If the drive seal/seals dislodge or pop out, it is due to an excessive build-up of internal fuel pressure in the high-pressure pump. The cause is either a blockage of the return from the pump to the tank or a problem with the Volume control/M prop. Valve.
Check the following.
Blocked/restricted return pipe.
The return pipe/pipes are all clear and not restricted.
The 3-way valve that connects the pump return and injector return to the tank is operational.
Volume Control Valve/M. Prop Valve.
Replace the valve.
Check with diagnostics what the rail pressure is at idle. Remove the electrical connection to the valve and see if the rail pressure increases. With no current to the valve, the pump should generate maximum rail pressure.
Check if you have an electrical input to the valve while the engine is idling.