I think the chief reason they are in the tank is to prevent vapor
lock. If the pumps are external they have to pull fuel through a line
from the tank to the pump. This "suction" lowers the fuel pressure and
can lead to vaporization in the lines, followed by vapor lock, and bad
things. By placing the pump near the bottom of the tank, there is very
little pressure drop from the fuel inlet to the pump, so little chance
for vapor lock.
You could pull a line off the bottom of the tank to accomplish the
same thing, EXCEPT, this would be a bad idea since everything in the
bottom of the tank would go directly into the bottom opening. In the
real world, the pump is slightly off the bottom, allowing water, rust,
dirt, whatever to fall to the bottom and not be picked up.
My sister had a 1986 Jetta. It had two fuel pumps, an internal and an
external. The external pump was a high pressure positive displacement
pump needed to feed the Bosch CIS fuel injection system. The in-tank
pump feed the external pump. She had the in-tank pump fail twice. Both
times it was in hot weather. The car would start fine in the morning,
but the first time you stopped in traffic during the heat of the day,
the car would vapor lock and stall.
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