compares the calculated vacuum to the actual system pressure if the engine is determined to be in a sensitive fuel control state i e low fuel consumption the purge flow is reset and begins to slowly increase flow thereby slowly drawing vapor from the fuel tank 12 and avoiding a rich engine condition the system and method of the present invention can best be described by an example of the operation of an engine as it cycles from a normal load to a heavy load and back to a normal load fig2 through 10 represent aspects of the engine system as the engine load is cycled over time the x axis in each of the graphs is representative of time measured in seconds referring to fig2 the engine load 36 in rpm 39 s is shown a normal i e light to moderate engine load 36 a is shown at about 700 rpm 39 s for a period of about five 5 seconds after about five 5 seconds the engine load is increased rather rapidly to a heavy load 36 b around 2000 rpm 39 s and held for about ten 10 seconds the engine load returns to normal 36 c at about fifteen 15 seconds on the graph 36 in general fig3 through 10 are 25 graphical representations of how the system reacts to the change in engine load shown in fig2 fig3 is a representation of the intake manifold vacuum 38 in inches of mercury as it corresponds to the changes in the engine load as shown by the first.