Mercruiser 5.0 mpi surges under load

[Jmcon]

Hey guys, I got a 2002 mercruiser 5.0 MPI 0w024288 Just picked up this boat a few months ago and it ran great, then one day I head out and it wouldn’t get above 3600 rpm. After doing some diag I was sucking water in through the exhaust due to rotted out risers. When I replaced the manifolds and risers I also noticed and replaced the FPR on the fuel cooler due to a rotted vacuum nipple. Repaired and it fired right up in my drive way. Now I put her back into the water I need to advance throttle to start but once running it idles fine no miss or hesitation but my voltage drops to 8v when starting. 2 brand new battery’s in her. I also think I have a intermittent charging issue because sometimes it will read 13.2v charging and I bump the rpms up and it goes back up to 14v. But once I get out to the bay to run WOT it’s sluggish and the rpms surge up to 3000 rpm and won’t go past that. Also no codes present. So far what I’ve replaced due to basic maintenance or malfunctioning parts is:

cap/rotor
plugs/wires
manifolds and risers
fuel pressure regulator on fuel cooler due to rotted vacuum nipple
2 battery’s
IAC muffler
fuel filter

42psi fuel pressure key on and drops to 38
40 psi running


Also adjusted shifter cable and intermittent switch due to stalling going into reverse.

 

[Searay205]

voltage shouldn't drop below 10 cranking. make sure your starter connections aren't corroded. they will be very accessible and easy to get to....... NOT. Most sensors work off 5 volts. Not much advice but your voltage shouldn't be dropping that much, starter may be going pulling to many amps while cranking

 

[alldodge]

Agree, clean the positive and negative battery cables at both ends. Grounds are most important

Once the motor reaches 1400 rpm the alternator should be producing at least 14.5V. Don't use the boat gauge, check power at the ALT orange wire

With having to give it throttle, check compression