It's not quite that easy. The
original fitting depends on a steel ball bearing to mate up with the flared end
on the fuel rail, providing the seal. The threads are straight onto the rail, so
there's no sealing available from them. Some folks have put layers of Teflon
tape on the rail threads, hoping that would be enough to seal. It might be "good
enough" for a quick pressure test on a cold engine, but not suitable for
longer-term use, like using it to support FMU tuning under extended load.
I drilled and tapped a stock cap to accept a very common GM fuel pressure test
connector, and use a commonly-available gauge on a short hose when I need to
check fuel pressure. The GM fitting uses a 1/4-28 straight fitting, and I added
an o-ring where the hose now butts up against the cap. Inside the threaded rail
cap, a 1/4" brass tubing ferrule sits over the stub of the fitting that passes
through the cap, effectively replacing the ball-bearing duty while allowing fuel
to pass through to the gauge port.
Theoretically, since the ball bearing does the original sealing at the flared
end of the rail, you could easily drill & tap your existing rail cap, then
install it again with the original ball seal after testing is done.
Theoretically. For me the cap was cheap and an easy add-on to a parts pick-up
from 928 International, plus it gets used on others' cars a lot more often than
on mine, so having a dedicated piece in the gauge kit made more sense for me.
__________________
'89 S4 Auto, black
Dr Bob.
=========
You can't just tap the original cap nut and install a fitting. The threads on
the existing cap nut are not designed to seal. So you MUST add some other means
for sealing it.
(like teflon tape or an o-ring, but both are makeshift).
The fitting you need is an 'AN' fitting with a 'globe seal'. (I can't remember
the exact designation at the moment).
It includes an internal shape that recreates that spherical seal. So it seals at
the same interface as the original ball bearing. These fittings are available,
but somewhat difficult to find.
I already own a couple of nice fuel pressure gauges in my tools. So I was
intending to just configure an adapter to use those. I finally gave up and just
purchased a complete gauge assembly. It includes the proper fitting and a
section of flexible hose. It looks exactly like the Porsche gauge (Factory Tool
# P378).
It seals properly and then I anchor the gauge with a tie strap. That allows me
to drive around with it in place for diagnosis. It is definitely not a permanent
installation for long term use. I have run around with it on the car for a few
days at a time. That allowed me to monitor/diagnose/remedy a restart problem.
The first gauge I bought now
resides over at Kevin's garage. So I just recently purchased another one to keep
at my place.
__________________
Jon,
there's a fuel rail at the passenger side of the engine (LHD) which has a pressure
test connection. Lift the plastic cover (2 6mm hex key bolts) and you will see
it. There is a big nut (19mm( covering the connection, and a steel ball that
closes the connection tightly. (don't loose that ball)
I made pressure gauge to connect whenever I feel I want to measure. I hate the
idea of a fuel leak right there. The threads are normal metric M12x1.5.
Since finding a coupler was sheer impossible I did have a special adapter made
to connect to a BSP thread on the gauge, and made a few units like this.
The pressure for a S4/GT/GTS should read 3.8 bar +/- 0.2 at standstill and relay
bridged and 3.3 bar when running idle. It should hold about 20 mins after
shutdown of the engine.
Theo
Venlo, the Netherlands
1992 Porsche 928 GTS midnight blue
http://www.jenniskens.eu
Mark in Atlanta
==========
----
Hallo zusammen,
da meine alte Benzindruck-Messvorrichtung mir zu unsicher war (einen Schlauch
über die Gewinde von Rail und Manometer gestülpt und Schellen drüber), habe ich
eine Weile recherchiert, um ein einfaches, günstiges und weniger unsicheres Tool
zu basteln.
- Manometer senkrecht (KU/Ms), 9,57 9,57, 40mm, 0 bis 6 bar, G 1/8" 10 EUR
- Aufschraub-Schlauchtülle G 1/8 MS 2,05 2,058"-6 (1/4")mm, Messing 2,50 EUR
und als wichtigstes Teil, der M12x1,5 Anschluss mit Schlauchtülle für das Rail
- Anschluss Kraftstoffleitung M12 x1,5 Innengewinde Artikel-Nr: 805032 bei "Fergyfan"
(wohl Traktorzubehör) 6 EUR/ Dann noch ein Stück Schlauch und Schellen auf die
Tüllen...
Inkl. Versand kommt man so auf ca. 25 EUR.
====
