Glow plugs are used to provide heat for combustion in compression ignition (CI) engines - Typically Diesel engines, but also small engines for radio controlled planes and cars which typically burn nitromethane. They consist of a resistive coil through which a current is passed, causing them to heat up. Their operation is completely different from a spark plug, as the engines in which they are used operate on different principles. In a spark ignition engine, the air/fuel mixture is compressed to a point in which rapid combustion can occur, at which point a spark is used to ignite it. In a compression ignition engine, the mixture is compressed in a hot chamber until it ignites on its own due to the heat of compression. Consequently, compression ratios are much higher in CI engines; gasoline engines are typically below 10:1, while diesels are usually over 20:1.
Glow plugs for diesel engines are screwed into a threaded hole in the side of the cylinder head or engine block that passes into the combustion chamber. Their energy consumption is generally self-regulating in that as they heat up their internal resistance increases, therefore limiting current. Diesel engine glow plugs are generally grounded through the engine, and as such require only one wire. They usually are only powered before starting; a light on the dashboard is generally illuminated until they are hot, and then goes out, informing you that you may start the engine. Some vehicles continue to power the glow plugs for several minutes after starting to assist the engine in burning fuel before the engine heats up. This decreases the amount of unburned fuel passed out of the engine while it is warming up, decreasing hydrocarbon emissions and reducing the amount of black smoke that comes out of the exhaust.
R/C engine glow plugs actually retain a great deal of heat and provide primary ignition temperature while the engine is running. They are usually initially heated through the use of a 1.5 volt battery, usually a C or sub-C cell, in a "starter" unit that clips onto the top of the glow plug. "Gas" vehicles with an electric start system (for example, most Traxxas RTR (ready-to-run) vehicles) have a lead running from the starter system to the glow plug, grounded through the motor like a diesel, that engages the plug while the starter button is depressed.
Failing glow plugs can be indicated by a "hard start" condition, but a better indication is when one cylinder is "missing" when the engine is cold, but the engine runs smoothly when warmed up. Glow plugs can be tested by comparing their internal resistance to one another, which can be done without removing them from the engine, by disconnecting the one wire from their exposed terminal and measuring resistance from there to the engine block or head. Abnormally low resistance indicates a full or partial short. Abnormally high resistance or infinite resistance indicates a broken glow plug. In either case, the plug should be replaced. Many glow plugs cost less than five dollars a piece, and in fact are typically cheaper than a so-called "premium" spark plug.