Short answer
Diesel contains paraffin wax that starts to form crystals as it gets cold. The temperature where it starts is the cloud point; the crystals plug fuel filters first. Winter-blend fuel, the right additive added before the cold, and keeping the filter warm are the main defenses.
Start with the easy checks
- Buy local winter-blend fuel ahead of a cold snap.
- Add a cold-flow (anti-gel) additive to the tank before fuel gets cold. Most do not reverse wax that has already formed.
- Keep a spare fuel filter in winter and drain the water separator.
- Use a block heater or fuel heater if your truck has one.
Likely causes, most common first
- Fuel wax at low temperature
- Wax crystals plug the filter.
- Water in fuel
- Can freeze and block flow.
- Summer fuel left in the tank
- Not blended for the cold.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
What not to do
Industry fuel guidance warns against adding gasoline or alcohol to diesel. Blending in #1 diesel (or kerosene where the engine maker allows it) is the method guidance describes for lowering the cloud point; flow improvers work differently and must go in before the fuel clouds.
When to stop and see a diesel tech
- A gelled tank or lines that will not clear.
- Hard starting continues after the fuel warms up.
Questions
Can I add anti-gel after my fuel gels?
Most cold-flow improvers must be added before the fuel reaches its cloud point. Some products are made to help re-liquefy gelled fuel; follow the label and your engine maker's guidance.
What is the difference between cloud point and pour point?
Cloud point is when wax crystals first appear. Pour point is when the fuel stops flowing. Filters plug near the cloud point, well before the pour point.
Related symptoms
Why is my Cummins hard to start when it is cold?
Troubleshoot →Fuel systemMy water-in-fuel light came on. What should I do?
Troubleshoot →StartingMy diesel cranks but will not start. Where do I begin?
Troubleshoot →Where this comes from
- Auto Care Association TSB 91-1: Solving winter diesel fuel and fuel filter problems
- Coordinating Research Council report CRC-671 (diesel low-temperature operability)
- American Chemistry Council: Overview of diesel additives and filtration issues
- Lucas Oil: Anti-Gel Cold Weather Diesel Treatment product data sheet
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.