Diesel fuel gels when paraffin wax in the fuel starts to crystallize in cold weather. The first stage is the cloud point, when the fuel turns hazy as wax forms. As it gets colder, the crystals clog fuel filters and lines, and at the pour point the fuel stops flowing. Untreated No. 2 diesel commonly begins to cloud somewhere around 10 to 20°F, though the exact figure depends on the fuel’s blend and season. No. 1 diesel and winter blends stay fluid at much lower temperatures.
Diesel fuel gelling temperatures
| Fuel | Cold-weather behavior |
|---|---|
| No. 2 diesel, untreated | Clouds in cold weather; can plug filters in a hard freeze |
| Winter blend (No. 2 with No. 1 or additives) | Sold seasonally in cold regions; much lower operability limit |
| No. 1 diesel | Much lower cloud point; less energy per gallon |
| Biodiesel blends | Generally cloud at higher temperatures than straight diesel |
Fuel suppliers adjust blends by season and region, so ask your supplier for the cloud point of the fuel you buy.
Cloud point, cold filter plugging point and pour point
Three numbers describe cold behavior. The cloud point is where wax first appears. The cold filter plugging point is the lowest temperature at which fuel still passes a standard filter, and it is the most practical number for drivers. The pour point is where fuel stops flowing altogether. Anti-gel additives mainly lower the filter plugging and pour points; they change the cloud point much less.
Diesel gelling symptoms
- Engine cranks but will not start, or starts and dies, on a cold morning
- Loss of power and stalling after a short drive as the filter plugs
- Cloudy fuel in the filter bowl or sample
- Low fuel pressure codes such as P0087
How to prevent gelling
- Buy winter blend fuel from a busy station in cold months.
- Add an anti-gel additive before the cold arrives; additives prevent gelling but do not reverse heavy wax once formed unless labeled to do so.
- Keep the tank full to limit condensation.
- Use a block heater and park indoors when possible.
- Change the fuel filter before winter and carry a spare.
How to fix gelled diesel
- Move the truck somewhere warm if you can, or warm the filter area gently. Never use an open flame.
- Add an emergency treatment labeled to re-liquefy gelled fuel, following its directions.
- Replace the fuel filter, which is usually packed with wax.
- Prime the fuel system and start the engine.
Related
See diesel hard to start when cold, diesel won’t start, diesel lift pump and fuel filter.
More guides: water in diesel fuel.
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