical ways to reduce the risk of repeat contamination
Preventing water contamination starts with good fuel management and routine maintenance. Building preventative checks into vehicle maintenance and depot procedures can help reduce the risk of contamination and catch potential problems before they cause disruption.
Maintain fuel filler components – inspect filler caps, seals and filler necks during routine vehicle checks, and repair or replace damaged parts promptly to reduce the risk of rainwater or wash water entering the tank.
Keep depot storage tanks maintained – follow a regular inspection and maintenance schedule, check for accumulated water and arrange its removal using appropriate procedures.
Monitor water separators – follow the vehicle manufacturer's guidance for checking and draining separators, and investigate recurring warnings rather than treating them as routine occurrences.
Check fuel deliveries – keep delivery records and follow established procedures for checking fuel quality, particularly when a delivery is suspected of being contaminated.
Keep clear refuelling records – record which vehicles use each bulk tank so that, if contamination occurs, potentially affected vehicles can be identified quickly and further exposure limited.
Protecting your fuel supply with TankSafe
At TankSafe, we’ve seen first-hand how disruptive water in the fuel tanks of your vehicles can be, something we’ve kept in mind when developing our pioneering anti-siphon devices. Our globally trusted TankSafe Optimum 9 product was developed with fuel security at its core, but we’ve also considered the risk of water entering the fuel tank. That’s why it features a patented head design that helps prevent water ingress, providing an additional layer of protection for your fuel supply.
To find out more about TankSafe Optimum 9 and how it can help protect your fleet, visit our website or call our team on 01253 400 401 today – we’re ready to help!