Why winter is harder on a diesel
A petrol engine ignites its mixture with a spark. A diesel has to compress air so hard that it heats up to the fuel’s ignition temperature. In the frost a cold engine draws that heat away, the battery has less power and the diesel fuel thickens. If just one of these is borderline, the car won’t start in the morning.
That’s why a winter diesel service isn’t about a single part, but several items that together decide whether the car starts from cold.
What to check on a diesel before winter
1. Glow plugs
They heat the combustion chamber so a cold diesel can start. One dead glow plug will go unnoticed in summer, but in the frost it means a hard start, smoke and rough running after starting. Glow plugs are checked with a diagnostic tester and a meter, and are best replaced as a set.
It’s not worth putting off replacing them for years – an old plug seizes in the cylinder head and can snap when unscrewed. There’s more on this in our guide to glow plugs vs spark plugs, and you can see where it leads in the story of a Vectra B 2.0 DTI.
2. Battery and charging
In the frost a battery delivers less current, while a diesel needs more. The starter has to turn over an engine with a high compression ratio, and the glow plugs draw a lot of current at start-up too. A battery that is “just about coping” in autumn often gives up on the first frosty morning. It’s worth testing it under load and, while you’re at it, checking that the alternator charges it properly.
3. Fuel filter
In a diesel, the fuel filter also traps water. In winter, water in the filter freezes, and paraffin wax comes out of the diesel and coats the filter element. An old or clogged filter is a sure way for the car to stall or refuse to start on a frosty morning.
If the filter is due in the next few months, or you don’t know when it was last changed, do it before winter. If the filter has a water drain, it’s worth draining it. Many diesels also have a fuel filter heater – it’s good to check that it works.
4. Engine oil
Cold oil is thicker, so the starter has to work harder and the engine runs with poorer lubrication for the first few seconds. If an oil change falls due during the winter, it’s better to do it in autumn – with oil that meets the manufacturer’s specification.
5. Coolant, thermostat and heating
The coolant has to withstand the frost. Its freezing point can be checked in seconds with a refractometer. Diluted coolant can freeze and damage the radiator, the pump and even the engine block.
A diesel warms up slowly. If the thermostat is faulty, the engine never reaches temperature, the cabin stays cold, the windows mist up and fuel consumption goes up.
6. Belt, leaks and fault codes
In the frost rubber hardens, so a worn auxiliary belt starts squealing on cold starts or snaps. While you’re at it, it’s worth checking the engine for leaks and reading the fault codes from the control units – the tester often shows a problem with the glow plugs or sensors before the car starts struggling to start.
7. Winter diesel
Diesel fuel thickens in the frost as paraffin crystals form in it. That’s why filling stations in Poland sell seasonal fuel:
| Diesel | When it’s sold | Frost resistance |
|---|---|---|
| summer | 16 April to 30 September | down to about 0°C |
| transitional | 1 October to 15 November and 1 March to 15 April | down to about −10°C |
| winter | 16 November to the end of February | down to about −20°C |
A few rules for winter:
- fill up at large stations where the fuel turns over quickly,
- don’t head into the frosts with a full tank of summer or transitional fuel,
- an anti-gel additive only works as prevention – it has to go in before the fuel thickens.
8. Short trips and the DPF
In winter we make more short trips, and the engine takes longer to warm up. The diesel particulate filter (DPF) in newer diesels needs longer drives to burn itself clean. If you mostly drive around town, take a longer trip out of the city every so often. And if the DPF light comes on, don’t put off a visit.
Book a winter diesel serviceCall +48 570 585 582
How to start a diesel in the frost
- Turn the key to the ignition position and wait for the glow plug light to go out. Only then turn the engine over.
- Switch off anything you don’t need while starting: the fan, window heating, the radio.
- Crank in short bursts – a few seconds, a dozen or so at most – and take breaks between attempts.
- Once it starts, drive off gently, without high revs. Don’t warm the engine up while parked: it warms up faster on the move, and in Polish built-up areas leaving the engine running while parked for more than a minute is an offence.
When to book the service
Ideally in October or early November – before the frosts arrive, and with them the queue of cars that wouldn’t start in the morning. That leaves time to order parts calmly and get everything done in one visit.
You drop off and pick up the car at a time that suits you – any day, including Sundays and public holidays, arranged in advance. Arrange a time by phone or on WhatsApp, in English.