Diesel engines rely on heat from compression to ignite their fuel, so everything about cold weather works against them: the air is cold, the metal is cold, the oil is thick, the battery is weak and the fuel itself can turn to wax. The good news is that every one of these problems has a straightforward fix, and most of them are cheap if you deal with them before the first hard frost.
Quick answer
Before winter: switch to winter-blend diesel or add an anti-gel treatment, change the fuel filter, test the battery and glow plugs, and use the oil viscosity your manual lists for low temperatures. On cold mornings: plug in the block heater two to four hours before starting, wait for the glow plug light to go out, and crank in short bursts. If the fuel has already gelled, warm the vehicle and filter and use an emergency de-gelling treatment; do not use starting fluid on an engine with glow plugs or an intake heater.
In this guide
Why diesels struggle in the cold
- Fuel gelling. Diesel contains paraffin wax. As the fuel cools, wax crystals form (the cloud point) and eventually clump enough to block the fuel filter (the gel point). Standard summer fuel clouds at roughly 15°F (−9°C); winter blends are treated to go much lower.
- Battery capacity. A lead-acid battery delivers only about half its cranking power at 0°F (−18°C), just when the engine needs the most.
- Thick oil. Cold oil drags on every moving part and slows cranking speed, and a slowly turning diesel does not build enough heat to fire.
- Cold cylinders. Compression heat leaks into cold metal, so the air may not reach ignition temperature.
Fuel: winter blend and anti-gel
In cold regions, fuel stations switch to winter-blended diesel, which mixes in lighter No. 1 diesel or additives to push the cloud point down. If you fill up before the switch, or buy fuel in a milder area and drive north, you can be caught out.
- Add an anti-gel additive to the tank at every fill in cold weather, at the dose on the bottle. It must go in before the fuel gets cold; it cannot un-gel fuel that has already waxed.
- Keep the tank as full as practical to reduce condensation, and drain the water separator regularly. Water freezes in the filter long before diesel gels.
- Change the fuel filter before winter. A partly clogged filter blocks far sooner.
- Biodiesel blends gel at higher temperatures than straight diesel. Check the blend if you have a choice.
See anti-gel additives on Amazon
Glow plugs and intake heaters
Most light-duty diesels have glow plugs, small heating elements in each cylinder, or a grid heater in the intake. The dashboard shows a coil symbol while they warm. Wait for it to go out before cranking, and in deep cold cycle the key a second time to run them again. A single failed glow plug makes for rough starts and white smoke; several failed plugs can mean no start at all. Test them before winter with a multimeter or have a shop do it. See diesel smoke colors for what the white smoke means.
Block heaters
A block heater is an electric element in the engine coolant or the block that keeps the engine warm overnight. Plugging it in two to four hours before a start transforms cold starting: oil flows, compression heat stays in the cylinder, and the battery has an easier job. A timer saves electricity by switching it on a few hours before you need the vehicle. Oil pan heaters and battery blankets do the same job for engines that lack a block heater. Check the cord for damage; a cracked plug on a wet driveway is a shock risk.
Batteries
Have the batteries load-tested each autumn, clean and tighten the terminals, and keep them charged with a maintainer if the vehicle sits. Many diesels run two batteries; a weak one drags the other down. If cranking is slow, stop and charge rather than grinding the starter until the batteries are flat.
Oil
Use the viscosity your manual recommends for the temperatures you expect. Many modern diesels specify a synthetic 5W-40 or 0W-40 for winter in place of 15W-40. Synthetic oil flows far better at low temperatures and shortens the time the engine runs with poor lubrication after a cold start.
Diesel exhaust fluid
DEF freezes at about 12°F (−11°C). Vehicles that use it have tank heaters, and the engine will start and run normally while the fluid thaws. Do not add anything to DEF to stop it freezing, and keep spare DEF somewhere it will not freeze repeatedly.
A cold-morning starting routine
- Unplug the block heater and stow the cord.
- Turn the key to run and wait for the glow plug light to go out. Repeat once in severe cold.
- Crank for no more than 10 to 15 seconds at a time, with a minute’s rest between tries, to protect the starter and batteries.
- Once running, let the engine idle a minute or two, then drive gently until the temperature gauge moves. Long idling warms a diesel very slowly and washes the cylinders with fuel.
- Keep an eye on the fuel filter warning light: a gelling filter often shows up as power loss after a few minutes.
If the fuel has gelled
- Get the machine somewhere warmer if you can: a heated shop, or even a few hours of sun.
- Replace the fuel filter, which is where the wax collects, and fill the new one with treated fuel.
- Add an emergency de-gelling product, which is formulated to dissolve wax already formed, at the dose on the bottle.
- Do not apply open flame to tanks or lines. Warm water on the filter housing is as aggressive as it should get.
Before winter: a checklist
- Fuel filter changed; water separator drained.
- Anti-gel additive in the vehicle and in any stored fuel.
- Batteries tested, terminals cleaned, maintainer available.
- Glow plugs tested; block heater cord checked.
- Correct winter oil viscosity.
- Coolant freeze protection tested with a refractometer or test strips.
- Spare DEF stored indoors.
The diesel engine maintenance schedule shows how these fit into the rest of the year.
Frequently asked questions
At what temperature does diesel fuel gel?
Untreated summer diesel begins to cloud at around 15°F (−9°C) and can gel enough to block a filter a few degrees below that. Winter blends and anti-gel additives lower those points substantially, often below 0°F (−18°C), depending on the product and the fuel.
How long should I plug in a block heater?
Two to four hours before starting is enough for most engines. Leaving it plugged in all night works but wastes electricity; a timer set to switch on a few hours before you leave is the usual answer.
Should I let a diesel idle to warm up?
Only briefly. A minute or two lets oil circulate, after which gentle driving warms the engine far faster than idling. Extended idling in cold weather can cause fuel to wash past the rings and dilute the oil.
