The return line from the injector pump is also attached to the top of the filter via an inline "tee-piece", held in place on the filter by a spring clip. The tee-piece is a temperature sensitive valve that opens when the fuel temperature is below about 15 C / 60 F, and closes fully when above 25 C / 80 F - the purpose of this is to supply some heat to the filter during cold conditions, to resist the tendency for cold diesel fuel to "gel".