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Fuel Bleed Back Mystery


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I NEED HELP. I want to thank everyone on this site. I wouldnt know as much as I do about this problem if it wasnt for you. So thanks, but I need more help. I will spare all of the info on how I got this far, but this is what I do know: I have a 94 Malibu Echelon with a EFI 454. It has a mechanical fuel pump (lift pump) and an electronic injector pump. The system is good fromt the mechanical pump to the engine. After the engine has been run and then shut down for awhile, fuel is draining out of the mechanical pump back towards the tank. When the motor is cold, the pump will overcome the air gap and begin pumping fuel, but after the engine has been run and is hot, the mechanical pump can not overcome the air gap and will not pump fuel. The engine will run on the fuel in the pressurized line between the mechanical pump and the engine, and then will die. It seems that the fuel line is vapor locking somewhere before the mechanical pump in the vacuum line. My question/concern, why is the fuel draining away from the pump? I think the air is causing the vapor lock. I installed a clear glass inline filter before the mechanical pump so I can monitor what is happening. When I first did this, I not only noticed the fuel quickly draining back after shut down, but I also saw air bubbles while running. I would guess this is bad. I suspected a problem with the anti-syphon valve and I have replaced it with a normal fitting (no check valve) for testing. This change eliminated the air bubbles and fuel did not drain back (at first). After running the boat up to temp, fuel again began draining back after shut down. There is no longer a check valve at the tank, but the line should be primed and not draining. I am not sure where the fuel was going when the anti syphon was installed. It could not go back to the tank (check valve installed), and I did not smell or see any fuel in the hull. I guess there was an air gap? I am leaning toward a hole in fuel line that is opening up when heated by the engine or when put in a vacuum with the fuel pump. With no anti syphon valve the pump isnt pulling as much vacuum, causing no air bubbles. Whith anti syphon the pump is pulling more vacuum and opening a hole and allowing air in. Any other thoughts? Is my pump going bad? I am not sure what is normal, but I would guess the pump should remain primed with no air bubbles and no air gap. I am sure there are some questions that I didnt include, so ask away or provide some help. I am very mechanical, but not a lot of experience with boat motors.

Thanks

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  • 2 weeks later...

After replacing the mechanical fuel pump, fuel lines, and anti syphon valve the problem was still not fixed. I finally installed an in-line fuel booster pump. This solved the problem. The fuel system remains primed and I have no starting issues. I highly recommend an in-line electric booster pump for anyone with a mechanical fuel pump.

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After replacing the mechanical fuel pump, fuel lines, and anti syphon valve the problem was still not fixed. I finally installed an in-line fuel booster pump. This solved the problem. The fuel system remains primed and I have no starting issues. I highly recommend an in-line electric booster pump for anyone with a mechanical fuel pump.

I have the exact same problem with my 92 Skier. Where did you get the in-line fuel booster pump and how much was it? Thanks

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I am sorry to hear you are having the same problem. Do you also have a mechanical fuel pump? I got the booster pump at AutoZone. It was a mister gasket pump. It was about $50. It is designed to be used in-line with the mechanical pump and mounted near the tank. They have a couple different models that have different flow rates and pressures. Mine was between 4 and 7 psi. I believe the mechanical pump runs about 5 psi. It puts out 25 gallons per hour, I think. I don't know what the mechanical puts out, I couldnt find that info, but this seems to be more. The only one they had in stock was for 5/16" fuel line. My boat uses 3/8". I just bought some brass fittings to make the connection. Since it has the same or greater output volume, I wasnt worried about the line size. I will worn you, this product is night marine rated. I wasnt too worried about it for my application, but you should know. If you buy a marine rated booster pump from Discount Inboard Marine, they are much more expensive ($100 to $200).

Hope this helps.

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I am sorry to hear you are having the same problem. Do you also have a mechanical fuel pump? I got the booster pump at AutoZone. It was a mister gasket pump. It was about $50. It is designed to be used in-line with the mechanical pump and mounted near the tank. They have a couple different models that have different flow rates and pressures. Mine was between 4 and 7 psi. I believe the mechanical pump runs about 5 psi. It puts out 25 gallons per hour, I think. I don't know what the mechanical puts out, I couldnt find that info, but this seems to be more. The only one they had in stock was for 5/16" fuel line. My boat uses 3/8". I just bought some brass fittings to make the connection. Since it has the same or greater output volume, I wasnt worried about the line size. I will worn you, this product is night marine rated. I wasnt too worried about it for my application, but you should know. If you buy a marine rated booster pump from Discount Inboard Marine, they are much more expensive ($100 to $200).

Hope this helps.

Yes, I do have a mechanical fuel pump. Thanks for the info. I can raise my hood and run cool water over my fuel pump when I know that the engine will be off for a while and it solves my proble. It is just a big hassle. There is always something in the boat that wants to slide over on the engine when the hood is raised, not to mention there is kids in and out of the boat and I am scared one of them is going to get burned. I will have to look in to doing something like this to mine..Thanks

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