I'm wondering if it need's to stay hooked up or not? I notice that as I get on the gas the fuel pressure increases when the hose is connected. If I leave it unhooked it just runs at a constant pressure all the time. Which is a better way to have it, and why? thanks

 

The purpose of the vacuum line is to allow the regulator to do what it is put there for-change the pressure. You don't need high pressure when the demand for fuel is low (like idle). High load situations make the vacuum go down. When this happens the regulator increases the pressure.

 

So I should leave this hooked up then?

 

I would. There is no purpose having high pressure at idle.

 

The vacuum "compensation" line is there to maintain a constant DIFFERENTIAL fuel pressure accross the injector. The injectors are rated at 43.5psi DIFFERENTIAL pressure.... the difference between the inlet to the injector - the fuel rail - and the outlet of the injector - the intake manifold.

At idle, there is a vacuum in the intake manifold, so in order to keep the DIFFERENTIAL pressure the same, the fuel rail pressure must be reduced in proportion to manifold vacuum. 36psi rail pressure minus the intake vacuum (remember... intake vacuum is a "minus" pressure, and minus a minus = a plus) will equal 43.5psi..... +36psi - (-7.5psi) = 43.5psi.

When you open the throttle, intake manifold vacuum decreases, so the fuel rail pressure must be increased to keep the DIFFERENTIAL pressure the same. At WOT, there is virtually no vacuum in the intake manifold, and the rail pressure must rise to 43.5psi..... 43.5 - 0 = 43.5.

So..... yes, you need to keep the vacuum compensation line connected, because the injectors are rated at 43.5psi DIFFERENTIAL pressure and the PCM is programmed with the flow rate for that pressure.