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
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.
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.
Venlo, the Netherlands
1992 Porsche 928 GTS midnight blue
Mark in Atlanta