How does ECU determine actual fuel parameters?

Does anybody know how the ECU on a Pump Deuse engine determines the "injection Quantity", and individual cyclinder fuel injection values that it reports for each cylinder? I know with air flow that you have a Mass Air Flow sensor that actually measures the air flow to the engine so that the Engine Computer just reports an actual measured value. With fuel flow however, there is no actual fuel measurement device that I am aware of so does the ECU just report what it is telling the fuel injection system to deliver (I would assume that it looks at the position of the gas pedal & tells the injectors how much fuel to deliver based on that). The ECU also has values for fuel delivery to each individual cylinder so how could it attempt to bias these or even them out if there is no actual fuel measurement device to base this decision upon?

Anybody have any words of wisdom on this? I know that with the Pump Deuse injectors that each time that the camshaft actuates a fuel injector that this injector gets moved through it's full stroke. The only way that the ECU can control how much fuel gets injected into the cylinder and how much gets returned to the fuel returned is by returning more to inject less. Am I correct in assuming that because there is no fuel flow measurement device that "Injection Quantity (mg/str)" just gets reported based on what the computer is expecting the injectors to deliver? I still don't see how the values for fuel injection to each individual cylinder could be calculated or measured. The reason that I am curious about this is because I am having a stalling at idle problem with my 2004 BEW PD Jetta that another TDI member in Texas is having. He caught a stall at idle with Vagcom logging various data & the one parameter that went from normal to zero while the engine was still coasting down was the fuel injection quantity. The problem that we are both having is a very, very intermittent problem (only does once out of about 20 times when I am at idle). Right now I am thinking it might be due to the tandem pump (we both have Luk tandem pumps which have sliding vane pump internals).

I have not much deep knowledge but I think it uses Duration of injection to "measure". There are no metering device...

All the ECU does is control the duration of injection electrically.
It does monitor for normal operation by using the shape of the current flow. This lets it determine if fuel is present or not, or if the solenoid valve has actually moved or not.

But it does not directly measure fuel flow.

There is a good write up in the training pdf on this, about page 45.

Search for the PDF "

1.9-Liter TDI Engine
with Pump Injection
(Pumpe Düse)​

Design and Function"

Thank You DanG144, the write up in that PDF File http://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf
does give an excellent description of the operation of the injectors, the tandem pump, and what the ECU is using to report injection values. I wouldn't have guessed that they could use the current flow to the injector solenoid to determine this.

"Pump/injector solenoid valve monitoring
The Diesel Direct Fuel Injection Engine
Control Module J248 monitors the electrical
current that actuates the solenoid valves at
the pump/injectors.
This provides feedback to the Diesel
Direct Fuel Injection Engine Control Module
J248 of the actual point in time when
injection begins.
The Diesel Direct Fuel Injection Engine
Control Module J248 uses this feedback to
regulate the beginning of injection periods
(BIP) during subsequent combustion
cycles and to detect malfunctions of the
pump/injector solenoid valves.
Start of injection is initiated when the
pump/injector solenoid valve is actuated.
Actuating current applied to a pump/injector
solenoid valve creates a magnetic field.
As the applied current intensity increases,
the valve closes; the magnetic coil presses
the solenoid valve needle into its valve
seat. This closes off the path from the
fuel supply line to the pump/injector
high-pressure chamber and the injection
period begins.
As the solenoid valve needle contacts its
valve seat, the distinctive signature of an
alternately dropping and rising current flow
is detected by the Diesel Direct Fuel
Injection Engine Control Module J248. This
point is called the beginning of injection
period (BIP). It indicates the complete
closure of the pump/injector solenoid valve
and the starting point of fuel delivery.
“Start of injection” is the point
in time when the actuating
current to the pump/injector
solenoid valve is initiated.
”Beginning of injection
period (BIP)” is the point in time
when the solenoid valve needle
contacts the valve seat.
With the solenoid valve closed, a holding
current is maintained at a constant level by
the Diesel Direct Fuel Injection Engine
Control Module J248 to keep it closed. Once
the required time period for fuel delivery has
elapsed, the actuating current is switched
off and the solenoid valve opens."
http://www.myarchive.us/richc/VW_TDI_with_PumpeDuse.pdf