Question: do glow plugs stay on all the time

Answer: No, In older generation diesel-engine vehicles, unlike in a gasoline-engine vehicle, for a 'cold start' the operator did not simply turn the key to the ``start'' position and have the engine immediately start. Instead, the operator activated the glow plug(s) for a time first. Early diesels used the 'Thermostart' type glowplug in the inlet manifold. These take 20 seconds to achieve working temperature and the vehicle operator had to manually time (or guess) when the 20 seconds had elapsed. With in-cylinder glow plugs, technological improvements included a warning light on the dash to indicate how long the preheating should last. The preheating phase was also made to be automatically activated when the operator turned the key to the ``on'' position for a long duration; the glowplug relay switches the glowplugs on, and a light (see picture at right) on the instrument cluster illuminates. This process is called ``pre-heating'' or ``glowing''. Many modern diesels automatically activate their glow plugs when the operator unlocks the vehicle or opens the door to the car, thus simplifying the process and shortening the time the operator has to wait before the engine will start. According to Bosch: ``Older engines with Thermostart manifold plugs used a glow period of up to 20 seconds whereas more modern engines use around a 6 to 8 second heat period and provide after glow at a reduced voltage.''