I wouldn't do it

on many cars there is a quick connect style fitting off the fuel rail
some other cars its a threaded fitting
I have never checked what my x5 has

but I wouldn't use a air psi gauge for a liquid application, the gauge will mess up from the inside

ontop of that a tire gauge is not a 100% sealed conction so it will mess with your diagnosis unless you can thread it to garentee no leaks than by all means try it out

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04 black on sapphire 4.8is personally put 90,000kms
fully loaded minus the fold out trunk floor
-20' DUB X10 3 piece rims staggered setup
-LED angel eyes + interior lights + e70 style shifter
-AFE stage 2 intake system
-x-pipe/resonator delete
-UV coat windshield
-rebuilt Pano roof
-dynavin N6
-all front and rear end suspension parts and both front cv shafts replaced
-OEM hitch

And 2007 GSXR750