In-tank Fuel Pumps
Bruce Buchanan says that the in-tank pumps were
fitted to 928s delivered to hot-climate markets to prevent cavitation of the
main fuel pump in hot weather.
My Australian-delivered '80 928 does not have an in-tank pump, although it
has two separate wires connected to each side of the main pump, so it may have
had one which was removed when it failed, and the wires attached to the place
where they were secure and couldn't do any harm.
Cavitation occurs when fuel vaporises in the fuel pump; this would occur on
the inlet side of the pump (in the wake of the impeller vanes), when the
pressure is low enough and the temperature is high enough for the fuel to vaporise.
The fuel vaporises as bubbles, so the impeller is subject to alternating
stresses as bubbles form and disappear. This manifests itself as noise, ie a
high-frequency vibration which will greatly accelerate the fatigue of metal
components.
The function of the in-tank pre-pump is to keep the inlet pressure at the
main pump high enough that vaporisation cannot occur, even at the elevated
temperatures a foot or so above our bitumen roads in summer.
When the in-tank pump fails, however, the main pump must draw fuel through
it. Happily, the main pump is up to the job for all but high fuel flows (high
load, high rpm) and its failure usually goes unnoticed (unless the failed pump
sheds debris into the main pump).
The significant pressure drop across a failed pre-pump greatly increases the
risk of cavitation, however, even in cooler weather. The main pump has to work
much harder and will fail sooner.
A failed in-tank pump, therefore, is worse than having no in-tank pump. - Glenn
Evans '80 928 Petrolblaumetallic 5 speed Sydney
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