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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?
Comments: To: Rocket J Squirrel <camping.elliott@GMAIL.COM>
In-Reply-To:  <502FC453.8090106@gmail.com>
Content-Type: text/plain; charset="us-ascii"; format=flowed

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


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