Or if you dont have a vacuum gauge, you could always pull the vacuum line off the regulator while its idling cold to confirm your base pressure. Then while idling hot, pull the line off again and see what the gauge claims your base pressure is when hot. This would subtract the variable of a difference in engine vacuum when cold/hot.

Pressure is set to 50 without vacuum source with vacuum source it’s is around 44-45 .... and while normal operating temps without driving vacuum is around 18 after driving it’s more around 16 and when hot fuel pressure is around 40 ..... but that’s going from the gauge gauge with the return kit .... I think it’s giving a inaccurate reading due to the AFR doesn’t lean out and if I try to increase pressure I start going rich

How fast should fuel pressure drop to zero with the return setup after the car is turned off

There are no intentions for fuel pressure once the pump is not running... so anywhere between immediately and a couple hours would sound about right to me.

Ok just checking all angles ..it drops to zero fast after engine is off... I still think pressure is dropping ...I never noticed my supercharged 370 changing this much ...I went to a cruise in today ...popped the hood pressure was around 37 ..was very hot ...I let it just sit and idle for about 5 minutes ...pressure came back up to 40....cold idle is 45 ....still need to verify pressure gauge but I feel it’s is fluctuation

So I have been looking at my fuel pressure as well...

Full blown fuel return kit, with a fuel pressure regulator, and pressure gauge. No vacuum line connected to the regulator from the manifold. The car is still stock other than the fuel return kit, which also has a 14 ga wire from the batter all the way to the fuel pump.

I had set the fuel pressure at 51psi, as per FSM, with the engine all warmed up. I did notice that the fuel pressure will go up as high as 56psi when the engine is cold, and it does take 10-15 minutes of driving for it to come back down to what I set it to. This is about a 9% fluctuation.

I also noticed that the OEM volt gauge will go from about 14.4, to below 13.6...ish cold vs warm, which is about 6%.

I think it is safe to notch the last 3% of fluctuation on the heat, and the influence it has on the spring in the FPR.

That’s strange ...mine is the opposite ...it’s dropping pressure from set point ....I had it set around 50 without vacuum line on it ...with vacuum line it dropped to around 45 ....after twenty mins of driving it would drop to 38 ish ...I watched it while idling and it would start to bounce from 35-38 .....sooooo i lowered the pressure down to 28 then increased it to 65 just to see if the regulator was hanging up ....I set it back to 52 without the vacuum it dropped to 50 with vacuum line on ....took it out for a drive for awhile got back home and checked it ..still holding 50 .....I’m wondering if there is a issue with the regulator that is magnified at lower pressure , now I’m just guessing ...there are really only a few things that can affect pressure ...vacuum source ..regulator ...pump ...voltage.....I’m using the vacuum lines off the throttle body for regulator source ..I’m gonna ty to switch it to another one to see if there is a change

I did find this info which almost fits my issues to a T .... Aeromotive AFPR problems | DSMtuners

and it came down to the gauge being liquid filled not being accurate and that aeromotive states not to use liquid filled gauges on their web site. And now that I think about it I had a fuel lab regulator on my other z with no fluctuations with a liquid filled gauge .....I’ll be adding a T in and add a regular gauge and compare the two

This may help as well. https://www.verociousmotorsports.com...-Filled-vs-Dry

That what the first thing I was saying... temperature effects the gauges.

You also need to always compare engine vacuum to your fuel pressure. Monitoring your idle fuel pressure without comparing manifold vacuum is sort of like expressing wheel offset without knowing the wheel width. Its mostly erroneous information when one of the major contributing factors is unknown.

Between temperature and engine vacuum fluctuation, you're probably fine. A loss of even a few psi will show a clear change in the A/F. So if you are already closely monitoring your A/F, you are already seeing the aftermath of the fuel pressure. I wouldnt dwell on the fuel pressure much at all unless I was first seeing a problem in the A/F that needed diagnostics. But if you really want to know for sure that everything is OK, you can get nice fuel pressure gauges for less than $200.

I was comparing the pressure with the vacuum and even with same amount of vacuum pressure I was seeing differences in fuel pressure ...reading from the gauge ...and I was never getting a lean condition ...I guese I was just paranoid ....better to double check something before hand than after a blown motor lol
What got me looking was a very light surging stumble during light acceleration ...checked everything couldn’t find anything ..changed plugs no change I thought the fuel pressure might have been bouncing causing it ....it is only under light throttle and very minor ..it’s around when the vacuum gauge is from 5-0 when I’m cruising at a steady speed .....no issue during boost

Ever find anything else about this or was it just the engine bay heat?

I chalked it it up to heat affecting the fluid in the gauge causing a inaccurate reading ....but one thing to note ...once you increase from vacuum to boost (0+ boost ) the gauge will go to original set point and increase as boost increases...doesn’t seem to affect the tune or set points.

UPDATE ..the surging I was feeling was in the tune ...I took John from Z1 for a ride and was able to show him what I was feeling while he had it connected to the laptop and he adjusted the fuel to eliminate it