Water is the real problem in a stored diesel tank
Diesel that sits for months picks up water. Water feeds microbes, makes sludge, and can freeze in a cold snap. Mid-fall is a good time to check the tank.
How water gets in
Warm days and cool nights make moist air condense inside a partly empty tank. Rain can get in through a loose fill cap or a worn vent. None of it is dramatic, but it adds up over a season.
Water is heavier than diesel, so it settles to the bottom under the fuel. A tank that is never checked or drained can hold a water layer for a long time without anyone seeing it.
What water does to diesel
Microbes grow where the fuel meets the water. They make slime and sludge that plug filters and can corrode the tank. In cold weather, free water can freeze and block lines and filters.
Generator tanks carry the most risk because the fuel sits. A short monthly test run may not show a problem that a long run during an outage stirs up from the bottom of the tank.
Check the tank before winter
Put water-finding paste on a gauge stick to check the bottom of the tank. Pull a sample from the bottom and look for haze, a water layer, or dark sludge. Drain the water if the tank has a drain or sump.
Keep the tank fuller in cold months so there is less room for moist air, and keep caps and vents sealed. Ask your distributor about winter blend and additives that fit your climate.
When to call for fuel polishing
Fuel polishing pumps the diesel through filters and water separators and back into the tank. Tank cleaning goes further and removes sludge from the floor and walls. Both are services you schedule.
Some distributors offer polishing or tank service and some refer it out, so ask. Check the tank before a fresh delivery, because new fuel landing on top of sludge stirs it up into the lines.
Questions
How often should a stored diesel tank be checked for water?
Follow the generator or tank maker's guidance. Checking after each delivery and again before cold weather is a sensible habit for any tank where fuel sits.
Does DEF have the same water problem?
DEF is not a fuel. It is already mostly water mixed with urea, but it still needs clean, sealed storage so dirt, fuel, and tap water stay out of it.