Diesel does not freeze solid at first. It clouds, then clogs filters. Knowing the terms and your options lets you plan for cold weather before equipment fails.
Diesel contains paraffin wax. As the temperature drops, that wax begins to form small crystals. At first the fuel looks hazy but still flows. As more crystals form and link together, the fuel thickens and can block the filters and lines that feed the engine.
That blockage, not a block of frozen fuel, is the usual reason a diesel engine stops in cold weather. It tends to show up as hard starting, loss of power under load, or an engine that starts and then dies.
Wax formation is gradual, which is why trouble often appears on the first very cold morning after a mild stretch. Fuel that behaved at one temperature starts to cloud as the overnight low arrives, and equipment parked outside suffers first.
Two terms matter. The cloud point is the temperature at which wax crystals first start to appear in the fuel. The cold filter plugging point, or CFPP, marks where those crystals begin to block a filter. CFPP is the number closer to real-world trouble, and it is usually lower than the cloud point.
Neither number is the same for every load of diesel, because it depends on the fuel's composition. Ask your supplier what the cold-flow properties of the fuel you are receiving are, instead of assuming a general figure applies.
Treat the CFPP as a margin of safety. Choose a fuel and additive combination whose cold-flow rating sits comfortably below the lowest temperature you expect, not right at it.
Diesel number 2 has higher energy content and better lubricity but performs less well in the cold. Number 1 is thinner, closer to kerosene, has fewer waxes and a lower pour point, and is less likely to gel.
To balance power and cold flow, suppliers often blend the two in winter. The share of number 1 in a blend varies by region and by how cold it gets. Because the term winter blend is loosely used, ask what is in the delivery instead of trusting the label.
Number 1 gives up some energy per gallon, so a machine may burn slightly more of it for the same work. The tradeoff is usually worth it when the alternative is a plugged filter.
When comparing suppliers, ask each the same question: at what temperature will this load start to cloud, and what is its CFPP? A supplier who can answer with test data for the fuel they actually sell is easier to plan around than one who points to the season.
Anti-gel and cold-flow additives work by lowering the temperature at which wax crystals cause trouble. They also help disperse crystals so they stay small. Many products can improve cold performance and some add lubricity.
They have limits. Additives are best treated as prevention, and fuel that has already gelled calls for the product label and your equipment manual. Dosing instructions on the product apply, and a supplier can often treat fuel at the time of delivery. If a product claims a specific temperature improvement, treat that as the manufacturer's claim and check it against your own conditions.
Check that an additive suits your engine and fuel system and does not interfere with emissions equipment, following the equipment maker's guidance.
Cold weather makes water in fuel a bigger problem. Condensation inside partly empty tanks adds water, and water can form ice crystals that also block filters. Keeping tanks reasonably full cuts down the air space where condensation forms.
Stored fuel that is held over from summer may be number 2 with no winter treatment. If you keep fuel in a yard tank, a fill of winter-grade product before the cold hits is more reliable than hoping an additive will fix it later. For keeping a tank stocked, see bulk tank delivery.
Standby generators matter here too. A generator that must start in a cold outage depends on cold-ready fuel. Read generator fuel planning and ask about winter treatment before the season starts.
Fuel in a vehicle's tank is only part of the picture. Fuel in lines, filters and an idle generator's supply system also sits in the cold. A machine parked for days in freezing weather is a candidate for trouble, and so is a generator that has not run since autumn.
Gelling usually shows up as an engine that cranks but will not start, runs rough and loses power under load, or starts and then dies. Because worn or dirty filters plug first, a machine with a marginal filter often fails before its neighbors do.
If you suspect gelling, do not crank the engine repeatedly. Keep open flames and unapproved heat sources away from the fuel system. Move the machine to a heated space if you can, and follow the manufacturer's instructions for thawing and for replacing filters. If you are unsure, call a qualified technician.
Run through this list before the first hard cold snap.
Write the checklist down and assign each item to a person with a date. A list that lives only in someone's head tends to be skipped the first week of a freeze.
There is no single temperature, because it depends on the fuel's blend and wax content. Cloud point and CFPP for your specific fuel are the useful numbers.
It is done, but it is easier to get a correctly blended product from the supplier. Wrong ratios can leave you with fuel that gels or lacks power.
Treat additives as prevention. For fuel that has already gelled, follow the product label and your equipment manual.
The dye is not the issue. What matters is the grade and blend of the fuel, which is covered in on-road versus off-road diesel.
Tell us what you run and where it sits, and we will come back with a delivery plan.