Short answer
Diesel fuel forms wax crystals as it cools, and the wax can plug the fuel filter. Buy winter-blend fuel, add an anti-gel (cold-flow) additive before temperatures drop, drain water from the fuel system, use an oil viscosity your engine allows for cold starts, check coolant protection, and plug in the block heater.
Why diesel gels
Diesel contains paraffin that begins to form wax crystals at its cloud point. As it gets colder the crystals grow until they plug the fuel filter, which is the point where the engine starves for fuel. Fuel can gel in the tank, lines and filter even when the engine starts and runs for a while.
Biodiesel blends tend to have a higher cloud point than straight diesel, and summer fuel left in a tank or a rarely used machine is the usual culprit when a truck gels on the first cold night.
Diesel gel point: No. 1, No. 2 and winter blends
There is no single gel temperature. Untreated No. 2 diesel commonly starts to cloud somewhere around 10 to 20°F (about -12 to -7°C) and can plug a filter a few degrees colder. No. 1 diesel stays fluid far colder, which is why northern stations blend it into No. 2 or treat their fuel for winter.
The fuel standard leaves cold-weather properties to the supplier and the season, so ask your supplier for the cloud point or cold filter plugging point of the fuel you are buying, especially for bulk deliveries.
Anti-gel additives and fixing gelled diesel
Anti-gel, or cold-flow, additives change how wax crystals form so fuel keeps flowing through the filter at lower temperatures. They work when mixed into the fuel before it reaches its cloud point; adding them to fuel that has already gelled does little. Treat at the fill-up, at the dose on the label for the temperatures you expect.
If the fuel has already gelled, get the truck somewhere warm or warm the filter and lines, replace the fuel filter, and use a product labeled for gelled-fuel recovery. Never add gasoline to diesel to thin it: it lowers the flash point and can damage the injection system.
AMSOIL Diesel Cold Flow is a cold-weather treatment, Diesel 4-IN-1 combines injector cleaning, lubricity, cetane and cold-flow functions, and Diesel Recovery is labeled for recovering fuel that has already gelled. Follow each label’s dose and temperature guidance.
| Symptom | Likely cause | What to do |
|---|---|---|
| Engine starts, then stalls or loses power in the cold | Wax plugging the fuel filter | Warm the filter and lines; replace the filter; treat the fuel as the gelled-fuel product directs |
| Hard cranking, slow start | Battery, glow plugs or intake heater, oil too thick | Load-test batteries; check the starting aids; confirm the oil grade allowed for the temperature |
| Water-in-fuel warning | Water in the separator | Drain the separator; check the fuel source |
| DEF warning on a cold start | Frozen DEF | Let the system heater thaw it; do not add anything to the DEF |
Source: AMSOIL Diesel Cold Flow.
Source: AMSOIL Diesel Recovery.
Water in the fuel
Water condenses in partly full tanks and freezes in filters and water separators. Keep tanks full when the equipment sits, drain the water separator on schedule, and carry a spare fuel filter in winter.
If you buy bulk fuel or store it on site, a fuel test shows whether water, sediment or microbial growth is the problem. AMSOIL sells a Diesel Fuel Contamination Test that checks for water, sediment and aerobic bacteria.
Source: AMSOIL Diesel Fuel Contamination Test.
Engine oil for cold starts
The first number of a multigrade, such as the 0 in 0W-40 or the 15 in 15W-40, describes how the oil performs at low temperature. Many diesel manuals include a chart of permitted grades by ambient temperature, and some allow a 5W-40 or 0W-40 in place of 15W-40 for cold climates. Use only grades your engine maker allows.
AMSOIL’s Signature Series Max-Duty diesel oils come in 0W-40, 5W-30, 5W-40, 10W-30 and 15W-40, each listing API CK-4 and engine-maker specifications on the data bulletin. Match the grade to the chart and the specification to the manual.
Source: AMSOIL Signature Series Max-Duty data bulletin G3467.
Coolant, DEF, batteries and block heaters
Test coolant freeze protection before the first hard freeze. A coolant analysis reports freeze and boil points along with pH and contamination. Use the coolant chemistry the engine maker specifies.
Diesel exhaust fluid freezes at about 12°F (-11°C). Trucks with selective catalytic reduction have heaters to thaw it, so do not add anything to DEF to keep it from freezing. Store spare DEF where it stays above freezing.
Batteries lose cranking capacity as they cool while a cold engine needs more of it. Load-test batteries in the fall, and use the block heater: a warm engine starts more easily and reaches operating temperature sooner.
Source: AMSOIL Coolant Testing Kit.
Sources checked . Prices and product guidance may change.
Frequently asked questions
What temperature does diesel gel?
It depends on the fuel. Untreated No. 2 diesel commonly starts to cloud around 10 to 20°F (about -12 to -7°C); No. 1 and winter blends stay fluid much colder. Ask your supplier for the cloud point.
Can I add anti-gel after the fuel gels?
An anti-gel additive works when mixed in before the fuel reaches its cloud point. For fuel that has already gelled, use a product labeled for gelled-fuel recovery and warm or replace the filter.
Should I switch from 15W-40 to 5W-40 for winter?
Only if your engine maker allows it. Check the manual’s temperature chart and the required specification, such as API CK-4.
Can I add kerosene or gasoline to keep diesel from gelling?
Never add gasoline. Some engine makers permit blending with No. 1 diesel or kerosene and some do not; follow your engine manual and fuel supplier.
General maintenance information. Always follow your vehicle or equipment manufacturer’s requirements and the current product data. Questions? Email [email protected].
