Starting a Car in Cold Weather

Why cold makes starting harder

A car’s engine needs three things to start: enough electrical power to turn it over, a suitable mix of air and fuel, and a spark in a gasoline engine or enough compression heat in a diesel. Cold weather makes each of these a bit harder.

The battery weakens. A lead-acid battery relies on a chemical reaction, and that reaction slows as the temperature falls. A battery that works perfectly well in mild weather can deliver noticeably less power in a hard freeze. At the same time, the engine needs more power to start, which is why an aging battery often fails on the first really cold morning.

Oil gets thicker. Cold oil flows more slowly, so the engine is harder to turn and parts take longer to be coated. This is one reason the choice of oil viscosity matters. Following the grade in your owner’s manual helps, and the “W” number in many grades relates to cold performance.

Fuel vaporizes less easily. Gasoline needs to vaporize to burn properly, which is more difficult when it is cold. Modern fuel injection systems handle this well in most conditions, though very low temperatures can still cause longer cranking. Diesel can thicken or gel in extreme cold, and suppliers often sell winter blends. Check what applies in your region.

Moisture can freeze. Condensation in the fuel tank, locks and door seals can freeze. A reasonably full tank leaves less room for condensation to form.

Precautions before winter

• Have the battery tested. Many shops can check the battery’s condition quickly. Batteries often last several years, but the age is only a rough guide. If yours is old or starts slowly, battery replacement before winter is cheaper than a tow.
• Keep terminals clean. Corrosion on the terminals can block current. Check that the cables are tight and clean, with the engine off.
• Use the right oil. Follow the manufacturer’s recommendation for the temperatures you expect.
• Check the antifreeze. The mixture protects the engine from freezing, and engine coolant is also important for cabin heat.
• Keep the tank at least half full. It reduces moisture and gives a safety buffer if you are stuck in traffic.
• Carry jumper cables or a portable booster pack, and learn the safe connection order. Follow your owner’s manual for the correct order of connections.
• Consider a battery maintainer if the car sits for long periods, as cold and short trips can leave the battery undercharged.

Some regions have block heaters, which warm the engine or coolant when plugged into an outlet. They are common where winters are severe. If your car has one, or you are considering installing one, a mechanic can advise.

A sensible starting routine

1. Turn off power-hungry accessories before starting: heater fan, lights, rear window heater, seat heaters and the stereo. This leaves all the battery’s power for the starter.
2. Turn the key or press the button and let the starter run. In most modern cars, you do not need to pump the accelerator. Check the manual for any special procedure, for example for diesels with glow plugs, where you may need to wait for a light to go out.
3. Crank in short bursts. Run the starter for no more than about ten seconds at a time, then wait at least half a minute before trying again. Long attempts can overheat the starter and drain the battery. If the car does not start after a few tries, stop and investigate.
4. Let the engine settle. Once running, give it a short time for oil to circulate. Prolonged idling is unnecessary for most modern cars, and gentle driving warms the engine more quickly than sitting still. Avoid hard acceleration until the temperature gauge begins to rise.
5. Clear the windows and check that everything is visible before you drive. The advice on defrosting windows covers how to do it safely.

How long should you warm it up?

Older cars with carburetors needed a long warm-up. Modern fuel-injected engines usually need only about half a minute or so before driving gently. Excessive idling wastes fuel and can leave residues in the engine, and it’s not good for the environment. Some places also restrict idling by law, so check your local rules. In extremely cold conditions, a little longer may help, and your owner’s manual is the best guide.

What to avoid

• Do not use starting fluid (ether spray) unless your owner’s manual and a qualified mechanic say it is suitable. It can damage some engines and is flammable.
• Do not pour hot water over a frozen engine or windshield.
• Do not keep cranking a car that is clearly struggling. Repeated attempts can flood the engine or deplete the battery.
• Do not leave a car running in a closed garage. Exhaust contains carbon monoxide, which is dangerous and can be fatal. Ventilate fully before running any engine.
• Do not ignore warning signs, such as slow cranking, clicking, dim lights or a dashboard that flickers.

If it will not start

Listen for clues. A rapid clicking sound often points to a weak battery. A single click or no sound can mean a failing starter or loose connection. A car that cranks normally but does not fire might have a fuel or ignition issue. A basic checklist for an engine not starting can help narrow things down.

If the battery seems low, a jump start can get you going. Once running, drive for a while or have the charging system checked, since a battery may not recharge fully on short trips. A charging warning on the dashboard indicates a problem with the charging system.

If you cannot get going safely, roadside assistance or a mechanic is the next step. Make sure you are in a safe place and do not take risks with a frozen or leaking fuel system.

Electric and hybrid cars

Electric cars have no oil or starter motor, but cold still affects their battery, reducing available range temporarily and slowing charging. Many can preheat the cabin and battery while plugged in. Electric range in winter depends on how much heating you use. A hybrid has both systems, and the small 12-volt battery that runs accessories can still fail in the cold.