Strange fuel tank issue
 

I have a sonic 35ss, 725's super charged, bravos. The boat has 2 70 gal aluminum tanks which were replaced in 1997. I have only owned the boat for 2 years and bought it as a complete project.
The port side tank uses much more fuel than the starboard . Each engine runs independent on its own tank. There is no fuel valves or crossover that I can see. In fact it is a very simple fuel system . 1/2" line with AN fittings straight from tank to mech fuel pump then to fuel filter and then to carb. Port tank drains 1/3 faster than other.
Here is a list of the tests I have done so far:
Filled both tanks to the top, ran the boat then topped both tanks again, results 45 gal used port . 30 gal used startboard.
I did that test several times with always the save results.

Next I swapped the fuel lines so the port engine now draws from the starboard tank. The problem does not follow the engine it stays with the port tank. Again I filled both tanks to the top, ran the boat for a while and topped the tanks both off again . Port tank took 1/3 more fuel .

There is NO smell of gas at any time and my bilge has no signs of fuel.

Any ideas??????

Billy, are you 100% positive the tanks are not connected to one another ? I have 2 -100 gallon tanks and I always notice one empties a little faster then the other. Not as fast as yours but noticable by time I run low on fuel. I always "assumed" that they are connected somewhere and one just gets pulled from a little more then the other.......

That is a lot of difference. Have you noticed a difference in RPM, thorrtle response, power, etc. on the starboard engine? Have you changed both engine mounted fuel filters? Did you run rhe starboard motor out of fuel while still running the port motor? If so, you may have clogged the starboard fuel filter. Check your fuel pressure on both engines. Previous owner may have replaced the starboard fuel pump with the wrong pump.

Ed
'99 Sonic Prowler 260
502 MAG/380 Hours/B1 Drive/B1 24P Prop
Dart PRO-1 Aluminum Heads/SS Intakes/Inconel Exhausts
Comp Cam Roller Rockers/Push Rods (.375")
Cometic SS Head Gaskets
ARP Studs
IMCO Aluminum Exhaust
Hardine Polished SS Risers
Complete MSD Ignition/Cooling Fan
Denso Iridium Plugs/MSD Plug Wires
K&N/Hardine Marine/PWR Air Intake
ECM Upgrade
San Juan FWC
RACOR Water Separator

Thanks for your response Ed, if you read my post you will see that I switched tank pick ups from one tank to the other so that port engine now pulls from starboard tank and vice versa . That eliminates any variables of engines or fuel system. It's is tank related

The more I read the more I feel there connected....... I'm no expert though.....

Weird problem....., you don't have the fuel tank valves under the rear seat marked right/left?
I think someone asked if the vent lines were tee'd together ?
Can you see the entire fuel line from the outlet of the tank all the way to the pumps.

The vent lines are not tee'd together and the fuel lines go directly from tank to pump. I made the lines and I stalled them myself. I didn't use the fuel selector switch because it only has 3/8" fittings which are not enough for 725hp. I might just install a transfer pump between the 2 tanks to manually keep them even . But the big question is , where is the fuel going?

OK, let's review. The fuel systems are not connected in any way. The fuel is not being pumped into the boat. That would be a lot of BOOM!. So I believe the fuel is some how being syphoned out of the port tank while you are running. That is the only way it can get out. Question: are the tank vent lines located in the same place on both tanks? I don't see how this could happen but it is the only way you could be loosing the fuel. You are not burning it, it is not leaking into the boat and the systems are totally isolated. I'm still scratching my head on this one!

Vent line is pinched or clogged causing the tank to pressurize and push more fuel to the motor?

Can't push more fuel to engine, I have fuel pressure regulators

Could fuel be sloshing around and thrown out the vent
while underway? No other way out

I agree with can't push more fuel to the motor. The only way that could happen is if the FPR diaphram is ruptured and sucking fuel back up to the intake plenum. But then, I would assume, the motor would run sh**ty or not at all. Have never experienced that data point so I don't know. So the only way it is getting out of the tank is the fuel fill cap or the fuel tank vent. And since the fuel cap is sealed with an "O" ring, it has to be the fuel vent; unless someone installed a "mystery" line between the tanks and that does not seem possible since the tank loosing fuel is not dumping it into the other tank. If you ever find out the cause, please post what it was!

Ed

Obama?

Was the boat out of your sight at any time between the "top off, run, top off again" test? Maybe someone is stealing gas and that side is easiest to get to?

I see you didn't use the Sonic factory fuel valves, however are you sure that is the only part of the system? Not sure exactly how it works but it seems there may be a crossover line involved somewhere.

My boat always uses more fuel starboard, but I know it is because that engine does work slightly harder, and is usually opposite with boats that spin out. However, your situation is just unexplainable with your tests. Has to be a link somewhere.

If you were losing 15 gallons out the vent in the short time that you burned 30 gallons, you would know. A gallon of gas on the water looks like a tanker spilled. :D

Granted I just got my 358 but my Port side uses a fair amount more (according to the guage) than my Starboard side.

Mine is full factory stock 496 HO's but I filled it up a few times and I would say the tanks are about 1/8th tank off from one another when pumping the same amount of fuel into both.

Thanks for all the replies and ideas. But so far I've not been able to figure it out. It's still happening and driving my crazy. BTW once the problem is solved the boat is going to be for sale

Have you had to put more fuel in 1 tank vs the other since you bought the boat? Could it be that 1 tank is larger than the other? ...thus not really burning more, but takes more to fill to top?

His tests negate that because he is starting from full, running down, then filling back to the top. Wouldn't matter

He is either leaking gas, or he is linked and has uneven equalization. I think the tanks are equalized with a hose.

A friend has a trip engine boat with center engine drawing on both tanks and tanks equalized, doesn't work. He always draws more from one tank.

This is a very interesting challenge! It seems odd that you run the fuel to the mechanical pump and them a fuel filter. Do you have a filter in the line before it gets to the mechanical pump? Is it possible you have an air leak in the fuel line from the port tank that is allowing less fuel to flow. This is a crazy plan that would require minimal effort but might isolate the problem. Take 2 - 6 gallon outboard tanks and plumb each into separate motors and give it a quick run. This will isolate your entire fuel system leaving you a hose to the filter -mechanical pump- motor. If you were in my neck of the wood I have 2 tanks and everything you need to make a test. This would definitely give you some additional data to resolve this issue once and for all. Good luck and place keep us posted.

Any pictures of the setup?

It would seem there has to be an equalizer or some sort between the tanks. With that much horsepower, I assume you removed the anti-siphon check valves from the fuel tanks or are they still in place?

With everything you have already done, that's the only other thing I can think of. Perhaps one of the check valves is stronger than the other so you are pulling 66% of the fuel from the weaker sprung valve, and 33% from the other.