This concept is why the factory has a test spec for fuel delivery! If
you measure the flow of excess fuel returning to the tank you can tell
if there is more than enough flow for maximum engine fuel demand.

The fuel pump runs full blast 100% of the time. Most of this fuel runs
through the fuel rails and right back to the tank, passing through the
pressure regulator. Only a small amount is needed by the engine and gets
injected. As demand increases the amount returned to the tank is a
little less. As long as plenty is returning you can know that a
restriction isn't limiting the engine performance or the fuel pressure.

You don't measure the fuel output of the pump directly, you measure the
amount returning to the tank from the pressure regulator while the pump
is hot wired on. If this flow exceeds by a wide margin the amount the
engine could ever need, you are good to go.

VW says the return line flow should be 1 liter per minute.

Mark

neil N wrote:
> On Sat, Jun 27, 2009 at 10:41 AM, David Beierl<dbeierl@attglobal.net> wrote:
>> ......
>
>> If anything, flow would be reduced when the engine is working, since MAP
>> would rise causing a corresponding rise in regulated fuel pressure.
>
>
> Thanks David.
>
> Ok, so I had to google MAP....  :)
>
> I'm sure I could google these principles, but if you or someone has the time....
>
> Assuming:
>
> at higher engine RPM's, (intake) manifold absolute pressure rises,
> more vacuum applied to diaphragm side of FPR, more fuel "allowed" into
> fuel rail, more fuel used.
>
> Does this mean that the increased demand on what we assume is a
> constant rate of fuel flow, "pushes" ID of tank outlet past it's
> limits? i.e. ID should be larger (as later model year tanks are) to
> meet demands of this increased fuel use?
>
>
> Neil.
>
>
> --
> Neil Nicholson '81 VanaJetta 2.0 "Jaco"
>
> http://tubaneil.googlepages.com/
>
> http://groups.google.com/group/vanagons-with-vw-inline-4-cylinder-gas-engines
>