A fuel injector includes an injector housing having a plunger assembly disposed within a central axial bore and including a lower plunger, an intermediate plunger, and an upper plunger. The lower plunger reciprocates within the central bore to meter a variable quantity of fuel and to inject this fuel during downward portions of the reciprocating motion. A timing chamber formed between the upper and intermediate plungers receives an amount of timing fluid to create a variable length hydraulic link between the plungers. The fluid is drained from the chamber through a drain passage at the end of the injection cycle. The timing chamber has associated with it a mechanism for changing the effective flow function of the drain passage, at a point in the cycle following fuel injection, to maintain a large downward pressure on the lower plunger to hold the lower plunger down and thus prevent additional, undesired fuel flow into the combustion chamber. In a preferred embodiment, the drain passage is located along the central axis of the intermediate plunger, and the upper plunger is provided with a projecting portion which moves into position as the timing chamber collapses to effectively constrict the drain passage, thus changing the timing fluid flow function to a function which provides greater fluid pressure in the timing chamber.