Does Diesel Go Off?

Short answer: yes. Diesel stored in reasonable conditions stays in serviceable condition for roughly 6 to 12 months. It does not spoil suddenly, and it does not last forever — and what usually ruins a tank of diesel is not age at all. It is water.

Why diesel degrades

Diesel is a mix of hydrocarbons, and from the moment it is refined it starts slowly reacting with its surroundings. Four things drive that, and on a real site they almost always happen together.

• Oxidation. Fuel reacts with air, forming gums and varnish that darken it and eventually clog filters and injectors. Warmth accelerates it.
• Water. The big one. Condensation forms as a part-full tank breathes through the day–night temperature swing, and water also arrives through faulty seals, damaged caps or a careless delivery. Water is denser than diesel, so it settles at the bottom — exactly where the pickup usually is.
• Microbial growth — the "diesel bug". Bacteria and fungi live at the boundary between water and fuel. Give them a water layer and they will colonise it, producing a dark sludge that blocks filters and corrodes tank walls. No water, no bug: it is that simple.
• Temperature swings. Not just heat, but the cycling. Every warm day and cool night pulls moist air in and condenses it out, quietly topping up the water layer.

How long does it actually last?

Treat 6 to 12 months as your planning figure for ordinary on-site storage. A cool, clean, properly sealed tank that is kept dry can hold good fuel longer than that. A hot tank sitting half-empty in the sun through a Melbourne summer, with a water layer nobody has drained, can be in trouble in a matter of months.

Two things shift the number. Biodiesel blends absorb water more readily than straight mineral diesel and generally need closer attention. And how full the tank is matters more than most people expect — a tank kept near full has less air space to breathe, so it condenses far less water than one habitually run down to a quarter.

The warning signs

Fuel rarely announces itself. In practice the first thing an operator notices is a machine losing power or a filter needing changing far too often — and it gets diagnosed as an engine problem before anyone thinks to look in the tank. Worth checking for:

• Colour darkening from a clear straw toward brown or black
• A sour, stale or varnish-like smell instead of the usual diesel smell
• Cloudiness or a hazy appearance, which usually means suspended water
• Visible sediment, or a dark slime on a dipstick or in a drained sample
• Fuel filters blocking noticeably sooner than they used to
• Hard starting, rough running or power loss with no obvious mechanical cause

A simple bottom-sample from the tank tells you more in two minutes than a week of guessing. If it comes up cloudy, or with a dark layer, you have your answer.

Keeping fuel in good condition

Nothing here is exotic. The operators who never have a fuel-quality problem are the ones doing these unglamorous things consistently:

• Keep water out and drain what gets in. Check the water level regularly and drain it. Keep caps, seals and vents in good order.
• Size the tank to real usage. An oversized tank is a standing invitation to condensation and stale fuel. Better to hold less and be refilled more often.
• Keep it turning over. Fuel that moves does not sit long enough to degrade. A standing delivery schedule matched to your consumption does this automatically.
• Get it out of the sun. Shade or a light-coloured tank reduces the daily temperature swing that drives condensation.
• Treat genuinely long-term storage differently. Standby generators and seasonal plant may hold the same fuel for a year or more. That is where stabiliser additives and periodic fuel testing earn their keep.

If the fuel has already gone off

Aged fuel can often be recovered. Fuel polishing — filtering and separating out water and particulates — will bring a lot of tanks back into service. Where microbial contamination has taken hold, the tank itself usually needs cleaning too, because treating the fuel and leaving a contaminated tank simply restarts the clock. Heavily degraded fuel should be disposed of properly, not run through an engine and hoped for.

The decision is worth making on evidence rather than instinct. A fuel test costs very little next to a set of injectors.