2005 Mercruiser 5.0 MPI rough idle booking underload

[fotonews]

Hello,

My new to me 2005 Formula 280 ss has twin Mercruiser 5.0 MPI with Bravo III serial #0W04809 and # 0W04793. While bringing boat home to Miami from Deerfield Beach I noticed idle was rough and engines would stall sometimes. Also under load had difficulty getting boat to plane. At around 3000 rpm issue feels gone away.

A few weeks ago I refueled to the top and engines started right away and all issues were gone and i was very happy. After using boat for many hours, about 50% fuel down, issues popped up again. I changed both IAC Valve and no fix. Yesterday fuel tank was about 75% empty and boat was running like crap. Rough idle and difficult to keep steady, lots of vibration and very difficult to plane.

This morning I filled fuel tank to the top and all issues are gone again. Could this be weak fuel pumps? Both engines feed from same fuel tank. I checked fuel pressure with 75% empty tank and shows low, doesn't reach 43 psi but with full tank pressure reads good, 43 psi and 38 psi when going up on rpms. Could amount of fuel in tank create this scenario? bad antiphon valves?

Any help greatly appreciated

 

[alldodge]

Since they are both doing it at the same time I would check antisiphone valve, fuel pickup, and tank vent valve. It starts up in morning good, then gets worst, If it starts running bad, open the gas cap and listen for air rushing in

 

[fotonews]

Thank you AllDodge for your insight. When boat was running rough, vibrating, bogging I took off fuel cap and there was no changes in engines behavior. Can I then rule out a fuel tank ventilation issue? and concentrate in fuel pick ups and antiphon valves?

 

[alldodge]

Can I then rule out a fuel tank ventilation issue? and concentrate in fuel pick ups and antiphon valves?

I would agree but don't forget about fuel filter(s)

Maybe even a bad electrical ground connection if others don't pan out

Could run the motors on gas cans to see if they both still have the same or similar issue

 

[fotonews]

AllDodge, thank you for your help. The issue was the plastic fuel pick up tubes were a bit loose where the slide into the fitting on top of fuel tank. Thanks again!!