The invention relates to a method for operating a glow plug with running engine, with the engine having a crankshaft and at least one cylinder. An effective voltage is generated from a vehicle electrical system voltage, with the effective voltage being applied to the glow plug. A plurality of measurement values of the combustion chamber pressure prevalent in the cylinder is measured during each working cycle of the cylinder, with the angular position of the crankshaft being determined for each individual measurement value. A characteristic value of the combustion process is determined from the evolution of the combustion chamber pressure measured in relation to the angular position of the crankshaft, and the determined characteristic value is compared with a setpoint value, and the effective voltage is set to a minimum value required for reaching the setpoint value.