Twin Mercs 5.0 MPI running rough/stalling
 

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

fuel pick up issue gas tank side. cracked or broken off. Run engines on remote gas tanks like what outboards use these are usually 3 and or 6 gallon tanks - new fuel line as well. Did problem go away completely while testing this ??? Also make sure your fuel vent(s) are clean and free to vent with no restrictions.

Thank you BUP and Camalot for your insights. 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?

the cap remove while running is a good test for venting.

Anti siphon valve is not going to change whether you had 5 gallons of gas or 5000 gallons of gas. Gas tank Fuel picks will because of the fuel level inside the tank.

Possible crack or broken off if you are saying a full fuel tank, the engines run great without issues. Sure sounds like to me if that answer is a YES, for the fuel pick ups. You can test with remote fuel tanks. that will give you an answer to with side the issue ---- Boat side or engine side.

I agree, it sounds like cracked pick up tubes sucking air.
I just find it strange that both engines are doing it since they should have separate pick up tubes

Griff, there are separate pick ups, one for each engine from same fuel tank. I just spent a few hours removing both pick ups and anti siphon valves. Valves look spotless, ball springs fine when a few psi applied. Pick up tubes are made of hard plastic, they look perfect, a bit of pressure and no leaks. One area that looked a bit sketchy was where the plastic tube connects to aluminum piece on top of fuel tank that has antiphon valve on the outer side. I made sure it was tight and put everything back together. I have several trips booked for the coming days so by Sunday I should know and get back with results.

Gentlemen, thank you all 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!!

fotonews, any tips to get access to the tubes? without removing the whole floor? I have same issue with my 06 Formula 280. Thanks!