Hi All,

There was some discussion about fuel pressure regulator (FPR) testing and
manifold vacuum (MV) a couple of weeks ago on the list.  My impression from
that discussion was that people typically evaluated the performance of
their FPR by measuring fuel pressure on the low pressure side of the system
(i.e. at the test tee).  Two measurements are made:  one with the vacuum
hose connected to the FPR and one with the vacuum hose disconnected from
the FPR and plugged.  Bentley provides a spec. for these two measurements:
28 psig and 36 psig, respectively (IIRC).

The problem with this spec. is that we don't know (okay, I don't
know--maybe you do) what idle MV is for this spec.  As MV decreases, the
fuel pressure at the tee will increase.  In other words, if you have an MV
leak and perform this test, you'll measure something higher than 28 psig
for the hose connected test, even if your FPR is mechanically sound.

Frank Grunthaner suggested that the 10 to 12" of vacuum many of us consider
"normal" for our wasserboxers seems too low and that at least 15" is more
typical for modern gasoline engines.

Sooo... I performed an experiment to find out what the test tee fuel
pressure would be if my manifold vacuum was 15".  I did this by
disconnecting the FPR vacuum hose and plugging it.  Then I connected my
MityVac brake bleeder/vacuum diagnosis tool.  This tool is a simple
hand-actuated vacuum pump that can pull up to 30" of vacuum.  It can also
function as a simple vacuum gauge.  (The vacuum gauge on this new tool has
a stated accuracy of 2-3% but I cannot guarantee its calibration. )  I
measured 28 psig at the fuel line test tee with 15" vacuum pulling on the
FPR.

If I assume my FPR is mechanically fit (I get something like 36.8 with the
vacuum hose disconnected at sea level) then that would lead me to conclude
that a wasserboxer in perfect mechanical condition would pull around 15" MV
at idle (and thus yield 28 psig fuel pressure at the test tee).

Agree or disagree?  This is definitely not my field of expertise.  I could
have made some false assumptions or illogical conclusions.  Your comments
are welcome.

Scott Terry
'85 1.9L Westy "Pearl"
Santa Barbara, CA
Still parking next to other Westys despite the danger...