FAA Airworthiness Directive
The optional electric fuel pump should be installed after the fine filter (to protect the pump) and before the mechanical pump. The pump should then be operated just before engine start and during maneuvers, take off and landings. In case of engine failure you will switch the pump on too.
Make sure that you do not raise fuel pressure to high for the mechanical fuel pump as a fuel leak could occur.
Excerpt from the document:
Due to high fuel pressure, caused by exceeding pressure in front of the mechanical fuel pump (e.g. due to an electrical fuel pump), in limited cases a deviation in the fuel supply could occur. This can result in exceeding of the fuel pressure and might cause engine malfunction and/or massive fuel leakage.
We are issuing this AD to prevent the pump from exceeding the fuel pressure, which could result in engine malfunction or a massive fuel leak. These conditions could cause loss of control of the airplane or a fire.
To continue reading see the pdf below: