A coolant control system of a vehicle includes a coolant pump that pumps coolant to a second radiator that is different than a first radiator that receives coolant from an engine of the vehicle. A diesel exhaust fluid (DEF) injector injects a DEF into an exhaust system and receives coolant output from the second radiator. A fuel heat exchanger transfers heat between coolant and fuel flowing therethrough. An engine control module is configured to determine a temperature of the DEF injector, control a duty cycle of the coolant pump, determine a vaporized condition of the coolant based on a DEF injector temperature, optionally further, in response to determining a vaporized condition of the coolant, implement a vapor purge by oscillating the duty cycle of the coolant pump, and optionally further identify a low-coolant condition of the coolant control system based on the vapor purges implemented during a time period.