Quote:
Originally Posted by Cantray
On this point, can anyone explain why the UBP can cope without help from the the in-tank pump when the tank is full, but not when the level in the tank falls?
will post again when fixed.
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There are two valves on top of the in tank pump - one is normally closed and is an overpressure bypass, and the other is normally open and is a priming valve which closes when the ITP is running as it is closed by fuel pressure from the pump.
When the tank level falls below the top of the ITP it exposes the priming valve, and if the ITP isn't running it is still open so all the UBP sucks is air.