Why are cars sometimes harder to start when the weather is cold? Why does using the choke help? My car doesn’t seem to have one. --Bernard.
Mechanical things are profoundly unamused by the cold. As a chill dawn breaks, the oils in your car lie in cold and gooey pools in the bottom of your engine, gearbox, and differential. Much greater force is required to turn the cogs, shafts, and bearings through this thick morass than when the oils are hot, runny, and evenly smeared over every moving surface.
And the initial provider of that force, the battery that turns the starter motor, is also torpid. The electrolyte is cold, so the chemical reactions required to generate electricity are weaker and slower.
Still, if the oils are clean, the battery is strong, the starter motor is crisp, the electrical components are healthy, and the connections are good, your car will start perfectly well.
A car whose ignition system is in good condition might need only one bit of help on cold mornings. The choke. This is because the air-to-fuel ratio is set at about 15:1 - an ideal mixture for performance and economy when the car is up and running at normal operating temperature.
When the car is started from cold, a richer mixture (less air, more fuel) is needed for easy combustion, and using the choke reduces the amount of air to make the mixture “richer” in fuel. Many modern cars have "automatic" chokes (no lever on the dashboard), which make this adjustment for you.
Each car will differ in the amount of choke it needs to get started, but the general principle for those with driver-operated chokes is that, as soon as the engine is running, reduce the choke as quickly and completely as possible without spluttering or stalling.
Your car will still run adequately if you leave the choke in partial operation, but it is not good for your engine to do that for too long. The richer mixture degrades the oil on your cylinder walls, fouls your sparkplugs, and wastes fuel.
For all the preceding reasons, the sooner your engine gets up to normal operating temperature, the better, so once the engine has started, don't leave a cold engine just idling; it will warm faster if you work it immediately, but gently.
Apart from the engine, several other parts need to flex and warm up before they are used severely - the tyres and shock absorbers in particular, and even the brakes.
If, despite the use of a choke, your car does not start easily on cold mornings, then one or more of the elements of your ignition system are not in optimal condition. Battery, starter motor, spark (plugs, points, condenser, distributor cap (or Ei unit), HT leads, and coil.
A common presumption is that if the car starts perfectly well when it is warm, but won't start when it is cold, then the cold is the problem. Wrong. The cold has merely exposed a problem which exists at all times, but which is not severe enough to disrupt warm starting.
Another mistaken belief is that a car which starts easily in cold weather without the use of the choke is somehow in better shape than one which needs a little choke. If your car starts easily without a choke on cold mornings, the fuel: air mixture is possibly too rich (the same effect as having the choke very slightly engaged all the time).
If you have cold-starting problems, it is worth having the cause attended to. The weather is unlikely to get warmer for several months, and next on the meteorological agenda will be the short rains, and wet weather has many of the same effects as cold weather on how your car feels when you turn the key.
While on the subject of richer and leaner fuel mixtures, one of the simplest ways to judge whether your car's fuel mixture is correct is to look at the inside of the exhaust tailpipe (this applies to petrol engines only, not diesels).
Ideally, it should be majivu grey; very light after a high-speed long-distance journey; darker after prolonged stop-start driving at low speeds in heavy traffic. If the exhaust tailpipe colour ever gets so dark that it is almost black, then your fuel mixture is almost certainly too rich - a waste of fuel and additional engine wear. The buildup of soot in the exhaust will also be fouling things like piston crowns and valves.
If it ever gets so light that it is almost white, then your mixture is almost certainly too lean - this might make the engine run too hot and will certainly overheat the pistons and exhaust valves to a possibly damaging degree.
So, get set for the cold by having your car serviced – paying special attention to cleaning away all the wet-weather dirt, making sure no mud or moisture are trapped inside door panels, etc, and getting the ignition and tuning right for a fast start. If your engine takes more than three engine turns to fire up, there is room for improvement.
