Pull the secondary filter and carefully pour it off. Look for water at the bottom.

If you find water, you'll need to drain the fuel tank. If the W124 is like the W126, there will be a supply and return hose coming from underneath the vehicle directly above the differential. The supply hose has the tank screen above it which is evidenced by a 46mm nut above the hose.

Disconnect the supply hose at the metal line and run it into a container. Naturally, it's preferable to have minimal fuel in the tank.........or otherwise have sufficient container capacity to fully drain the tank.

You might wish to change the screen while you've got an empty tank if you can dig up a 46mm (1 13/16") socket.

Fill the tank with three gallons of fresh fuel and fill the secondary filter with fresh fuel before reinstalling it.