Fuel pump vent system question
 

Hi all -
I don't have an offshore boat but I do have a technical question that I bet someone here can address. I used the search but didn't find the exact answer so here goes:

Does anyone know how the fuel pump vent system works on the 7.4LX MPI (1996 year) Mercruiser engine? Specifically, if I stuck a compound pressure gauge on the "sight tube" at the sea water mounted fuel pump what would it read? I think it would read manifold vacuum but I'm not sure. The sight tube vents the mechanical fuel pump to the top of the VST and there's another hose from the VST top to the bottom of the intake plenum. However, inside the VST at the sight tube connection is, I believe, a check valve. This question is based on my intent to replace the mechanical fuel pump with an electrical pump to get away from the problem of leaked fuel into the gear oil. I started wondering if there is a vacuum on the vent side of the sea water mounted fuel pump that contributes to the problem. Thanks for your help.

If you are talking about the clear tube that comes off the mech fuel pump, that is for safety purposes. If the fuel pump diaphram ruptures, the excess fuel goes through the tube and dumps into the fuel system instead of into the bilge.

Hey Griff -

Thanks for the reply. I'm aware of the purpose of the tubing but you didn't answer my question. Is there some way I can clarify it for you?

Oh, and Happy New Year.

It is not a vent. It is an overflow tube.
I doubt you will see any vacuum. If it had manifold vacuum, you would easily be able to feel it sucking air.
The lube in the sea water pumps always get some contamination with fuel seepage from the mech pumps.
Its pretty minmal though unless you have an bad internal leak in the fuel pump.
Change the lube every season and it should be fine.

That's way we keep Grif around.

Hey Griff -

Well, vent or overflow tube I have to change the gear oil more frequently than once per season which is why I was planning on replacing the Mechanical fuel pump with an electrical one. When I got the assembly off the engine I started thinking that the overflow tube may be contributing to the fuel leakage since it is connected to the manifold vacuum via the VST. I'd have measured what's going on If I hadn't taken the engine apart already. I'd just like to know what's going on with this fuel pump to VST to intake plenum connection.

It is an overflow tube only. It is an overflow tube only. It is an overflow tube only. Not a vent. It is an overflow tube only. Okay?

The tube has nothing to do with your issue. How much fuel is in the lube??? How much extra comes out of the fill/sight hole???
I would just replace the 15+ year old mech pump if there is a significant amount of fuel in the lube.

Well, not really: Mercruiser calls it a "sight tube" (Service Manual 16 pages 5A-3 and 5A-7) and it's main purpose is to indicate a ruptured diaphragm. It serves no overflow function. Having said that I don't really care what you call it. now: can you answer my question, please, can you answer my question, please, can you answer my question, please? Thanks.

If the tube has manifold vacuum in it then the diaphragm would have a much larger pressure across it than if the tube was vented to ambient pressure via a hose barb on the flame arrestor as other marine engines I've owned have. At idle the Mercruiser scan tool indication for manifold absolute pressure is 6.5 psi which equates to approx. 8 psi vacuum. Add that to the 4-7 psi fuel pump output pressure and the pressure across the and you've better than doubled the normal pressure the diaphragm sees if vented to the atmosphere. So again, without knowing what's going on in the VST, the tube connection could, I believe, contribute to fuel leakage into the gear oil.
The fuel pump is 2 years old purchased to replace a fuel pump that was causing gear lube dilution.