Ill get a pic tonight

A lot of info and problematic flowcharts in here, see if any are your system:
http://www.weindex.info/pdf/FuelSystem/9.pdf

Little update... Pulled the old vacuum line off and it was completely packed with sludge. No way there was any air movement through that tube. Someone used crappy vinyl tubing and it was rock hard and loose around each nipple. Pulled plenum off to clean out the nipple passageway and came across another packed line (the short line that runs from the bottom of the plenum to the starboard front stack on the motor). Replaced that as well. It was too late to fire her up and recheck pressure. Won't get to that until Friday. Will a plugged vacuum line cause pressure issues??

Didn't get a chance to snap a pic of the pump before my phone died. Will grab later.

Vinyl tubing (usually yellow from the factory) is usually used for mechanical pump vents or vapor vents to go back to a non vacuum source...ie: just before the tbody/carb. Make sure your vacuum lines and vent lines (if any) go to their proper source.

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When a fuel pressure regulator is hooked to a vacuum source (most are, but not all) - this will change fuel psi and will change as vacuum changes.

The vacuum will be on same side as the spring in the fuel psi regulator. More vacuum opposes spring psi and thus the fuel psi will lessen. As vacuum get's lower, the net result will be more spring pressure which will cause more fuel psressure.

So, high vacuum conditions (idle, low speed, second or two right after lessening throttle , etc) will cause lower fuel psi.

Low vacuum conditions (high load + large throttle opening) will cause higher fuel psi.

Key on engine off has no vacuum, therefore the fuel psi read will be higher than with key on engine on. Unless the battery voltage at key on engine off causes pump to not pump enough to push against that pressure.

Ok, thanks for the explanation. That makes sense. I believe those were plugged vent lines that I replaced. I'm assuming the vacuum hose is the short hose from the regulator to the throttle body? I replaced that also. Still new to this side of things. Appreciate all the input.

You'd have to find a diagram for the vac and vent (if any) lines. Been a long time since I had a 502MPI intake manifold off. That one was interesting as a few vacuum ports on the intake pulled right out when pulling the vacuum lines off.

Remember, anything on the engine side of the throttle blades, will be vacuum source.

I'm trying to understand this hose routing. Wondering if something is hooked up incorrectly. I took some random pics of my brothers 97 '454 mag awhile back as a reference for re-installing mine. I just flipped through and noticed that his vacuum hose has a different configuration than mine.

Mine: I have one short line running from the pressure regulator straight up into the bottom of the plenum. Additionally, I have a separate line with three points of connection. One at the VST tank, one at the mechanical fuel pump, and one teed off into the bottom of the plenum near the mouth of the intake.

The pictures I have of my brothers configuration show a "T" running off of the connection between the fuel pressure regulator and the intake plenum.

Soooo...what is the correct routing? I have been searching all over trying to find a diagram to no avail. Just want to have this rigged up properly before I take it to the water tomorrow.

According to page #167 here, you have it correct. Take a look.
http://www.boatfix.com/merc/Servmanl/16/16C5R2.PDF

Please save and keep this .pdf in your computer. 190+ pages of Merc MPI / TBI fuel systems info. It has tons of info - even pulling VST apart , what's inside, and how to repair.
Fuel pressures, wiring, diagnostics,......you name it, it probably has it.

Thanks for finding that!! I'm gonna take her out tomorrow and test under load. I'll report findings.