Fuel gauge question - what to trust?

[kirk a]

Have a pair of Yamaha 225's on my 330 with the standard gauges. Did a long run the other day, and have a question about what I'm seeing. Prior to trip, I filled up both tanks. Both tanks started to bubble fuel up through the vents at the fill. I slowed down the flow and pumped a bit more into each. The main tank gauge read 7/8 bars, and the aux gauge read 8/8 bars. I reset the fuel used. My set up is to have one motor run off of each tank. The trip burned 175 gallons per the gauge. This is about what I expected, given we went ~75 miles each way, plus 7 hours trolling. Upon return to harbor, the gauges read very low. Aux was approx 2/8 bars, and the main was 3/8 bars. I would expect there is still approx 120/200 or 60% in main, and 70/150 or 46% in aux tank.

Short of topping off the tanks again, would you be more inclined to believe the fuel used, or the tank level?

Also, do others have to slow down when topping off the tank, due to the vent?

 

[seasick]

As the tank gets fuller, the vent air flow increases. As a result, fuel may surge up the fill and trip the nozzle. My advice is to listen and learn what the tank sounds like as it gets near full and don't top off. Just slow down the fill rate and when the nozzle shuts, you are done.

I never trust the gauge levels. If you have a fuel management gauge and it works, I would trust that a lot more

I am missing something in your math. One tank selected to a motor. If total burn was 175 gallons, then you can only assume ( and that may be a mistake) that each motor used the same amount of fuel. That would be 87.5 gallons each.
For the main, 200-87.5 = 112.5 or about 56% left. For the aux, 150-87.5=62.5 gals or 42%.
I think I got the math correct.
Note that EMPTY on the gauge doesn't mean the tank is empty necessarily. It only means that the sender is at the bottom of its range. For swing arm senders, that is wherever the arm was preset to. For rod type senders, there is still gas below the pickup and/or sender since neither touch the bottom of the tank.

Some motors have better systems for measuring fuel flow and therefore consumption. Each approach has its pluses and minuses but my advice is to know how much fuel you typically use, and how much you think you used and compare that to the gauge readings

I have mentioned this several times but on my 208 with an 82 gal tank, two bars (out of eight) on the gauge might lead you to conclude that the tank was 1/4 full or about 20 gallons of gas. In fact when I see a change form three bars to two bars, I have 40 gallons left, just about half a tank. When I see one bar, I can have 20 to 30 gallons. When that one bar starts flashing, it's time to panic

 

[kirk a]

As the tank gets fuller, the vent air flow increases. As a result, fuel may surge up the fill and trip the nozzle. My advice is to listen and learn what the tank sounds like as it gets near full and don't top off. Just slow down the fill rate and when the nozzle shuts, you are done.

I never trust the gauge levels. If you have a fuel management gauge and it works, I would trust that a lot more

I am missing something in your math. One tank selected to a motor. If total burn was 175 gallons, then you can only assume ( and that may be a mistake) that each motor used the same amount of fuel. That would be 87.5 gallons each.
For the main, 200-87.5 = 112.5 or about 56% left. For the aux, 150-87.5=62.5 gals or 42%.
I think I got the math correct.
Note that EMPTY on the gauge doesn't mean the tank is empty necessarily. It only means that the sender is at the bottom of its range. For swing arm senders, that is wherever the arm was preset to. For rod type senders, there is still gas below the pickup and/or sender since neither touch the bottom of the tank.

Some motors have better systems for measuring fuel flow and therefore consumption. Each approach has its pluses and minuses but my advice is to know how much fuel you typically use, and how much you think you used and compare that to the gauge readings

I have mentioned this several times but on my 208 with an 82 gal tank, two bars (out of eight) on the gauge might lead you to conclude that the tank was 1/4 full or about 20 gallons of gas. In fact when I see a change form three bars to two bars, I have 40 gallons left, just about half a tank. When I see one bar, I can have 20 to 30 gallons. When that one bar starts flashing, it's time to panic

Your math is better than mine, as I used 80 on each tank not 87.50.

I've never really filled the tank - at least have never had the nozzle click off. Just slow down flow and keep going - listening, as you say. Prior to this boat, I did not have a fuel gauge, I had relied on a Garmin sensor where one added the volume of new fuel. It will be interesting to fill up to see what it will take, I'm hoping the fuel used data is accurate, as it does correlate to the 1mpg most with this boat report.

Thanks

 

[Fowl Hooked]

On my 265 the fuel gauge levels are good for general estimation but I don't trust them, am much more confident in the fuel flow meter and the total it reports. It's still not dead on accurate based on the fill up but it's much closer than the fuel level gauge indicates. On the upside, the bar indicators err on the good side and tell me I have less gas in the tanks than I truly do so they provide a sort of built in safety margin of reserve. Similar to Seasick's 208, on my 120 gallon aux tank each bar, if accurate, would theoretically indicate 15 gallons but I'll get down to one bar and that one will be flashing but I'll burn another 40-50 gallons out of the tank before anything starts sputtering. The main tank has demonstrated the same behavior.

 

[SkunkBoat]

don't forget, if the tank has a V bottom the gauge is inherently inaccurate because the sensor is linear and the tank volume is not linear.

 

[ScottyCee]

I don't even have gauges - Yamaha Fuel Management is what I use and nothing else. My girl holds too much fuel to run full - 1600 lbs of gas. I ran my aux tank empty, and put the desired amount into it as "backup / get to the gas dock" fuel. Then put a known amount into the main tank and reset the counter. My main runs out within ~2% indicated vs. what I put in. When it runs out I reset the counter, change to backup tank, run to get fuel, and when I get there I know exactly what each tanks contains. (Main is empty, aux holds known - indicated on gauge). Restore backup to desired, fuel up main, reset counter again.

Helps to have a pen and paper nearby if you are memory challenged like me. You have to remember / record how much you put in or this system falls on it's face! 8^)

 

[seasick]

On MyOtherBoat, I have Merc Optimaxes with Smartcraft gauges. They display instantaneous fuel consumption as well as cumulative fuel use since last reset. When I got that boat, I didn't find any fuel flow metering devices and wasn't sure how they determined consumption. It turned out that the Optimaxes and I suspect other Mercs calculate fuel burn and consumption based on how long each fuel injector is activated using that in combination with the injector flow rate. So I guess as long as the injectors work correctly, the estimates are pretty accurate.
My actual gas guage is all over the place but the fuel consumption cumulative number seems good.

 

[grady23]

Not sure how old your sending units are, Bu I replaced both of my original sending units with WEMA senders. Have NOT questioned them since. They were replaced in 2004.

 

[SmokyMtnGrady]

I have found my Yamaha fuel flow and consumption meter is accurate within a gallon or two. I will fill up the tank full. reset the guage on fuel burned and start my trip off at zero. The next time I fill up I will compare what I burned to what I just put in to get it full and the difference is often just a gallon, maybe 2 tops. My fuel gauge on the instruments is a good way to get an idea at a glance where you are more or less,but the fuel burned is far more accurate and trust worthy.

 

[Blaugrana]

As one that does “trust” their fuel gauge being new to the boat, I can say that fueling (aka listening to the gas flowing) and where the gauge was at prior the fueling shows how inaccurate the gauge is.

I have had the gauge at 2 bars on several occasions and each time the amount I put in seemed to vary by 10 gallons. For good measure, the last time out I fueled until I got the click and it made no difference.

I’ll address in the future when I repower but I somewhat ignore the fuel gauge and run it down to whatever knowing I have the auxiliary filled

 

[seasick]

As one that does “trust” their fuel gauge being new to the boat, I can say that fueling (aka listening to the gas flowing) and where the gauge was at prior the fueling shows how inaccurate the gauge is.

I have had the gauge at 2 bars on several occasions and each time the amount I put in seemed to vary by 10 gallons. For good measure, the last time out I fueled until I got the click and it made no difference.

I’ll address in the future when I repower but I somewhat ignore the fuel gauge and run it down to whatever knowing I have the auxiliary filled

The actual number of bars doesn't tell you how much gas is in the tank. If gives you an indication of a range of gas levels. When the number of bars change for example from two to one, that is a better indication but you don't really see that change since 1. you probably aren't looking and 2. your hull is at a different angle that when sitting at the dock.
On my 208, one solid bar means 20 to 30 gallons of fuel.If the display just changed from two to one bar, that means I have the upper end of the range, about 30 gallons When that one bar starts flashing, I have 20 gallons or less.

 

[Kizuna]

On my previous boat I always used the Yamaha fuel consumed/burned meter to let me know how much I have left in the tank(s). As said above, it's accurate within a few gallons depending on how empty the tank is. I currently have a Suzuki set up, and the fuel level senders seem more precise with % of fuel and # of gallons in each tank, but those numbers fluctuate as the boat moves, and is up on plane versus sitting still. I still use the gallons burned readout for my remaining fuel levels.

 

[g0tagrip]

i have found over the years(almost 13) I have had my Marlin with twin Yamaha F-250 my fuel used is pretty accurate, by that I mean +/- 1 gallons. The bars on the fuel gauge are not nearly as accurate as the fuel used. I keep a log of fuel used and record it and compare it to when I fill up the tanks. Like I said I am within a gallon of the fuel used. I trust the fuel flow and fuel used over the tank indicators. As you get to know your boat keep a log of fuel burned and hours so you have a good feel for your fuel used.

 

[kirk a]

Follow up and closure for now. The fuel consumption gauge showed we burned 173 gallons. I filled up with 168 gallons and both tanks are reading 8/8. That is within 3% or so, which is plenty good for me.

Interesting. Main tank is 200 gals, took 76 to fill, but read 3 bars.

Aux tank is 150 gals took 93 to fill and was reading 1 bar.

Now I know that the levels display is "off" and will rely on the consumption.

It is possible that the Aux was not completely full prior to last trip, so I'll double check that next time, but otherwise it looks like I have a fuel consumption discrepancy between motors. The display shows the flow for each motor, so I can get a sense of that, but otherwise think I'll have the injectors and VST serviced this winter.

 

[seasick]

Do your same test again but with the tanks switched between motors. That will tell you if burn rate is different.