Hyundai Sonata 1991 Shop Manual page 300

FUEL PUMP
Relief Valve
If, for some reason, fuel is not expelled at the discharge port,
the pressure within the fuel pump would increase. In such a
case, the relief valve opens in order to relieve the pressure,
preventing fuel pressure in the fuel line from rising above the
specified pressure.
Check Valve
When the pump stops, the check valve is closed by spring force,
thus assuring that residual pressure will remain in the fuel line.
By maintaining fuel pressure in this way, restarting of the engine
becomes easier. In addition vapour locks at high temperature are
prevented.
REPLACEMENT
1. Disconnect the fuel pump connector.
2.
Remove the fuel tank cap.
3. Drain the fuel.
4.
Disconnect the fuel high pressure hose.
Y311BOA
CAUTION
Cover the hose connection with rags to prevent splashing
of fuel that could be caused by residual pressure in the
fuel pipe line.
5. Remove the fuel pump installation screws and the bolt for
holding the fuel pump (at the lower side of the fuel tank),
and then remove it from the fuel tank.
6. Align the three POSitioning projections of the packing with
the holes in the fuel pump.
Positioning projection
31Y155
31-75