A cold start is an attempt to start a vehicle's engine when it is cold, relative to its normal operating temperature, often due to normal cold weather. A cold start situation is commonplace, as weather conditions in most climates will naturally be at a lower temperature than the typical operating temperature of an engine. Occasionally, the term also refers to starting the engine of a vehicle that has been inactive or abandoned for a significant amount of time such as months, years or even decades.

Causes of cold starts

Cold starts are more difficult than starting a vehicle that has been run recently (typically between 90 minutes and 2 hours). More effort is needed to turn over a cold engine for multiple reasons:

• The engine compression is higher as the lack of heat makes ignition more difficult
• Low temperatures cause engine oil to become more viscous, making it more difficult to circulate.
• Air becomes more dense the cooler it is. This affects the air-fuel ratio, which in turn affects the flammability of the mixture.

Solutions to cold starting

The problem of cold starting has been greatly reduced since the introduction of engine starters, which are now commonplace on all modern vehicles. The higher revs that can be achieved using electric starter motors improves the chance of successful ignition.

Starting fluid, a volatile liquid, is sometimes sprayed into the combustion chamber of an engine to assist the starting procedure.

Diesel engines make use of glow plugs to heat the combustion chamber prior to ignition, improving the conditions inside the engine, while certain manufacturers have incorporated a block heater, which heats the engine block prior to ignition to reduce the problem of cold starting.