Originally Posted by lr172
One thing to consider, is that fuel tank fill height will change that amount of suction required to pull fuel. The fuel pump draws a good bit less suction when pulling fuel from a full tank than a near empty tank. Something to consider when reviewing symptoms. I would expect a small air leak to show more significant symptoms on an empty tank compared to a full one.
YES. The suction head available is related to the head of liquid in the tank. The problem should be worse when the tank level is low and it might appear to be non-existent with a full tank.
A combination of a filter introducing an excessive dynamic head loss (pressure drop when flowing) and a low tank level will result in too low a suction head at the pump.
This will be particularly evident for a positive displacement pump, like the engine driven fuel pump. If the suction head is too low, a volatile fluid such as gasoline will tend to vaporize on the pump's intake stroke.
A pump with an internal recirculation may not show this, as some fluid on the discharge side will return to the suction side and increase the average suction pressure. Think of a jet style well pump here. I'm not certain of the configurations of the various electric fuel pumps, but it's likely they have some sort of internal recirculation setup to allow flow past the pump if it fails/is turned off.