On further inspection of the manual it seems that on some models the fuel regulator is connected to the vacuum from the plenum , but on the throttle body EFI engine i think it is a ruptured regulator diaphagm line from regulaor to flame arrestor? is this correct plese.

My symptoms have changed slightly it now wont idle at all low down even holding it on the throttle . with throttle closed turning the key it starts easily every time idles good for 4 seconds then the MIL alarm goes and it cuts out.
After this there are no codes thrown up. It can be kept running by opening the throttle to about 25% but on reving the engine is holding back at about 4000 rpm symptoms as though its not getting enough fuel.

My fuel pressure sensor on the throtlle body has grey , black and green wires not blue as in the wiring diagram ? Also the main ECM plug wire positions are COMPLETELY different from the wiring diagram ??

Agree it its TBI then there is not a rubber line. If its MPI then it does. If its carb then there is no IAC

The serial number is the issue as SB mentioned: L09691
A Merc number is a number, a letter and then 6 more numbers. Looking up using 0L09691"X" all are carb motors

So it looks like they have put in a carb motor when changing the engine? could this be my problem ? if they put all the ancilaries including the TBI , loom manifold etc will this not work on a carb motor? thankyou for you help I think we maybe on to something !

There it is the pesky little "2"
Not L09691 but L096291

Yes this is a TBI
https://www.mercruiserparts.com/5-0l...-0l012052-thru

Guess I need to see a fuel pressure reading

I am really sorry i made a mistake when i posted the serial number ! today I am checking wiring plugs and earths again ! I read the section on' intermitents ' in the manual this seems like my problem ( Mil sounding , engine cutting but no codes thrown up .)
can you tell me what is the purpose and how does the circuit breaker that is on top of the engine work ?mine is rusty but it wont come appart , can i test it ? it show continuity.

The breaker supplies main power to the motor and the helm, it protects against a short

Okay, so the boat now has a 5.0 vortec longblock and the boat originally had a 5.7 vortec longblock w/tbi ?

oh boy. That’s not so good.

The ecm has no clue 50cid’s where taken away. So that’s another issue. Are the injectors same on 5.0 vs 5.7 ? Doubtful. Will look that up in a bit.

You sure SB, looking at the BAM link (not the best) it does list 5.0 on top, but under list it does list both 5.0 and 5.7

When looking at parts for Block and Pistons there is 2 listings each which would indicate for me that it can be either

No, not sure. Thus the ? Looks like it from listing.

cont Mercruiser to double check cause that would suck bad.