Mercruiser 350 MAG MPI with cool fuel not siphoning fuel

[stevekehoe]

Been trying to troubleshoot this for 2 weeks! Engine initially had been stopping after running various periods of time (2 minutes to 1.5 hours). Changed fuel filter and filter disc and added some fuel stabilizer hoping this might correct problem but still had the same problem. After being towed twice I decided to get some tools and try to diagnose. Service bulletin 40 has some instructions for diagnosing cool fuel pump problems. At key up I get 43 psi on the fuel rail and it drops back to about 37 psi at idle. Generally will run okay for about 3 or 4 minutes but then gets starved for fuel and quickly fuel pressure drops and engine stalls. Ran clear hose directly to fuel container to eliminate problems with on board tanks and hoses from tank to fuel filter. With clear hose I could see fuel is not being siphoned so thought low pressure pump must be bad because it was not pulling a vacuum. So I bought a new low pressure pump and installed (very difficult to get to cool fuel module). With new pump I am still having the same problem. I bought a vacuum gauge and teed into the clear fuel line (between fuel container and fuel filter). At key up a small amount of fuel can be drawn from the fuel container (vacuum gauge flickers slightly) however after about 2 minutes the fuel pressure gauge on the rail starts to cycle from 30 to 35 psi as the fuel is not being drawn from the tank. At the same time you can see air bubbles coming back against the fuel flow direction (from engine to fuel container). When you shut off the engine the bubbles continue to come back from the engine to the fuel container. Pretty sure I shouldn't be seeing air bubbles going backwards? Constricted fuel line to see if vacuum could be created and it is inconsistent. Generally only 1 or 2 can be drawn. A couple of times vacuum did go up to 5-10 but also had decent fuel flow at that time. So the problem seems to be an inconsistent vacuum but I think it has something to do with the air bubbles. I can keep engine running if I hook up a hand pump in line and manually pump fuel when fuel pressure starts to drop. Any ideas? Thanks.

 

[Fun Times]

Ran clear hose directly to fuel container to eliminate problems with on board tanks and hoses from tank to fuel filter.
I bought a vacuum gauge and teed into the clear fuel line (between fuel container and fuel filter).

Hi there, welcome to iboats!

Do you have an inline filter between the boats fuel tank and Gen 3 cool fuel module? The Gen 3 module isn't supposed to have an inline filter...One of the issues suggested by Mercruiser is it could affect the vacuum due to restrictions.

If that's the case or even if it isn't, Try by passing the boats fuel system and get yourself an external fuel tank, shove the hose in the small fuel tank and connect it to the cool fuel module and see how it runs.

Also check the fuel line hose barb fittings for air leaks. Are they tight and sealed properly.

 

[stevekehoe]

No I don't have an additional fuel filter between tank and cool fuel module. I did try running directly from an external fuel tank (what I called fuel container in post) but still can't get a vacuum and have air bubbles coming up fuel supply hose.

 

[jimdigug]

I have same problem . I replaced fuel pump in cool fuel module. Now motor will run on muffs for about 2-2 1/2 minutes then stalls . At idle about 37 psi at rail at about 2 minutes pressure starts to drop then stalls ?.

 

[Zoubi]

Same problem, any solutions ?

 

[GA_Boater]

Same problem, any solutions ?

Start a new thread, please.

Closed.