Re: Mercury 50HP EFI Bigfoot Spraying Fuel in Water
I've got a 2005 Mercury 50HP Bigfoot with the same problem - pumping fuel out the VST tank vent into the water. I'd had addtional fuel leak problems which I'll explain later.
I've had the VST tank apart and replaced the needle valve. The old needle with the rubber face looked perfect to the naked eye but under 30x magnification, you can see slight manufacturing imperfections - nothing that should cause the needle/seat interface to leak. Also, the rubber face on the needle was not hardened. I had been running oxygenated fuel and stabilizing with SeaFoam over the winter. I would also drain the main fuel tank and start with fresh gas every spring. I did not drain the VST tank. This year I started running non-oxygenated 91 octane fuel. Anyway, I replaced the needle and it stopped leaking for a hour or two. When I first opened the VST tank at the start of all this I notice tiny black particulates at the bottom of the tank and cleaned the tank thoroughly. I made a mental note that those particulates would probably ulitmately be a player in the root cause. There was no varnish buildup in the VST tank.
I also check the fuel pressure from the mechanical pump and is was 2-3 psi, just where it's supposed to be. Not that it matters for this problem but the high pressure is also where it's supposed to be at 42-44 psi. I also experimented by disconnecting the mechanical fuel pump and used a pressure gauge and a large syringe full of fuel to pressurize the needle/seat at 2-3 psi, with the VSP tank full, and was able to push fuel right through the needle/seat. The float appears to be in perfect condition as is the needle seat.
While I was waiting for the new needle to arrive, I disconnected the overflow tube from the VST tank and replaced it with a fuel line that went all the way back to the main fuel tank rather than dump in the lake. Now overflow from the VST went back into the main fuel tank. This actually works great and I don't know why it's not part of the original design.
I've not yet determined the root cause of the problem. My VST is leaking again so I guess my next step will be to start replacing fuel lines, possibly go though the mechanical fuel pump so see if there is some rubber degrading in there that is flowing downstream into the VST needle/seat. When I replace the fuel line between the mechanical fuel pump and the VST tank, I'll probably add an inline filter to catch any particulates that come through until I can determine the source of the particulates.
The other leaks I mentioned previously are quite disturbing to me and makes me wonder how a manufacture can release such a design. Anyway, there is a fuel cooler/pressure manifold bolted to the side of the VST tank that acts as the high pressure manifold that directs fuel to the fuel injector system. Fuel is pumped from the high pressure electric pump in the VST tank into this manifold then back from the manifold against a pressure regulator inside the VST tank to maintain the correct fuel pressure to the fuel injectors. Also fresh water flows through this manfold to cool the fuel and reduce vapor generation. The manifold appears to be some kind of machined aluminum block (possibly a simple aluminum extrusion) with a stainless plate on either end. When I first started noticing fuel in the water around the engine, I ran the engine with the cover off and found that raw fuel was spraying at a high rate between the manifold body and the stainless plate. The service manual states this is a non-servicable part (new one is $375, really?). I remove the screw in the center of the stainless end place and found four o-rings. Two on the fuel side and two on the fresh water side. The o-rings on the fresh water side were in good condition but the o-rings on the fuel side had hardened. I replace the o-rings and the leak stopped. As an engineer by trade, this kind of failure mode is unacceptable, i.e. spraying raw fuel inside the engine compartment just below the starter motor. I'm glad I didn't burst in to flames while I was on the lake with my family. Again, keep in mind that the high pressure is regulating correctly so the leak is not a result of over pressure.
In my opinion, this whole fuel system design is a kluge, I can see how it could evolve that way from a carburated system with a mechanical fuel pump to the whole VST kluge then adding two fuel coolers to resolve vapor lock problems. They run the fuel back and forth across the engine twice. No big surprise they may have had some problems with vapor lock. Easy for me to say but does there really need to be two fuel pumps, two fuel coolers, multiple filter points, a needle/seat and float, fuel lines run back an forth across the engine twice! If this is the best they can do, put the carburator back on, it would be much simpler and much more reliable, less likely to empty my fuel tank in the lake or burst into flames should a spark occur inside the engine cover.
Are the other OEMs this complicated? Once I get past this I think the ultimate solution will be Honda or Yamaha. Oh, by the way, this all started with less than 50 hours on the engine. And now the o-rings on the bottom of the fuel cooler are spewing fuel into the engine compartment. My bad for not replacing them when I replaced the others.
