Date: Sat, 18 Aug 2012 15:07:11 -0400
Reply-To: David Beierl <dbeierl@ATTGLOBAL.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: David Beierl <dbeierl@ATTGLOBAL.NET>
Subject: Re: Cheap and easy fuel pressure gauge?
In-Reply-To: <502FC453.8090106@gmail.com>
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At 12:35 PM 8/18/2012, Rocket J Squirrel wrote:
>suspect there are less-expensive alternatives. Generic FLAPS unit or
>something?
Less expensive, yes - in my tiny experience VW special tools come
with the dealership purchase. My dealer didn't seem so much
disinclined to sell me a {I-forget}, as to be puzzled as to how to get it.
Any pressure gauge that will measure 50 psi and won't dissolve in
gasoline will do nicely, with whatever plumbing it takes to accept a
piece of fuel hose. US Gauge makes nice ones. If you have a well
pump you could probably borrow the one off the tank/switch
arrangement temporarily. Mrs. Squirrel might have something to say
about the taint of gasoline in your water later.
What you're really looking for here is reserve pressure capacity,
which you're not going to find with a regular Bentley-type
test. What you'd like to know is the highest pressure that the pump
can push a liter per minute against, and if it's a good pump that's
probably over 100 psi. I wouldn't care to subject the fuel lines to
that without a very good reason, so that means pulling the pump to
test on the bench, with an outlet valve that can be closed gradually
against the pressure. Completely closing off the outlet might stall
the pump, not the best idea.
As a quick-and-dirty, pinching the outlet line from the fuel pressure
regulator while watching the gauge ought to give you an idea of
whether it's happy with additional restriction (and possibly uncover
a weakness in your hose-clamping if you pinch too enthusiastically -
the actual hoses/pipes ought to be ok. Might blow the end off a fuel
rail if the retaining band is on the edge of letting go. The system
*was* designed with full knowledge of the pump's high-pressure
abilities in a pinch, so to say).
If the pump is so worn-out that, far from delivering a liter per
minute against pressure it can barely supply the engine's needs, that
could perhaps leave you open to vapor lock troubles. A precarious
situation though.
Suction pressure is limited by the vapor pressure of gasoline, which
is why suction-line restrictions will cause cavitation and that very
distinct buzzing noise (which I suspect will go away if the line is
completely blocked). Cavitation bubbles won't survive on the outlet
side of the pump, so cavitation as such isn't your issue.
This is a roller pump, incidentally (not the peristaltic kind of
roller pump, this is related to vane pumps). About 50 seconds into
this video
http://www.hypropumps.com/FileAttachments/spray/en-us/Service_Videos/Roller_Pumps/1_7560_Series_Disassembly.wmv
is a nice view of the chamber and roller assembly on a much larger
agricultural spray pump. Each roller seals the open end of its slot
against the chamber walls; the open area of the slots actually carry
the fluid from the inlet around to the outlet side of the pump, which
for minimum internal leakage will probably be 180 degrees
away. Compared to this one the chamber and rollers in the Bosch pump
are much shorter, about equal length vs diameter on the rollers; and
it spins at thousands of rpm with its internal motor, instead of
hundreds from a tractor's PTO coupling.
Yours,
David