Fuel gauge working?

[kitebuz]

This may be a stupid question - but I'm going to ask it anyway.

What is the best way to tell if the fuel gauge is accurate?

I recently purchased a used boat w/ a 100 gal main tank, and a 40 gal aux tank. There are two fuel gauges. The main tank gauge is accurate. The aux tank shows full, but I am not sure it actually is, and have not added any gas to it yet. The previous owner thought the tank was empty. The gauge "behaves" the same as the main tank gauge in that it goes to zero w/ the ignition off, and gradually rises to full when on.

Thanks.

[rockn n rollin]

not sure about accuracy but my yamaha guage falls to empty when switched off. if you want a more accurate reading and your boat is on the trailer now, put a level on the boat and level it with the trailer jack. more rise by the trailer will push the guage towards the full mark.

[Monstro]

The fuel sender is a variable resistor that adjusts the voltage measured at the fuel gauge.
Battery voltage will also affect the level shown on the gauge. The best way to test the gauge
is to fill the tank with the ignition on and watch the needle deflection.
Too bad my CIGAR post is buried. I mentioned fuel gauges and not to rely solely on them.
You might drain the aux tank anyway. Who knows how old the gas is.

Monstro
Been there, Done that, Got the scars.

[tinmarine]

It may also be stuck depending what type of sending unit you have. If it's a hollow tube with a float in it, it may be prone to sticking. Pull the sending unit out of the tank and see if it moves freely with no hesitation.

Jim

[Curmudgeon]

You can't possibly know if the gage is accurate until you know how much (if any) fuel is in the tank! Try switching the sender wires between gages and see what happens. If both gages read as before (except reversed), you can assume the gages are functional and can then proceed with determining amount present.

[jethro1]

The best way to tell how accurate a gauge is is to observe its indication while filling the tank.

Start with the tank being empty. The gauge needle should be on the empty position.

With the boat in the water, or on a trailer at the same pitch angle as it rides in the water, add gasoline in 1/8th increments (12.5, 25, 37.5, 50 gallons, etc). Observe to see gauge each time you add gasoline and see if the gauge needle increases accordingly. If so, you know you have a correct system. If not, you could make a quantity correction card so that you know how much to add or subtract from indicated readings to determine actual fuel quantity.

Once you get tired of the conventional fuel quantity indication system and want to go to the "real deal" I recommend installation of a Centroid Solid State fuel sender and a Cruzpro Digital Fuel Quantity Gauge. I installed these two devices and they are 100 % accurate.

[kitebuz]

Thanks for the replies.

I am trying to avoid adding more gas to this tank. I want to keep it empty most of the time, and only use it for the occasional longer runs (keep the weight down & use fresh fuel).

They are new tanks this year, so hopefully the sending unit is good (could still be stuck though). I think Monstro has the right idea in checking the sending unit 1st. This should also allow me to just look inside the tank and see what is there. Start at the source and work back. Swapping the relay wires is also a good idea to see if the gauge reads the same, but I think it would read the same if the sender was stuck.

I was hoping to avoid removing any tank fittings, but this may just be the easiest thing to do. It's that, or just add gas as others have suggested. I was hoping for an easy solution like running some kind of dipstick down the fuel fill line, but doubt that would work very well.

Draining is another option, but since I know the tanks were installed @ 1 yr ago, this gives me an idea of the fuel's age. If I have 40 gals of fuel that is say 10 months old, can I get away w/ just adding Sta-bil, and running thru that tank 1st in the Spring? (engines are '91 Evinrudes).