When a Fuel pump fails, or gets to a point where it should be replaced, what are some of the symptoms?

The engine in my car runs great from a full tank, till a little bit past a quarter. beyond that, the idle is lumpy, power is slow and anything but smooth.

I'll put it this way, I can haul ass up a hill in 3rd, with a FULL tank, but at 1/4 I'm downshifting just to maintain 50Km/h.

EVERY tank, and I run 91

Is it possible that my fuel pump is getting weak, and with the added pressure against the pump with a full tank, it works well, but it's ability to deliver the correct amount of fuel pressure diminishes as the weight of the fuel in the tank decreases?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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