In this paper, the fuel excursion and cyclic variation of the fuel/air equivalence ratio have been studied by means of a catalytic hot wire sonic nozzle probe. This probe, developed in our laboratory, is simple to construct and very sensitive to fuel/air equivalence ratio with a time response less than 400 μs. Transient fuel excursions were studied on an engine with multipoint fuel injection during different transient operations (starting, fast throttle opening and closing). The influence of the coolant temperature was also investigated. The measurement of cycle to cycle variation of the fuel/air equivalence ratio was carried out on both a multipoint stoichiometric fuel injection engine and a lean burn SI engine (direct fuel injection). The influences of engine speed, load and injection timing were investigated. The fuel used was unleaded gasoline and all the measurements were carried out without combustion.