Diesel Goes Off in the Tank, and Takes the Generator With It

A standby generator is tested monthly, serviced on schedule and recorded as ready. The one thing nobody examines is the fuel it will have to burn, and fuel kept for emergencies spends almost its whole life sitting still in a tank, which is the worst thing you can do to it.

Where the water comes from

Every tank breathes. Warm air enters as the fuel level drops and as the day cools, and moisture condenses on the inside of the tank walls and runs down into the fuel. Water is denser than diesel, so it settles at the bottom, exactly where the suction line draws from.

That layer is also where biological growth lives. Microbes feed at the fuel-water interface and produce a sludge that blocks filters, corrodes the tank floor from the inside and clings to the pickup. None of it is visible from the filler cap.

What it does on the night

The failure is almost always the same. The set starts cleanly, runs for twenty minutes and then begins to hunt and lose power as the filters load up with sludge that the vibration of running has stirred off the tank floor. A monthly no-load test never disturbs it; a nine-hour outage does.

What to actually do

• Drain the water off the bottom on a schedule and record what came out. A tank that keeps producing water has a breathing or sealing problem worth fixing.
• Sample and test rather than eyeballing. Clear bright diesel can still carry water in suspension and a microbial population.
• Keep tanks full between uses. Less air space means less condensation.
• Polish or replace fuel that has been standing for a year or more, rather than hoping.
• Carry spare filters on site and know who can change them in the dark.

The test that proves something

Run the set on real load for as long as a real outage lasts. The monthly exercise proves the starter, the battery and the control panel. Only a sustained run on load proves the fuel, and the fuel is what fails.