I would guess that you have a loose/pinched and/or lockring/o-ring.
Hope some of this helps.
SECTION 310-00: Fuel System — General Information 2007 Mustang Workshop Manual
GENERAL PROCEDURES Procedure revision date: 06/19/2006
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Fuel Tank Draining Printable View (311 KB)
Special Tool(s) Fuel Storage Tanker
164-R3202 or equivalent
Socket, Fuel Tank Sender Unit
310-D006 (D90P-9275-A)
Release the fuel system pressure. For additional information, refer to Fuel System Pressure Release in this section.
Disconnect the battery ground cable. For additional information, refer to Section 414-01 .
Release the fuel tank filler cap and position aside.
Insert a suitable fuel drain tube into the fuel tank filler pipe until it reaches the fuel tank inlet tube.
CAUTION: The fuel pump (FP) module and the fuel level sensor will be below the fuel level in the tank when the tank is full. Make sure to drain 1/8 of the fuel capacity through the filler pipe prior to removing the FP module or fuel spillage will occur.
NOTE: When draining a completely full fuel tank, this step will remove approximately 1/8th of fuel from the LH side of the fuel tank.
Attach the fuel storage tanker to the fuel drain tube and remove as much fuel as possible from the fuel tank filler pipe.
Remove the rear seat cushion. For additional information, refer to Section 501-10 .
Position any insulation padding covering the FP module access cover aside.
Remove the FP module access cover and position aside.
NOTE: 4.0L and 4.6L fuel pump (FP) module shown, 5.4L similar.
Disconnect the FP module electrical connector.
NOTE: Place absorbant pads in the general work area in case of fuel spillage.
Disconnect the rear fuel supply tube-to-FP module quick connect coupling. For additional information, refer to Section 310-00 .
Using the special tool, remove the FP module retaining lock ring.
Position the FP module aside. Using the fuel storage tanker and a suitable fuel drain tube, completely drain the LH side of the fuel tank.
CAUTION: The fuel pump (FP) module must be handled carefully to avoid damage to the float arm, filter and the convolute hoses.
Carefully lift the FP module out of the fuel tank enough to access the fuel cross over tube-to-FP module quick connect coupling.
Disconnect the fuel cross over tube-to-FP module quick connect coupling. For additional information, refer to Section 310-00 .
CAUTION: The fuel pump (FP) module must be removed and handled carefully to avoid damage to the float arm.
Completely remove the FP module from the fuel tank.
Insert a suitable fuel drain tube into the cross over fuel tube quick connect coupling.
Attach the fuel storage tanker to the fuel drain tube and completely drain the remainder of the fuel from the RH side of the fuel tank.
