[Gunter]

Dirty fuel would clog up the filter.
DO NOT use the new pump to get the fuel out of the tank, the pump is very sensitive to dirt.
Use gravity to empty the tank.
If the new pump has an external check valve, unscrew and clean.
To make sure that the tank is clean, empty the tank and open the cover on top that holds the fuel level sender and use a flash light to look at the bottom.
Careful! Have a fire extinguisher nearby.
Fuel delivery components:
Pump delivers to Accumulator which is next to the filter, then into the filter, then into the fuel distributor.
Flush out the lines from the pump to the Accumulator, disconnect and empty out the Accumulator, disconnect and clean all fuel lines.
There is a tiny screen in the line on top of the WUR, open the line and check/clean.
The injectors also have a tiny screen inside.
Consider disconnecting the injector lines for cleaning, then soak the injectors overnight fully submerged in injector cleaner, blow out with air.

It's a lot of work but needs to be done.
Get a new fuel filter.
I still recommend the Bentley, lots of good info you'll need.