Fuel pump vent system question
01-04-2014, 04:50 PM
#11
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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.
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.
01-05-2014, 03:31 PM
#13
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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.
01-05-2014, 04:01 PM
#14
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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
01-05-2014, 05:24 PM
#15
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? 
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Last edited by US1 Fountain; 01-05-2014 at 08:39 PM.
01-05-2014, 05:49 PM
#16
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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.
01-06-2014, 01:58 PM
#17
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.
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.
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.
01-06-2014, 04:32 PM
#18
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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.
Last edited by keith2500hd; 01-06-2014 at 04:35 PM.
01-07-2014, 05:45 PM
#19
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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.....
Great place, lots of great people. Try and be one.....
01-07-2014, 06:03 PM
#20
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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.
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.
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.
The fuel pump is 2 years old purchased to replace a fuel pump that was causing gear lube dilution.
Have a look here
http://forums.iboats.com/mercruiser-...ce-630623.html
