I have a 1998 Marine Power 390hp 454 MPI motor, it's an mefi cpu. The engine has 150 hours on it. My problem is when starting the motor after the batteries have been turned off. It will fire immediately but then stall anywhere from 2 to 4 times before it will idle. Once it seems to "learn" it instantly stops cycling the idle rpm and smooths to a rock solid rpm needle. It will start fine the rest of the day, even after stopping and letting it cool off. It seems to loose it's profile after you disconnect the batteries. The boat is new to me, the previous owner had it into the shop a couple times for this issue but it was always no trouble found.

What I have done so far:
Plugs
wires
Cap and rotor
fuel filters
timing checked
air cleaner
no codes beng thrown
cleaned IAC no change
replaced IAC no change
IAC is moving

Any input? My next thought was the coolant sensor telling the motor it was warm but again no codes. I have not checked fuel pressure yet.

1) Idle vacuum is only 11 , best vacuum occurred around 2000rpm and that was 15inches.
I sprayed starter fluid all over the manifold and intake areas looking for a leak, nothing found.
Now I'm assuming the cam in this motor isn't big enough to run that low a vacuum, I'll have to call Marine Power again to ask that question.

2) Checked the fuel pressure, 41 psi at key on (41-47 spec) and high 30's idle. Full range is supposed to be 34 to 47. Idle it was running high 30's.
Fuel pressure is also leaking down, a few minutes and it will drop to 35psi, 30 minutes to 30psi. Injector or regulator leaking

3) Pulled plugs, they have 10 hours on them now and are not too bad but maybe a tad rich. Kinda makes sense, low vacuum will raise the fuel pressure and give extra fuel. No O2 sensor on this motor, open loop.

4) Next pulled the plugs and did a wet/dry test after the motor was warmed up. Motor is 8.8 to 1. Everything is within 10% dry and wet.
1 140 160
3 135 170
5 135 165
7 130 160
2 140 165
4 145 175
6 130 160
8 145 170

When pulling the plenum to remove the injectors found a bunch of silicon used around the o-rings. Looked like 6/8 was leaking. Plenum was warped, it's at the machine shop now, injectors are also sent off for cleaning/flowing.

Other than your wayward silicone, it sounds like a classic case of a ECU learning its environment in a car. I know my 2000 7.4 L 320 hp does not do this even after a week with batteries disconnected, so just my 2 cents.

hi having the same prob did you fig it out

leave the battery switch on 1 battery,as stated the pcm wont hold the information unless theirs power going to the ecm..the Fixx if you want to turn off your batterys is to run the ecm power wire directly to the batterys on a fused sorce then your problem will go away ..

Before you do any of the above try leaving your batterys on for a few days and see i it reacts the same,if it does then disreguard the above statment and look into vacume leaks and fuel delivery problems.