A flat battery gets all the blame in winter, but it isn’t the only reason an engine refuses to fire on a frosty morning. Both diesel and petrol cars have their own cold-weather quirks, and knowing which is which can save you a lot of fruitless cranking. Here’s what’s really going on under the bonnet, and how to get started again.
Why diesels struggle in the cold
Diesel engines rely on compression and heat to ignite fuel, and cold works against both. Two things commonly go wrong in winter:
- Glow plugs. These pre-heat the combustion chambers so a cold diesel will fire. Wait for the coil/glow-plug light to go out before cranking, and if a plug has failed, cold starting becomes a real struggle.
- Fuel “waxing”. In very low temperatures the paraffin in diesel can gel, clogging the filter so fuel can’t flow. UK winter diesel is blended to resist this, but a near-empty tank of older summer-grade fuel in a hard frost can still wax up.
If the engine turns over strongly but won’t catch, and the battery is fine, suspect glow plugs or fuel rather than charge.
Why petrol engines play up
Petrol cars are less prone to fuel gelling, but the cold thickens oil and saps battery power, so the starter has to work harder. Over-cranking a reluctant petrol engine can also “flood” it with unburnt fuel, which then won’t ignite. If you smell fuel and it still won’t start, stop cranking, wait a few minutes, then try again with the accelerator held gently down to clear it.
Cold-start checklist for both
- Turn off all electrics before cranking to give the starter maximum power.
- On a diesel, cycle the ignition on and off a couple of times to let the glow plugs pre-heat, then start.
- Crank in short bursts of no more than ten seconds, with a rest between, to avoid flattening the battery or flooding the engine.
- Keep at least a quarter tank through winter, it reduces condensation and, on diesels, keeps more anti-wax additive in play.
If the battery itself is the weak link, a jump start may be all you need, especially after a few days parked up in the cold.
When it’s more than a cold start
If you’ve ruled out the battery and the engine still won’t run, forcing it rarely helps and can cause damage. That’s the moment to call for recovery. K9 Recovery and Transport can get you and the vehicle to a garage or back home wherever you are, with 3,000 vetted drivers nationwide, no membership, full insurance and the keenest price in the UK because drivers compete for your job. Any time, any place, any vehicle, any situation, car, van, 4×4, motorbike, prestige or anything else, if it’s broken down we can move it. Our breakdown recovery service covers it all, rated 4.9/5 from 200+ Google reviews.
Broken down this winter? Get the keenest price in the UK, fast.
Frequently asked questions
Why does my diesel start fine in summer but not in winter?
Cold exposes tired glow plugs and can gel the fuel. If summer starting is fine but winter is a struggle, have the glow plugs checked and make sure you’re running fresh winter-grade diesel with a decent amount in the tank.
I can smell petrol and it won’t start, what now?
The engine is probably flooded from over-cranking. Stop, wait five minutes, then try again holding the accelerator gently down. If it still won’t fire, don’t keep draining the battery, call for help.
Does letting the car warm up help on cold mornings?
Modern engines don’t need long idling, but giving a diesel a moment for the glow plugs and driving off gently is kinder than revving hard from cold. Idling for ages just wastes fuel.
Can you recover a diesel that’s gelled or won’t start at all?
Yes. Whatever the cause, we’ll recover the vehicle to a garage or your home. One call, a nearby driver, and a fair price, no membership required.