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Fuel pump vent system question

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icu812

New member
Hey all -
Does anyone know how the fuel pump vent system works in the 7.4LX MPI (1996 year) Mercruiser engine? Specifically, if I stuck a pressure gauge on the "sight tube" at the sea water fuel pump what would it read? I think it would read manifold vacuum but I'm not sure. The sight tube vents the mechanical 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 pump with an electrical pump to get away from the problem of leaked fuel into the gear oil. I started wondering if I there is a vacuum on the vent that contributes to the problem. Thanks.
 
You say " fuel leaking into the gear oil " -not sure how that could happen.-------Mechanical fuel pumps need to control leakage by sucking leakage from a diaphragm into the motor intake.----This prevents fuel vapour build up in the boat.--------There are some very strict rules on boat fuel systems enforced by Coast Guard regulations. -Please read the rules.--------Then again you could wear a parachute if you make changes to a boat fuel system.
 
racerone said:
You say " fuel leaking into the gear oil " -not sure how that could happen.-------Mechanical fuel pumps need to control leakage by sucking leakage from a diaphragm into the motor intake.----This prevents fuel vapour build up in the boat.--------There are some very strict rules on boat fuel systems enforced by Coast Guard regulations. -Please read the rules.--------Then again you could wear a parachute if you make changes to a boat fuel system.
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Well, thanks for the response but I really can't make sense of it. This is a well known problem with the sea water pump/fuel pump design and mine is a simple question. What is the pressure or vacuum in the fuel pump "sight tube"? Anybody?
 
there is 0 pressure in the tube. It is called a "tell-tale" to indicate fuel from a ruptured diaphram. the tube carries the fuel up to the carb and dumps it in.mechanical pumps need this vent. it`s what allows the diaphram to work properly.
 
You will do away with the clear hose when you install the electric pump. You will need to hook the electric fuel pump up with an oil pressure switch to conform to Coast Guard Regs.
 
Bt Doctur said:
there is 0 pressure in the tube. It is called a "tell-tale" to indicate fuel from a ruptured diaphram. the tube carries the fuel up to the carb and dumps it in.mechanical pumps need this vent. it`s what allows the diaphram to work properly.
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Thanks for the reply. "sight tube" is the Mercruiser terminology (Service Manual 16 pages 5A-3 and 5A-7).
There's no carb; it's an MPI type engine. I'm willing for there to be "0 pressure" (by which you mean atmospheric pressure I presume) in the tube but please explain how this could be with the vent system of my engine that I described; mech. pump to VST, VST to intake plenum. There is no vent to the atmosphere. And yes, I understand the multiple uses for this tube. Thanks.
 
Docksidemarineservices said:
You will do away with the clear hose when you install the electric pump. You will need to hook the electric fuel pump up with an oil pressure switch to conform to Coast Guard Regs.
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Thanks Chris. There is a Holley (712-801-1) that's in the running and it needs the vent line. If I do install this replacement pump I'll slave a relay off the existing ECM controlled fuel pump relay.
I'm trying via this post, to figure out if maybe all that's required is to move the mechanical pump vent line from the VST to an atmospheric location. I'd try this first if I knew this line is usually at manifold vacuum. If so, that could put a very high pressure across the pump diaphragm. I'd go make this simple measurement if I hadn't taken my engine apart already. Any knowledge about the vent system, Chris? Thanks.
 
Get the serial number off the engine. The line you are speaking of that has manifold vacuum may go to the fuel pressure regulator. I will no exactly what you have after you post the serial number.
 
We`re all confused now3. You dont have a mechanical fuel pump if you have an electric pump for the fuel injection?This question is based on my intent to replace the mechanical pump with an electrical pump to get away from the problem of leaked fuel into the gear oil. I started wondering if I there is a vacuum on the vent that contributes to the problem. Thanks.
 
Get the serial number off the engine. The line you are speaking of that has manifold vacuum may go to the fuel pressure regulator. I will no exactly what you have after you post the serial number.
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No, the fuel pressure regulator is at the end of the fuel rail nowhere near this sight tube and having nothing to do with the mech. fuel pump/VST vent system. I've given you the year and model of my engine but please do not expend any further effort on my behalf. It's obvious you're unfamiliar with my engine.

We`re all confused now3. You dont have a mechanical fuel pump if you have an electric pump for the fuel injection?This question is based on my intent to replace the mechanical pump with an electrical pump to get away from the problem of leaked fuel into the gear oil. I started wondering if I there is a vacuum on the vent that contributes to the problem. Thanks.
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Well, I'm confused by your including part of my response in your response, however, there are two fuel pumps. The low pressure to fill the VST and the injector pump in the VST.

Thanks to you both, BT and Chris, but neither of you are familiar with my engine so please concern yourselves no further. I appreciate the replies but I need someone who knows this engine.

Oh, and Merry Christmas.
 
kimcrwbr1 said:
If your getting fuel in the oil it could only be a couple things. Fuel washing down the rings either faulty injector/s not closing all the way and when you shut it down the fuel pressure bleeds off thru the injectors or a leaking diaphram in the fuel pressure regulator dumping raw fuel in the intake. If it has a regulator take a glass jar and put two nipples in the lid and connect it in line on the vacuum tube if its sucking fuel the raw fuel will drop into the jar or make shift sump. If the fuel pressure drops rapidly as soon as the engine is shut off the injectors are suspect.
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It's a Raw water pump mounted fuel pump.
 
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