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Date:         Fri, 17 Aug 2001 18:43:38 -0700
Reply-To:     "Scott G. Terry" <sgterry@ENGINEERING.UCSB.EDU>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         "Scott G. Terry" <sgterry@ENGINEERING.UCSB.EDU>
Subject:      Re: fuel pressure regulator and manifold vacuum
Content-Type: text/plain; charset="us-ascii"

Fuel pressure at the test tee should ONLY be "dead on [the Bentley] spec" IF your manifold vacuum is "dead on spec" AND you are at sea level. If you don't have proper manifold vacuum you can buy all the new fuel pressure regulators (FPRs) you want but you'll never get 28 psig at idle. A functional FPR simply maintains the fuel pressure a constant 36 psi above manifold vacuum [as pointed out by Beierl].

That said, Michael's case is kind of special. He is about 3 psi ABOVE specs with the vacuum hose connected AND disconnected. Provided he doesn't live well below sea level, this SEEMS indicative of a faulty FPR rather than a manifestation of poor vacuum. If one has a good FPR but inadequate manifold vacuum the fuel pressure measured with the vacuum hose connected will be above spec, but the fuel pressure measured with the hose disconnected should be close to 36 psig at sea level.

Scott Terry '85 Westy "Pearl" Santa Barbara, CA

P.S. I think Michael meant to write 39 psi unconnected and 32 psi connected below? P.S.S. psig means psi "gauge". If your vacuum/pressure gauge reads -0- at sea level then the units are psig (g for gauge). If your instrument indicates +14.7 (psi) at sea level then the proper units are psia (a for absolute). PS3: Plugging the manifold vacuum hose during the hose-disconnected FPR test shouldn't affect the measured fuel pressure--just maintains manifold vacuum and keeps engine from surging, keeps engine happy.

Michael wrote: >> I have one of these vacuum guages and I have borrowed Robert Lilley's fuel >> pressure guage, so I will give your test a try next week. I measured it >> last weekend at 32 psi unconnected and 39 psi connected -- slightly too >> high but not enough to worry about, I hope. I forgot to plug the manifold >> vacuum hose, though, so I need to repeat this measurement. (What does the >> g stand for in psig?)

Matthew responded: >Wait! Slightly too high? Like how much? 3psi is about 10%... follow that >through and it means 10% more gas in the injectors.... It should be dead >one spec. A bit high (on two independent gauges) is not acceptable. I've >been down this long road before....


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