Apparatus (201) for testing fuel flow in a return or returnless fuel system. The apparatus comprises a housing (205), a primary flow passage (209) in the housing having an inlet (211) for connection to the fuel system for receiving fuel pumped by the fuel pump (41), and an outlet (213) adapted for connection to the fuel system for flow of fuel to one or more fuel injectors (21). The apparatus includes a first device (221) for measuring a rate of fuel flow through the primary flow passage, and a second device (227) for measuring a fuel pressure in the primary flow passage. A bypass valve (255) in the primary flow passage is movable between a first position in which a bypass passage (231) is closed and the primary flow passage is open to permit the flow of fuel to the outlet of the primary flow passage and a second position in which the bypass passage is open and the primary flow passage is closed whereby fuel is directed through a bypass passage for transfer to a fuel receptacle. The bypass valve is movable to its first position for testing fuel flow in a return flow system and to its second position for testing fuel flow in a returnless fuel flow system.