Combustion engines, such as diesel or gasoline engines, may employ emission control systems that are configured to reduce the level of pollutants emitted from the engine. Some combustion engines utilize exhaust gas recirculation (EGR) systems for reducing nitrogen oxide (NOx) emissions. The EGR system diverts exhaust gas through a cooling mechanism, such as an EGR cooler, and recirculates the cooled engine exhaust gas back into the combustion chamber of the engine to reduce the volume of air and/or other combustible matter in the cylinder or the pre-combustion mixture. By replacing a portion of the oxygen or combustible material present during the combustion process with cooled, spent exhaust gas, EGR systems may reduce the temperatures obtained during the combustion process in the engine and thereby reduce the amount of NOx that is generated by the combustion and emitted from the engine.
Vehicle on-board diagnostics (OBD) including EGR flow diagnostics are enabled and run continuously when the engine is operating until such time a “hot lamp” is lit on the vehicle's instrument panel, indicating to the vehicle operator a hot lamp set-point has been exceeded. The hot lamp set-point typically corresponds to a hot engine coolant temperature limit, where the temperature of the coolant in the engine (the engine coolant) is measured while the engine coolant is in the engine or proximate to the engine, for example, in a thermostat housing attached to the engine, and compared to the hot engine coolant temperature limit, for example, by a controller in communication with the engine coolant temperature sensor. When the hot lamp set-point is exceeded, e.g., when the measured engine coolant temperature exceeds the hot engine coolant temperature limit, the engine controller disables EGR usage and the OBD system generates a diagnostic trouble code (DTC) which is recorded by the OBD system to indicate EGR usage has been disabled. The hot lamp set-point may correspond to a engine coolant temperature limit established based on the configuration of the engine, the type of coolant and coolant material characteristics, and other factors such as a engine coolant temperature level which has been determined to correspond to probable localized boiling of coolant in the EGR cooler, where localized boiling of the coolant in the EGR cooler may be detrimental to the EGR cooler by generating thermal excursions in the EGR cooler which can induce thermal fatigue of the cooler. The hot lamp set-point may, therefore, correspond to a hot engine coolant temperature threshold which is limited to the engine coolant temperature level corresponding to coolant boiling in the EGR cooler, where the temperature limit triggering the hot lamp is set below a maximum coolant operating temperature and/or below a maximum engine operating temperature, thereby limiting the operating engine coolant temperature range in which EGR usage may be enabled to a hot lamp set-point which is below the maximum coolant operating temperature, such that EGR usage can be disabled at a hot lamp set-point when EGR coolant boiling is probable without triggering lighting of a malfunction indicator light (MIL). Adjusting the hot lamp set-point to correspond to an engine coolant temperature limit above the engine coolant temperature level corresponding to localized boiling of coolant in the EGR cooler increases the probability of EGR damage in use, and/or requires disabling EGR usage below the hot lamp set-point to prevent localized boiling of coolant in the EGR cooler which would disable EGR related diagnostics of the OBD system at engine coolant temperatures below the hot lamp set-point.