The most common reasons a diesel won't start in cold weather are weak batteries, failed glow plugs or an intake grid heater, gelled fuel, water or ice in the fuel system, and engine oil too thick for the temperature. Diesels rely on heat from compression to ignite fuel, so anything that robs heat or cranking speed on a cold morning can stop them.
The common causes
1. Weak batteries
Cold cuts battery output while thick oil makes the engine harder to crank. Many diesel trucks use two batteries; one weak battery can drag down the other.
2. Glow plugs or grid heater
These preheat the combustion chamber or intake air. If several glow plugs fail, or the grid heater or its relay fails, cold starts get hard or impossible. Long cranking with white smoke is a common sign.
3. Gelled fuel
Paraffin wax in diesel fuel starts to cloud and then gel as temperatures drop. Standard #2 diesel can start clouding around 10 to 20°F, depending on the blend. Gelled fuel clogs filters and lines, and the engine starves.
4. Water in the fuel
Water collects in tanks and the fuel-water separator, then freezes and blocks fuel flow.
5. Thick oil
Oil that's too heavy for the temperature slows cranking. Check your manual for the right viscosity for winter.
Gelled fuel or glow plugs?
- Gelled fuel: the engine may start and then stall, or crank strongly but not fire; the fuel filter may look waxy. Problems often show up after a very cold night.
- Glow plugs or grid heater: the engine cranks at normal speed, maybe sputters and puffs white smoke, then catches once it's warmer or after several attempts. A wait-to-start light that acts oddly is a clue.
Prevention
- Use winter-blend fuel, or fuel treated for cold. In very cold regions, stations sell blends with #1 diesel
- Add anti-gel additive before it gets cold. It prevents gelling but usually can't reverse fully gelled fuel
- Plug in the block heater a few hours before starting
- Drain the fuel-water separator regularly
- Test batteries before winter, and replace weak ones in pairs
- Keep the tank fuller to reduce condensation
Gas car owners: our winter car checklist covers your side.
DEF in winter
DEF freezes at about 12°F. Systems have heaters, but heater failures are a common winter fault. See DPF and DEF problems.
When to call a mobile diesel mechanic
If the truck still won't start after charging the batteries and plugging in the block heater, a mobile diesel mechanic can test batteries, glow plugs or the grid heater, check fuel flow and filters, and treat or replace gelled fuel filters on site. See mobile diesel mechanics.
Frequently asked questions
Should I use starting fluid on a diesel?
Many manufacturers warn against it, especially with glow plugs or grid heaters, because it can cause damaging early ignition. Check your manual.
How do I un-gel diesel fuel?
Warm the truck (a heated garage is best), use a product made to re-liquefy gelled fuel, and replace clogged fuel filters. A mechanic can help.
How long should I plug in my block heater?
A few hours before starting is common. Many owners use a timer.
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