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Glow Plugs
This page  gives a description about Glow Plugs, used in used in indirect injection Diesel engines. Around 90% of all cold starting problems are due to faults with glow plugs and incorrect preheating procedures.  Glow plugs operate when ignition is turned on prior to starting, during starting and for a short while after the engine has started. A glow warning light illuminates as soon as the ignition is turned on. When the warning light goes out, in theory the engine should start, some car manufacturers e.g. Ford the warning light goes out after a couple of seconds, in which case does give the glow plugs chance to preheat enough before starting of the engine, while other manufacturers e.g. Rover have the warning light on for 10seconds.
 Most Glow Plugs require 8 - 10 to fully preheat before the engine can be started. I personally allow 10 seconds from turning the ignition to allow the glow plugs to warm up enough before engaging starter motor,  This method ensures immediate cold starting even in freezing conditions


 Glow Plug pre heatedGlow Plug Cold

Glow Plugs may also be found on the modern small capacity Direct injections (e.g. TDdi & TDCi) for extreme cold starting conditions and aren't usually needed. For very cold countries, a block heater is fitted which pumps and keeps the engine coolant hot, it is plugged in to the electric but is not fitted to cars for the UK market