Started taking temperature readings at different places on the engine. The extreme rich idle is telling you something about what the computer is trying to correct.

......Not sure if I agree, Merc 1075/1300 run REAL well using EFI, I have also been on numerous 500/525EFI boats that ran like "swiss watches" too.

Not saying the Carb motors dont have a place in boating, but, not all the EFI stuff is "crap" either

Just my .02

its a trick question thier is really nothing wrong with it just see how many suggestions or answears people would give

IAC replaced or checkedd?
Map replaced or checked?

Water seperator changed or cleaned out?

Good thing about 2 motors is you can swap parts instead of buying.

Ah heck, if you want to go backwards, why not just get some oars or paddles

EFI is by far a better, no comprimise fuel system compared to carburetors.

The absolute first step is to hook a fuel pressure gauge to the rail and check your fuel pressure at an idle, you should see approx 40psi, now increase the rpm's to 3000 and check the psi and you shouls see the pressure regulator bring the psi to 35 or so...return the engine to idle and recheck the pressure it should be around the 40 mark again. shut the engine off and watch the fuel pressure if it drops off quickly you most likely have a stuck injector. or a ruptured diaphram in the regulator, pull the vacume signal line off the regulator and and smell the rubber commector...if it smells like gas you have a regulator problem, injectors can be difficult to diagnose and "Maltec" injectors are not very condusive to cleaning where as a pintle injector can be cleaned successfully. Look at the tip of the injector "the part in the intake tract" if there are a bunch of small holes around the inside of the tip it is a maltec if there is 1 hole in the center of the tip it is a pintle style. Check the fuel pressures first and report back. Advance auto rents fuel pressure gauges for free so get one and don't just start replacing parts.

fuel pressure regulator

Fuel Pump Is Going Bad

he is going to xpend the $100 on fixing his boat then forget about this trend

Not forgotten, did check fuel pressure today seems good at 40psi with no leakdown, timing ok, IAC working fine. Still a day or so away from scan tool check. I'm leaning towards something temp related because it runs fine on the hose, put it in the water for a test and did the same sh*t. Might not be seeing the temp sensor.