Open might be better then capped though the difference will be slight. I have mine capped cause I am funny that way, and I assume that INJ PW is based on MAP and is already factors ambient air pressure. Bottom line, best to go with the tuner on this.

Videoing your gauges is FAR better than just observing them, HOWEVER, there may excessive "smoothing" built in (for a stable display) that detracts from accuracy . But, I expect you are fine.



how close is your regulator to your rail?
are your gauges sealed? perhaps one has a different internal ref?

It is not critical that the 2 agree (as long as they are consistent). I am guessing they are both mechanical ? The one on the regulator is handy for observing any adjustment delta ... not sure how useful a second mechanical gouge on the rail is in this set-up.

On my set-up, I have a return line coming out of the forward end of each side of the rail and then on to the regulator which is about 24" of fuel line away. It has a mechanical gauge mounted to it and then excess fuel (bled off to maintain pressure adjustment) returns to the tank through the original fuel line. The new supply line comes from the SL55 pump and filter (mounted where original pump was) and enters the fuel rail at a fitting I added at the rear/center. The original rail inlet NOW serves as a mounting location for a 100 psi fuel pressure sensor that goes to my data logger. The log-ability is nice as I can see if I am having a fuel pressure issue while driving (eg as I approach WOT), but this sensor has failed twice over the years (in a spectacular , fuel all over the place, fashion .... ), so I often have it plugged off.

Chris