Fuel sender accuracy issue

MRWood

Well-Known Member
Last winter while redoing my boat, pod etc, I also changed fuel tank and gauges. My fuel gauge reading is not matching what is in the tank. It’s been a bit of a head scratcher for me this summer as I took a while to realize it was off. Fuel shows up as a percentage of full on my Mercury smartcraft gauge. As an example when I would go and fill the tank the gauge could say 65% full but the actual fuel added would total up to an amount greater than the 35% required to fill the tank. The gauge should have been reading more like 47% full.
After doing this a number of times this summer I thought the sender must be wonky. It is a Moeller fuel sender with float on end of metal rod. Then after more thought I came to the realization that my irregular tank shape is the cause of the issue. The bottom portion is shaped to the hulls V. Therefore the lower section of the tank holds less fuel per inch of height than the upper section.
Yesterday I went out and measured how many inches of fuel was in the tank. It was 9” which is about 52% full, if my math is correct, while gauge showed 68% full. The issue is that the sender thinks that there is twice as much fuel in the triangular bottom than there actually is. (See my pic of drawing and notes). Today I bent the sender arm up somewhat and now fuel % is at 60%, which is getting closer to the actual of 52%. This will also help me if I was to get near empty as the float will sit a bit higher when the reading shows empty.
Are there fuel senders that can be calibrated to take irregular tank shapes into account?
Have any of you had this issue and were able to rectify it?
I guess it doesn’t need to be perfect but close is best.
 

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A fuel sender is just a basic potentiometer, all it cares about is how much it is deflected. It will not take into account the oblong shape of different fuel tanks. More expensive fuel sender /gas tank setups can be calibrated for the odd shape by making a calibration curve.
 
A fuel sender is just a basic potentiometer, all it cares about is how much it is deflected. It will not take into account the oblong shape of different fuel tanks. More expensive fuel sender /gas tank setups can be calibrated for the odd shape by making a calibration curve.
I think I did a half azz guess calibration curve when I put a bend in the rod. Now as the fuel gets lower my gauge fuel % will get closer to the actual fuel % until it catches it and passes it. Once my gauge gets to zero % full there would still be some fuel in the tank.
 
Are you not supposed to calibrate the fuel levels on your smartcraft? Start with an empty tank at 0% and add fuel adding in the percent. Been a few years since I have done one but they were accurate.
 
Are you not supposed to calibrate the fuel levels on your smartcraft? Start with an empty tank at 0% and add fuel adding in the percent. Been a few years since I have done one but they were accurate.
I’m not sure how that works. They didn’t calibrate it when they first filled it up when installing motor etc. Would that take into account the fact that the tank is smaller as it gets emptier.
 
I 100% rely on what my ECUs say i have burned for fuel. My boat has no gas gauge, seems to work. I carry a spare 65 litres and if I burn off the 240 in my tank i drop in another 65 litres.
 
I 100% rely on what my ECUs say i have burned for fuel. My boat has no gas gauge, seems to work. I carry a spare 65 litres and if I burn off the 240 in my tank i drop in another 65 litres.
I have discovered that I can do that through my HDS pro. It shows fuel used for trip and for season. I’m going to start using that in conjunction with the gauge. Mine will only give fuel consumed for the big motor, it doesn’t account for the fuel the kicker uses. I guess no big deal about the kicker as it’s fuel consumption is quite low.
What and where do you store your spare fuel in?
 
Are you not supposed to calibrate the fuel levels on your smartcraft? Start with an empty tank at 0% and add fuel adding in the percent. Been a few years since I have done one but they were accurate.
I checked that video and it’s not quite the same one for calibration. It’s for resetting the fuel each trip.
I dug abit deeper and found an area in settings on the HDS Pro that deals with the Mercury engine, see pic. I got there by going to…..Settings/mercury/vessel/tanks/fuel tank1/calibration. This should have been done by the Merc guys when they filled the tank from empty. Now if I wanted to do it I would need to empty the tank and then go through the steps as I fill it up. This consists of (I think) of seeing what % full the Merc gauge says at empty, quarter full, half full, three quarters full and full and enter what % the gauge says at each level 😳.
 

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I checked that video and it’s not quite the same one for calibration. It’s for resetting the fuel each trip.
I dug abit deeper and found an area in settings on the HDS Pro that deals with the Mercury engine, see pic. I got there by going to…..Settings/mercury/vessel/tanks/fuel tank1/calibration. This should have been done by the Merc guys when they filled the tank from empty. Now if I wanted to do it I would need to empty the tank and then go through the steps as I fill it up. This consists of (I think) of seeing what % full the Merc gauge says at empty, quarter full, half full, three quarters full and full and enter what % the gauge says at each level 😳.
That sounds about right. Guess your going boating to empty the tank
 
No fuel gauge in mine.
I personally just zero off my smart craft huge/fuel burned.
Easy for me cuz I was there when it was filled the first time so I know how many gallons it holds
 
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To me its a guage and use it as such, I go tops 4 trips before I fuel up full.
In summer I take down fuel everyday I prefer to be safe and close to full, Ive rescued a few people from being out of fuel... dont be that guy!!!
 
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