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.

It absolutely serves a purpose as an overflow tube. If the diaphram ruptures, the fuel has to go somewhere so it does not dump into the bilge.
It goes through the "sight" tube so you can see it and dumps into the fuel system.
It does not have any measureable vacuum in the tube and certainly does not have full manifold vacuum.

Your question has been answered in post 4, 7, 8 and now.

Pop.....

if the sight-vent-overflow tube is hooked up to anything that has vacuum on it, it is either hooked to something that has malfunctioned and opened to vacuum, or somebody hooked it to the wrong place. if it had vacuum on back side of pump diaphragm, the pump would not work very well, and would probably cause it to leak fuel through prematurely. BUT, the vacuum would suck up the excess fuel and not let it contaminate the lube oil. it might also suck out the lube oil.

Iirc, the hose is tough to see and there are a couple vacuum hoses that are easy to put in the wrong place on these engines. The line from the pump does not go to a vacuum source, it goes to nipple that goes up into the flame arrester. Ill see if I can get a pic tomorrow

just run that line back into your gas tank. No vacuum, no pressure, (for the engineer minds or professors to fret over) and no worries about gas getting dumped into your motor or bilge. Diaphram ruptures, you just keep on truck'n. What safer way to deal with it? ;)

fuel pump with vacuum won't pump, diaphragm at top of stroke = no pump. marine fuel pump has seal to prevent gas(fuel) from getting into hot crankcase, engines tend to go boom, fuel needs place to go. directing fuel to airhorn, should stop engine.

Yes, the tube will act as you say.

My question has not been answered because you keep ignoring my explanation of the routing of the sight tube. I'll try one more time: it connects from the mechanical fuel pump to the top of the VST and the top of the VST also connects to the intake plenum ergo how could it not have a vacuum in it. If you can't answer that you don't have to post.

they are both for rupture, should go to throttle body. fuel pressure regulator should connect to manifold. if you put gauge inline you should have no vacuum or pressure. not unusual for someone that has no reason to touch marine engine to screw them up for lack of understanding system and reason for design.

I count 7 answers.....do you need someone to draw u a pic? In crayola? How about you start over by introducing yourself, look around, do a search....and wait a few years to create a "stir"......

Great place, lots of great people. Try and be one.....

If you getting gas in the raw water lube area the pump needs to be replaced. The issue is not going from the VST to the lube area, it is a problem with the fuel pump. When the pump leaks and fills sufficiently to backup the tube you will see fuel in the tube going to the VST not the other way.
Have a look here

http://forums.iboats.com/mercruiser-...ce-630623.html

I've asked "How could the sight tube not have a vacuum on it with the connection I've described?" and that question has not been answered and now I've created a "stir". This engine is Mercruiser original and this configuration has been discussed in this forum in the past. Unfortunately I've not connected with anyone that knows this engine. I wish I knew what the problem was. Maybe reading comprehension combined with a need to up your post count.

Well, I'm out.

Oh AllDodge, I have read that thread. It just doesn't help with my question.

Sorry to not be of help. I have the same engine setup you do with mechanical fuel pump and VST. So to answer the vacuum question, the line runs from the VST to the fuel pump. Under the plenum the line is teed and connects to a port which has about a 1/32 hole in the center of it. If the line was removed the hole is so small your engine wouldn't know it was off. The vacuum at the point should equal intake vacuum but since about anything can plug it up. I'm guessing here but since the hole is so small it could also take some time to draw enough vacuum to show up on a gauge, so you may or may not get a reading.

To answer your original question...

I've seen guys just lay the vent tubes from the mechanical pumps along where the intake and head meet and the gear lube still gets contaminated with fuel. So no matter where the vent tube is hooked up (VST tank for EFI, carb/flame arrestor on carb motors, or flopping around in the bilge) the pumps still weep fuel into the gear lube even though the pumps are otherwise "fine" and their diaphrams are not ruptured.