Fuel tank and Sending Unit questions

[UB]

I just returned from a long (160 mi) trip down the river to the C. Bay and back. Everything onboard works fine except all of a sudden the gas gauge always reads close to full. If I disconnect either of the 2 wires to the sending unit, the gauge goes to empty so I figure the gauge and the wiring to the tank is ok. Here's the thing...This boat (97 Chris Craft Sea Skiff 20) has a 64 gallon plastic tank. When I opened the access cover in the deck over the sender and looked in, With about 30 gallons in the tank I saw the gas tank top is concave as if as the gas is used the tank top collapses down??? IS THIS OK?? As the tank top surface goes down only in the middle and not near the corners. Rectangular tank about 4 or 5 feet long by 2 feet wide by I'm not sure how thick. After a fill-up the tank looks nice and square (top flat). It's like there's a vacuum created inside the tank as gas is consumed. I tried opening the filler cap and I heard air rushing into the tank but it did not re-expand until actually filling with gas. To make a long story short, Is this nornal? Not a hiccup from the engine. By the way, the gauge was working fine up until this week. Maybe ok and just a stuck float?? Or maybe the float/sender is getting mashed to bottom of tank as the top gets sucked down. There is a vent hose from tank up to filler hole but once cap is on the system seems sealed. ??? Sorry for the long questions and thanks to alll your help.

 

[pistol37]

Re: Fuel tank and Sending Unit questions

As for your fuel gauge testing it is simple. Key up (not start) the ignition switch, as this likely powers your gauge. If you were to ground your sender wire to a good gauge it should, if working correctly, peg to full. With a fuel gauge this sender wire should be pink. If you ground this pink wire and your gauge goes to full, I'd say that you have a faulty fuel sender. If you have a mulitmeter, you can take the sender out of your tank and connect your multimeter to each pole of the sender. Now move your float up and down and check for changes on your multimeter as this is now representing a fuel gauge. If you need to replace the sender don't settle for one that is "close enough". Either get another sender exactly like the old one or measure your fuel tank depth and go from there. Your tank sucking in could be because of a clogged vent hose, but sometimes this is just normal for plastic tanks.

 

[Gary H NC]

Re: Fuel tank and Sending Unit questions

I would check the fuel vent line for restrictions.If you open the cap and hear air rush in you have a blockage.

 

[UB]

Re: Fuel tank and Sending Unit questions

I believe that the sender is working, It's the fact that the tank collapses as fuel is used essentially keeps it reading full because there is no air in the tank. I will look into what allows air into tank as gas is used. The filler vent is ok but only vents to outside when gas cap open...so...there must be another way for air to enter tank as fuel is drawn off. I will research this. There must be a check valve or something. Maybe it's not working.???

 

[UB]

Re: Fuel tank and Sending Unit questions

I have done a detailed inspection of the fuel system lines to and from the gas tank and there is no vent to the outside for the tank. There are 2 hoses at the input side of the tank, 1 for fuel fill and the smaller 5/8" for the vent. The vent line runs to the fuel filler and merges into the filler neck just below where the cap threads end. When the cap is on, it essentially seals the system. Would it be a good idea to install an outside vent (Thru-hull with 5/8" hose forming a loop higher than the vent and filler) to allow air into the gas tank as it is drawn out under operation??? This would stop the top of the tank from being sucked down due to vacuum. The loop would prevent any water intrusion as it would just run back down and out if any got in at all. A vented gas cap would solve this but I've never seen one for a boat. I'm worried that it was designed this way for a reason and doing so might cause some other problem. The other end of the tank has single hose outlet with what looks like an in-line anti-siphon fitting, then runs to engine. If the tank top stays flat then the fuel gauge sending unit functions perfectly. Thanks for any help or advice.

 

[erikgreen]

Re: Fuel tank and Sending Unit questions

Is this a new boat? Who did the install of the fuel tank?

I'm pretty sure that CG regs require a vent on the tank, and your tank doesn't have one - what it does have is called an overflow tube or fill vent, and it doesn't count. Did someone replace the filler and cap recently?

Erik

 

[UB]

Re: Fuel tank and Sending Unit questions

1997 CC Sea Skiff 20, 64 gallon plastic tank. Original I assume. I did replace the cap chain. Nothing else. Boat was given to me over the winter. I know the owner history but first owner deceased and second owner never replaced tank or filler neck.

 

[Silvertip]

Re: Fuel tank and Sending Unit questions

Did you ever suck the air out of a soda bottle -- either as a kid or an adult -- and notice the bottle collapsing?? That's what's happening to the fuel tank. You can't suck fuel out of the tank without letting air into it. The result is a collapsed tank which is what you are experiencing. The tank MUST have an atmospheric vent. It is either built into the fuel cap or its a separate line to the fitting on the hull.

 

[UB]

Re: Fuel tank and Sending Unit questions

Just wanted to post a follow up. I installed a proper vent line to the outside and it fixed the collapsing tank issue and now the fuel gauge sending unit functions as it should. It's hard to believe that there was never an external vent installed by the factory. Thanks again to everyone for your help.