It depends on the engine fitted, if the PA28 is NOT a turbo it will be fitted with a Lycoming, this fuel injection system is a derivative of the pressure carburetor and requires the electric pump to be on during critical phases of flight as fuel metering is unaffected by extra pressure delivered by the electric fuel pump.
Aircraft fitted with the Continental engine have an engine driven pump that provides a basic metered flow ( trimmed by other means) if you use the electric pump in flight under normal operation it will result in the delivery of too much fuel and the engine is likely to stop.
On these engines the electric pump is used only for priming or if the engine driven pump fails, even under under an EDP failure situation the pump provides a very rough and ready fuel flow, the engine runs but not very well.
Last edited by A and C; 23rd Oct 2012 at 08:56.