i'm thinking it's suppose to be on. but just want to double check. thanks.

and at idle, an LS motor should be around 40 psi, right?

 

yes the motor has to be on

 

thanks. also do i have to take off the vacuum line and cap it off? and a LS should be at 40 psi?

 

stock is around 40 yes. you can either take the vaccum line off, tighten the screw, then put the line back on, or tighten the screw with the line on. when you take the line off the fuel pressure will go up a bit, but dont worry it will come back down as soon as you re-attach it.

 

when the line is off, are you suppose to cap off the FPR? or just leave it open?

 

To test static fuel pressure:

Attach a fuel pressure gauge.

Remove the vacuum line from the FPR and place your thumb over it to prevent a vacuum leak.

Read the gauge.