A liquid fuel detection system for a fuel vapor system of a vehicle providing fuel vapor to an engine operating in closed loop includes an oxygen sensor that generates an oxygen signal based on an oxygen level in engine exhaust. An engine speed sensor generates a speed signal based on a speed of the engine. And a control module receives the oxygen signal and the speed signal, determines a fuel control factor based on the oxygen signal, determines a long term modifier based on long term changes of the fuel control factor, and detects the presence of liquid fuel in the fuel vapor system based on the fuel control factor, the speed signal, and the long term modifier.