On our '88 E34 I just discovered that both the fuel vent hose and the filler hose are routed loosely way downwards under the cockpit sole after they exit the tank top. I think this is not good. This means that the low loop in both hoses is full of fuel. To fill the tank, the low loop in the vent hose has to burp air before more fuel can be added.
From a quick look it appears that there is room to re-route the hoses upward towards their deck fittings as they should be. Perhaps the fuel system designer wanted the proper routing but the builder didn't do it correctly.
I've had this boat for two winters. To fill it I add fuel slowly with a small siphon hose from a 5 gallon portable tank until the fuel backs up the filler hose and then I assume that the tank is full. After filling I have looked at the manual gauge on the tank but it showed half full. I thought it was broken. Anyway, each fall I top-off the tank before haul-out for the winter. This spring we've only done about 6 hours since de-winterizing with a "full" tank, but after removing the manual gauge and examining the tank I saw that it really is only half full !
One clue was that when I removed the first fuel gauge screw (with the engine off), there was a hissing sound which I determined to be from releasing a vacuum from the lift pump previously sucking from the tank. If I had just "filled" the tank, this would have been releasing pressure from the weight of some fuel in the filler hose. With the engine running, the fuel lift pump must be strong enough to lift the fuel in the vent hose and let more air in when the tank vacuum gets too high.
I removed the fuel vent hose from the tank end and could hear fuel sloshing around in the vent hose. Then I pressurized the tank end of the vent hose and there was significant backpressure from the fuel in it. When I released the pressure, fuel came out the tank end of the hose.
I noticed similar tank pressurization behavior in my last boat, an '88 E32-200 but didn't look into it or consider the consequences.
I think my next task is to re-route the vent and filler hoses continuously upwards to eliminate the problem. I also will put one of those Attwood fuel vent hose check valves in the vent hose.
Is my analysis here correct ? Any fuel system professionals out there ?
-- neal
From a quick look it appears that there is room to re-route the hoses upward towards their deck fittings as they should be. Perhaps the fuel system designer wanted the proper routing but the builder didn't do it correctly.
I've had this boat for two winters. To fill it I add fuel slowly with a small siphon hose from a 5 gallon portable tank until the fuel backs up the filler hose and then I assume that the tank is full. After filling I have looked at the manual gauge on the tank but it showed half full. I thought it was broken. Anyway, each fall I top-off the tank before haul-out for the winter. This spring we've only done about 6 hours since de-winterizing with a "full" tank, but after removing the manual gauge and examining the tank I saw that it really is only half full !
One clue was that when I removed the first fuel gauge screw (with the engine off), there was a hissing sound which I determined to be from releasing a vacuum from the lift pump previously sucking from the tank. If I had just "filled" the tank, this would have been releasing pressure from the weight of some fuel in the filler hose. With the engine running, the fuel lift pump must be strong enough to lift the fuel in the vent hose and let more air in when the tank vacuum gets too high.
I removed the fuel vent hose from the tank end and could hear fuel sloshing around in the vent hose. Then I pressurized the tank end of the vent hose and there was significant backpressure from the fuel in it. When I released the pressure, fuel came out the tank end of the hose.
I noticed similar tank pressurization behavior in my last boat, an '88 E32-200 but didn't look into it or consider the consequences.
I think my next task is to re-route the vent and filler hoses continuously upwards to eliminate the problem. I also will put one of those Attwood fuel vent hose check valves in the vent hose.
Is my analysis here correct ? Any fuel system professionals out there ?
-- neal