I was wondering if the throttle body injection on the 575SCI is an aftermarket item or is it Mercs own system.

Kurt.

I beleive that the throtle bodys are after market and the electronics is gm dirivitive uses gm sensors

The ECU is from Delco/GM, you can get this from Az. Speed and Marine. The throttle bodies are off 95 454 TBI GM truck engines as well as the injectors. You should be able to get all this from Az. Speed and Marine.

Thanks,
Dustin

Do they use a return line to the tank?

No, I don't know of any OEM Coast Guard approved system that does but I'm sure there out there. Mercury motors all have cool fuel systems now and go in a loop.

Thanks,
Dustin