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Joe, Marine Mechanic
Category: Boat
Satisfied Customers: 7251
Experience:  ASE, electronics, Marine mechanical and electronics, pwc, wiring, all around repair tech,
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Have 2007 Mercruiser 350 MAG MPI with gen3 cool fuel module

Customer Question

Have 2007 Mercruiser 350 MAG MPI with gen3 cool fuel module that won't siphon fuel from tank. Replaced low pressure fuel pump but still not siphoning fuel (even from an external tank). Noticed I am getting air bubbles coming from engine back to external tank when I hooked up a clear fuel supply hose.
Submitted: 2 years ago.
Category: Boat
Customer: replied 2 years ago.
Hello anyone out there???
Expert:  Joe replied 2 years ago.
Thank you for allowing me to assist you, my name is ***** ***** I will be happy to assist you with your concern today.
I apologize that no other experts have been able to assist you sooner.
Things to start checking, fuel pressure regulator (leaks), injector o-rings, Fittings in line. If needed I would drain fuel from line, plug tank side and then smoke test the fuel supply system, where smoke starts coming out is the source of the leak.
If it is sucking air then it will have a problem, but what you would first need to do is block off the tank side and make sure the leak is still there and not instead in the fuel tank pickup assembly, also try pulling the fuel fill cap and see if that changes anything.
Injector o-rings and regulator are the most common places.
Thank you!
- Joe
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Customer: replied 2 years ago.
I am getting 43 psi on fuel rail at key up and it will idle at 33-37 psi for 2 or 3 minutes (occasionally longer), wouldn't it lose positive pressure right away if I had a leak on the positive side (regulator or injector)? Also no fuel is coming out around cool fuel regulator or injectors that I can see. For whatever reason it is not consistently pulling a vacuum to siphon fuel from tank. I've tried just running a clear hose from external tank to cool fuel module and generally can't get fuel to siphon. If I hook up a hand pump in line between tank and cool fuel module I can keep engine running by hand pumping fuel when fuel pressure starts to drop. I've tried plugging fuel line and measuring vacuum and generally can't get a vacuum, however, every once in a while it will draw a full vacuum of 11 in Hg. It seems to be intermittent. I replaced the low pressure fuel pump thinking this pump is responsible for the vacuum. I didn't change the bottom seal of the low pressure fuel pump because I didn't take off the seawater cooling lines and it was really hard to get to. The existing seal seamed quite tight as I had to work the pump back and forth to pull it out. I'm wondering though if this seal is compromised if it would cause the pump to not create a vacuum? From the Gen iii cool fuel pump drawing it looks like this seal isolates the low pressure pump inlet to draw fuel from the fuel filter chamber. I guess if it wasn't sealing it would draw some fuel from inside the cool fuel pump chamber which would probably not be good.
Expert:  Joe replied 2 years ago.
Fuel pump will create a suction, if you disconnect and put your finger over while its pumping do you feel a suction and will it hold a vacuum?
Customer: replied 2 years ago.
Generally no suction and no vacuum. Every once in a while it will draw fuel for a little while and have a vacuum. This started as a sporadic problem where I would take boat out for anywhere from 10 minutes to 1 hour before it would not draw fuel. Now it is pretty consistent where it generally won't draw fuel. What's weird is even with holding a clear fuel supply hose (filled with fuel) up 3 or 4 ft above engine, most of the time it will only run for a couple minutes and not draw any fuel. It's like it is using the fuel in the cool fuel module and that is it. I have checked for restrictions and pulled the hose fitting off the cool fuel module to make sure it wasn't plugged and there is nothing stopping the fuel to get into the filter and cool fuel module. As I said before, I can use a hand pump and force fuel into cool fuel module and it will keep running.

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