Abstract: A filter system forming part of the fuel system of a compression ignition engine includes a housing which carries a filter element. The housing has a first inlet which opens into a chamber defined between the housing and chamber the first inlet being connected to a fuel tank, and a first outlet for fuel which has passed through the element and which is connected to a fuel injection pump. The housing has a second inlet which is connected to the chamber and receives heated fuel from the fuel pump and a second outlet which is connected to the chamber through a port. The second outlet is connected to the fuel tank. A third outlet is connected to the second inlet and is connected to a flame type starting aid of the engine. Valve means responsive to the temperature of the fuel in the chamber controls fuel flow through said port.