Okay all, having a fuel problem on my rebuilt 1999 502 Mpi. Not making enough pressure. It s bouncing around 30-38 lbs but when you pinch off or remove the vacuum line going to the plenum it gets a steady 43 lbs. I have replaced the fuel pump, regulator, anti siphon/ fuel shut off valve, all injectors, and the pressure line from the seperator to the pump. There isn't a whole lot of stuff left to replace except the fuel cooler and the line going from pump to rail. HELP! I've spent a year and a half rebuilding everything on this boat and now it's done and this dumb little issue is keeping me from running the hell out of it. Can any of you 502 Mpi experts help me out?

Have you checked the vacuum reading from the manifold itself? Is it bouncing around? If so then you likely have a valve problem and the bouncing vacuum signal is effecting the regulator, Put a vacuum guage on it and see what it looks like.

Thanks Ted. Will do

Thats my first thought as well,,,,,good luck !!!!

Okay. Didn't get a vacuum gauge yet but I unplugged the vacuum line and ran the boat today. Ran like a bat out of hell, 4800 rpms. So that brings the question, why does bypassing the fuel regulator by unplugging the vacuum line make it run right? It is a new regulator, do you think the regulator I got could be bad? Since it ran so well I don't believe I have a stuck valve or a valve issue. What do you think?

the way its designed its soppose to up the fuel pressure as you loose vacuume to it...i would also check your timing to make sure its spot on..did you change the cam? also id so you need to reprogram the ecu..

I did not change the cam. So without vacuum it was just running full fuel pressure no matter what throttle? How do you reprogram the ecu?

Can you swap the ECU with the other engine easily?

I thought about doing that but didn't figure it was an electrical or computer issue. Blackwood told me to send my ecms out to that place in Arizona to remap them.

if you knew 100% it was the ECM's i guess that would make since, did you get to run at all this weekend?