[Vwdiesel] Pump pressures, banjo jet sizes and injector leakage...

[Sandy Cameron]

At 01:21 AM 22/10/04 +0100, you wrote: 
>Moving on to Hagar's method of measuring pump pressure; I
>dont see how it can be measured accurately (absolute
>pressure) when its the removed banjo bolt that has the
>metered bleed hole in it that creates the pressure in the
>pump. Replacement with a drilled out bolt would change pump
>pressure. Of course it could be used to compare pumps but
>this wouldnt help those of us with only one pump unless the
>'home kit' was passed around the members as required and the
>data thus strengthened by cumulative readings....

I think the way Hagar did it was to use a bolt long enough to accomodate 2
banjoes (?)
 One was an input type, open free flow, to which he attached the guage, and
the other was the output "metering" banjo with the tiny hole.

The transfer pump is capable of much more volume than the tiny hole can
pass, and the pressure inside the pump is controled by a spring loaded ball
relief valve on the top at the sprocket end of the pump that allows the
excess fuel to recirculate back around the transfer pump (the vane pump) ,
maintaining a constant pump internal pressure, not much affected by speed.
There is some pressure change with speed change, , and that is used to
operate the timing advance, but I don't think it's affected by the small
size of the hole in the return banjo.
That's there mostly to bleed air from the pump in an ongoing basis (thank
goodness!), and to some extent, providing cooling for the pump and heat for
the filter (in later models, after 88) and the tank. The tank pickup/guage
unit is built in such a way that the returned fuel passes into a small
chamber near the bottom of the tank, to mix with the fuel traveling to the
front of the car, warming it a little, and helping limit cold weather
gelling in the lines.

Sandy