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Hi Goofee691,
On the top of the engine, a loop of steel tubing connected to the fuel line and the injectors is the fuel rail. Towards the front of the fuel rail is a pressure regulator that uses manifold vacuum. The regulator has a fuel return line, don't cut that. Put a tee in the regulator vacuum line for your gauge. The vacuum line is a slip fitting and smaller than the fuel return.
Also most vacuum gauges measure in inches of mercury. Air pressure is about 30 in of hg (29.92 at std...). So a vacuum will measure as about 30 in. on the guage. A car engine with a PCV valve will have manifold vacuum at idle of about 17 to 22 in. The Volvo doesn't have a PCV but does have a metered leak to vent the crankcase to the intake so idle vacuum is about 20 in. or a little lower depending on wear. As the engine powers a larger load the manifold vacuum drops almost to 0 at full throttle.
If your guage is in psi then the normal engine idle will be about 5 psi and drop to 0 at wot. But 15 psi is unlikely even for the vacuum resevoir of the hvac.
-mort
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