Have spent a lot of the summer trying to get my new to me 502 MPI running right.
Previous owner put the engine in as a take out from a 3rd party, so history / age of the engine is kind of a mystery. There is no serial number plate on the engine or anywhere else I can find. From the parts lists it appears to be a "newer" ( 1999 - 2000 ) model. It does have the cool fuel system.

Engine would max out at 4400 RPM with both a 21 and 23 pitch prop.
At this point, I have had the injectors cleaned and flow tested, installed new mechanical fuel pump. ( Plugs, distrubutor cap )
Fuel pressure holds 42 psi from idle to about 3500 rpm ( 1/2 throttle ) then starts dropping down to around 30 at WOT.
I'm guessing / assuming the electric fuel pump / pressure regulator need replaced.
Anything else common I should look for?

Serial # info would of been on the original valve cover sticker, my 2000 mpi didnt have a mechanical pump, just the high pressure cool fuel pump, you sure its not VST ??, but I agree to change the high press. pump

Edit: is the vac hose for the fuel regulator hooked up to the intake manifold ?

Your injectors I flowed and cleaned were the old style so unless its a hodge podge its older than 99/2000, does your motor have v belts or serpentine belts?

Had dash mounted fuel pressure gauges on mine, when fuel pump was going away it would hold 43# to about 3900 and pressure would drop rapidly. Replaced pump and cured it, regulator was fine

It is a cool fuel system - fuel line goes right from fuel cooler to fuel rail. ( No VST tank ).
I checked the vacuum hose from intake to pressure regulator - it is connected on both ends.
It does have a mechanical fuel pump on the sea water pump ( which seems odd, since non of the Mercruiser parts manuals show it with the cool fuel system . . )
Motor does have v-belts. 1 for the sea water / fuel pump, I think only a second belt for everything else ( alternator, power steering ).
I did install an new gaffrig fuel pressure gauge - it was actually reading 60psi - but I assumed that was because the sender was a 0-10bar ( 150 psi ) connected to a 0-100 psi gauge ( I have no idea why they did not come matched from the factory. ) If its truly a mis-match, then the actual pressure would be 42psi. I should probably get a mechanical gauge to verify the electronic one . . . either way, the pressure is dropping at higher loads / rpm.
Does anyone know if there were more than one high pressure pumps for these? I don't have a serial number, and suspect that the engine is somewhat of a hodgepodge so don't know if the serial number would match the actual part any way. . .

this is it, if yours fits inside the black plastic housing mounted below the motor mount

http://www.ebay.com/itm/NEW-MerCruis...hU27Xh&vxp=mtr

Being you have v belts ,old style injectors, a mechanical fuel pump tells us it started as a mefi-1 or 2 and had cool fuel added. As mentioned, probably needs a pump as long as it has solid voltage at pump snd isnt dropping under a load

Did some more "fishing around" the mercruiser online parts manuals - best guess is that its around a 1996-1997 502 that originally had the VST tank ( there are 2 fuel lines on the back of the fuel rail, one is connected to the cool fuel system the other was plugged and is now my fuel pressure attachment. ) Appears that it has been upgraded to cool fuel. I assume the mechanical pump feeding the main pump is not causing problems - it was used with the VST system, but I cannot find it on the cool fuel systems.

Thanks for the link to the pump.

Got it into the local marine repair shop - 4 weeks for an appointment - drop off on Monday - Wednesday at 6 they call to tell me that the fuel pressure is 60psi and I need a new pressure regulator and fuel pump - I told them when I dropped it off that those needed to be replaced . . (sigh).
I'm assuming that they used a mechanical gauge to check the pressure and not my electric one.
Now they have to order the parts ( which Mercury doesn't make any more ) - still waiting for an update of when they think they will have it done. I'm hoping I can have it back for the weekend repaired or not . . .

Is it "safe" to run with 60psi injector pressure - other than I will probably use a lot more fuel than needed?

Should have just bought the parts last week and would have it done by now, but figured it was worth having someone else contort themself getting the pump changed . . .

Read some threads on here , remove the cool fuel , install Weldon or Aero pump , install regulator on the plugged line and return to tank.
All problems solved.