(Because it's important)
Let's talk about fuel pumps
The fuel pumps found in most OEM fuel delivery setups today are driven by an electric motor that is running at a fixed speed. This effectively fixes the flow rate and discharge pressure provided by the fuel pump.
The flow rate and pressure deviate over time due to several reasons which include:
- clogging of the inlet filter
- worn seals
- the brushes in the electric motor wear out (though this wouldn't explain a low flow rate)
Fuel regulators in a return-style setup
Without going into the nitty-gritty of how fuel regulators operate, the key thing to remember here is that it alters the amount of fuel returned back to the fuel tank to ensure that a fixed pressure drop is maintained across the fuel injectors.
This means that the classical relationship between pressure drop and flow rate is not respected. In fact, fuel rail pressure is almost constant regardless of flow rate.
Why measuring pressure isn't sufficient
A low fuel pressure measurement at the rail might indicate a clogged fuel filter, some obstruction in the fuel line or a fuel pump that just can't deliver a high enough pressure.
However, the pressure-stabilizing effect of the fuel regulator means that the fuel rail pressure measurement doesn't tell us anything about the adequacy of the fuel flow rate (from the perspective of the flow demanded by the fuel injectors).
This is the reason for the flow rate test; you can have good fuel rail pressure but insufficient flow.
For this test, my Bosch book says that most manufacturers specify:
"[fuel] delivery of one litre in 30 seconds or less, but check your manual."