mefi-1

thats the ecm, is it TB EFI.........502 mag mpi ?? 500 EFI

Throttle body EFI, Tune port injection. These are the best pics I have for now.

https://scontent-atl3-1.xx.fbcdn.net...30&oe=60C210DE
https://scontent-atl3-1.xx.fbcdn.net...5a&oe=60C06A12
The engine bay was going througha big cleanup in this pic.
https://scontent-atl3-1.xx.fbcdn.net...72&oe=60C1C063

Custom MPI setup
Since your getting spark and have fuel pressure, something is telling the the ECM to not turn ON the injectors

Normal for a MEFI-1 with Merc program
Once the ECM see's that the distributor is firing the spark plugs, then it will turn ON the injectors. They injectors are batched fired so have fires then the other half.

Your getting spark but can you see the tachometer show at least 300 rpm while cranking?
If it shows 300 rpm or more then the ECM should be turning on the injectors

If somehow it is vapor locking, you can releive the VL by pressing in on the fuel rail schrader valve. Use a rag and safety glasses and press in to see if you get nothing but fuel, or there is some air

thanks. I'm going to look into the ECM some more. I did hit that valve and fuel shot out. Valve for this set up is on my main pump, though.

It still makes no sense why it would shut down the injectors after getting up to temp?

D'oh!!!!! 8 bad injectors and I could see where they were leaking going into the manifold. Guess what.....16 new injectors are on the way and the 8 good ones from the port motor will be kept as spares. When I hit the shrader valve, fuel would shoot out, but over the course of 10-20 minutes my fuel pressure would drop to 15 psi and it's never done that before.

The ECMs came back from repair and reprogramming yesterday, so I'll put those in along with injectors, cross my fingers, and hope I don't need to commit insurance fraud to fix this issue with a small can of gas and a match.

Wishing you a long and trouble free season.

Thanks. This boat has been one nightmare after another, it seems.

8 leaking injectors?? Sounds like your fuel sys is contaminated.

Not sure abt your setup, but some of the fuel inj Mercs had an issue with paint coming loose contaminating the fuel sys. I think it was in the cavity where the hp pump resides.
Regardless, you better get the fuel sys sorted out before installing new injs.

Do your injectors have screens on the inlet? Were they dirty?

Sounds like you have some other issues, but I had a friend with a 400SS with gas motors in it (believe they were MPI motors) and he would experience vapor luck with winter blend fuel. He would always have to wait until after the summer fuel was brought to the marina before he filled up...