Hard starting in winter: the causes in order of likelihood
In short
The most common cause of hard starting in cold weather is the battery: at −20 °C it delivers roughly half its rated cranking current, while the engine needs two to three times more torque to turn over through thickened oil. Check in this order: battery and terminals, the first number in the oil viscosity, the glow plugs on a diesel, then the fuel and the ignition system.
The battery: by far the first suspect
Cold attacks the battery from two directions at once. The chemical reactions inside slow down, so at around −20 °C a battery delivers about half the cranking current it gives in summer. At the same time, cold oil is far more viscous, and the starter needs two to three times more torque to turn the engine over. A battery that barely coped last winter will not cope at all this one. The classic sign is slow, laboured cranking, with the headlights visibly dimming as you turn the key. A repeated click with no cranking means there is not even enough voltage for the solenoid — again a battery or a poor terminal. The check takes a minute with a multimeter. After standing overnight, a healthy battery reads above 12.6 V; below 12.4 V it is partly discharged, and below 12.0 V it is seriously flat. Resting voltage does not tell the whole story, though: a battery can read 12.6 V and drop below 9.5 V on the first crank, which points to sulphated plates. A load test at any workshop gives the definitive answer. Before replacing the battery, clean the terminals. A white-green layer of corrosion can block enough current to give a perfect impression of a dead battery. Batteries usually last 4–5 years, and short urban trips, where the alternator never fully replaces what the start consumed, shorten that life.
The first number in the oil viscosity
The canister says, for example, 5W-30. The first number, the one before the W for winter, describes precisely how the oil behaves in the cold: the lower it is, the more fluid the oil stays at sub-zero temperatures and the less resistance it offers the starter. The difference is physically obvious at −20 °C. A 15W-40 becomes almost paste-like at that point, and the starter must overcome that resistance before reaching the speed at which the engine can fire. A 5W-30 or 0W-30 flows far more easily and, just as importantly, reaches the bearings in seconds rather than tens of seconds — which matters enormously, because most engine wear happens in the first moments after start-up. The practical rule: in Moldova, where winter regularly drops below −10 °C and occasionally below −20 °C, a 5W or 0W oil is the sensible choice, provided the specification your carmaker demands (ACEA, API or a brand approval) is respected. Viscosity is never chosen separately from the specification, and an oil thinner than the manual allows can create problems of its own.
Diesel: the glow plugs
Diesel does not ignite from a spark but from the heat generated by compressing air. In the cold, the combustion chamber loses too much of that heat to cold metal, so glow plugs pre-heat the air before and during cranking. The signs that one or more have burnt out are typical: difficult starting only from cold, very rough running for the first tens of seconds, thick blue-white smoke from the exhaust immediately after starting and, sometimes, a preheat lamp that flashes or stays lit. In summer the same failed plugs can go completely unnoticed — which is why the problem seems to "appear" suddenly in November. Glow plugs are replaced as a set, like spark plugs, because they wear together. One important detail: if they have seized in the head through carbon build-up, forcing them out can snap them off in the thread, so the job is never rushed and never done on a fully cold engine. A useful habit in the cold: turn the key to ignition and wait for the preheat lamp to go out before cranking. At very low temperatures, two consecutive preheat cycles help noticeably.
Fuel: gelling, water and condensation
Diesel contains paraffins that crystallise at low temperatures. Summer diesel starts to cloud at around −5 °C and can block the filter before it reaches −10 °C, while winter-grade diesel is additised to cope down to roughly −20 °C. If you filled up in autumn and the car then sat until January, this is exactly the scenario you get: the engine cranks normally but receives no fuel. Water is the second problem. It accumulates from condensation and freezes in the filter or in the lines, cutting off supply entirely. Many diesel filters have a settling bowl with a drain tap that needs emptying periodically; some cars have a water-in-fuel sensor with its own warning lamp. Two simple habits reduce the risk: keep the tank fuller in winter, because an empty tank condenses more water on its walls, and refuel where stock turns over quickly, so you actually get seasonal fuel. Petrol has no gelling problem, but water in the tank is just as unwelcome. A fuel filter near the end of its interval passes unnoticed in summer and becomes the bottleneck in winter, when viscosity rises. If the hard starting appeared along with the cold weather and the battery is sound, the filter is worth checking.
The spark and the remaining causes
On a petrol engine, if the starter spins briskly but the engine will not catch, attention moves to ignition. Worn plugs demand higher voltage exactly when the battery has least to give, so a plug that just about coped in summer may not produce a usable spark at −15 °C. Cracked leads and hairline-cracked coils leak current to earth, especially in damp weather: on a foggy night, or after washing the engine bay, a stray discharge is visible in the dark under the bonnet. Other causes are rarer but real: a faulty coolant temperature sensor that feeds the control unit the wrong reading and ruins cold enrichment; a dirty mass airflow sensor; leaking injectors that flood the cylinders; or low compression, which shows up first in winter, when losses past the rings weigh more heavily. If the engine will not start after a few attempts, do not keep cranking continuously: you will overheat the starter and flatten the battery to the point where none of the simple causes can be tested any more.
Narrowing it down with three questions
Diagnosis becomes simple if you start with the right question: what does the engine do when you turn the key? If the starter turns slowly or you hear only a click, the problem is almost certainly electrical — a flat or aged battery, corroded terminals, a loose earth strap, more rarely the starter itself. None of the other causes is worth investigating until cranking voltage is healthy. If the starter spins briskly and the engine still will not fire, either spark or fuel is missing. On a diesel, suspect the glow plugs first, then the filter and gelled fuel. On a petrol engine, suspect the plugs and coils, and if there is a strong smell of fuel, an over-rich mixture. If the engine does start after prolonged cranking, runs roughly for a minute and then settles, you have a cold-start problem specifically: worn plugs, partially failed glow plugs or a temperature sensor that is lying. Note the conditions in which it happens — only below a certain temperature, only in the morning, only after a night outside — because those details are exactly what shortens the diagnosis.
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Frequently asked questions
Does switching the headlights on for a few seconds before starting help?
Sometimes, yes. A small load for 10–15 seconds slightly warms the electrolyte of a marginal battery and can make the difference on the first crank. It is not a cure, just a one-off trick for a battery that still needs testing.
Which oil should I choose for Moldovan winters?
One starting with 5W or 0W, provided the specification and approval your carmaker requires are respected. The second number stays as the manual states. Do not switch to a thinner oil purely because of the cold if the manual does not allow it.
Is it fine to jump-start from another car?
Yes, if you follow the correct connection order and do not improvise with thin cables. But remember that a jump-start treats the symptom, not the cause: if a battery has died once in the cold, it will do so again on the next cold night.
How long should I warm the engine up in winter?
Long enough for oil pressure to stabilise, roughly 30–60 seconds, then drive off and keep it gentle until the temperature gauge starts to rise. Extended idling burns fuel, dilutes the oil and warms the engine more slowly than driving smoothly does.