Fuel gauge inconsistencies...

Gunn

Well-Known Member
TECHNICAL Contributor
Oct 4, 2006
2,335
Potomac River - MD
Boat Info
2003 280DA and 1995 Sea Ray 175
Engines
Twin 4.3l and 3.0l, all w/ AlphaI GenII drives
I'm wondering what to do over the winter about my fuel guage being inaccurate. As far as I know, its the stock sending unit in the tank and gauge in the dash. I have a 20 gallon tank. Here is the situation that I have found out after having the boat out and refilling...

When the gauge reports 1/2 full, when I fill it up, I only need 4-5 gallons to top it up, maybe less. The gauge goes from full to 1/2 very quickly when running.

It stabilizes at 1/2 for quite a while compared to how long it took to get there from full. The last day we took it out for the year was the lowest I ever let the gauge go. It was reporting about 1/8 of a tank left. When I topped it up for the winter, I used about 12 gallons of fuel.

So,
gauge reads full: 20 gal. tank is full
gauge reads 1/2: needs 5 gal. to fill
gauge reads 1/8: needs 12 gal. to fill

What can I do to make it more accurate? How can I tell which part is the problem? Are there aftermarket sending units that are better than the OEM one?

The problem with this is it limits how far we can travel; I generally would like to stick to the rule of using 1/3 to go out, and a 1/3 to get back wiith some in reserve. But I end up playing a guessing game with how much I have left! And being out in the Chesapeake in my boat, well, I'd rather not do that. :)

Hope everyone is having an enjoyable winter...

Tom
 
Fuel gages are notoriously unreliable. My 02 176 with 21 Gal tank had the same problem and at about the same level. I kept a GPS with me and did some figures and pretty much kept them in my head. I had 100 miles easy in a tank with roughly 30 mile reserve based on actual running time. I did get a 138 miles out of a tank in one particular trip and that included getting beat up pretty badley in some 2-3 foot chop. When I filled up that next day it took 19.5 gallons. :smt009 That was close. :grin:
 
Gunn said:
Are there aftermarket sending units that are better than the OEM one?
Tom

You can use your GPS, hourmeter, speed, knowledge of fuel burn, and how much fuel you started with to figure how much fuel you SHOULD have left. No guarantee it is how much fuel you DO have left. An unacceptable difference in my boat.

I think there are aftermarket units that are much better, but I have not used one yet. I may this year. The one you have now, if stock, relies on a wiper on a wire wound rheostat inside the tank to vary the resistance of the sender. The gauge responds to this and tells you the fuel level. Over time, the windings wear. In particular, the wiping action may wipe metal from one winding to the next, shorting them together. What that happens, both winding report the same fuel level. If thas happens over several windings, then you get a flat spot in the sending curve, and a wrong reading.

The newer units have no moving parts. They are a tube that goes down into the tank and measures the fuel level by measuring how the fuel changes the capacitance between and inner rod and the tubes outer body. The have electronics in the head of the sender, and usually there are adjustments there that let you set the empty and full points on your gauge. This also allows the tube to be cut to length to properly match your tank.

Gunn said:
How can I tell which part is the problem?
Tom

I think it is most likely the sending unit, due to the problem I described above. But you can check it yourself if you have a good multimeter. Take the sender out of the tank after disconnecting the wires. There may be only one or two. Cover the hole in the tank where you removed the sender so your boat does not fill with gas fumes. Keep in mind you are playing with a gas tank in an enclosed space. Take all precautions, and I don't know enough to tell you what they all are. But be especially careful that you don't cause any sparks when removing or replacing the sender.

With the sender out of hte tank, mount it somehow in a fixed position. Connect an ohmmeter between the sending terminal and the ground side. You should measure between 20 and 250 ohms, depending on the position of the wiper. Now move the wiper to a location that corresponds to a fixed distance in the tank, say full down. Measure and record the sender resistance and the number of inches the sending unit float is below the sender mounting plane. Move the sender float up one inch and repeat the measurement. Keep doing this until you get to full up. Now plot a curve of the results. You might get something like this.

FuelSenderCurve.png


This is the sign of a worn sensor. You can see that between 5.5 and 6.5 inches down for the float, the ohms don't hardly change, so the gauge won't either. Then all of a sudden the ohms starts rising rapidly and the gauge starts falling quickly. Not good.

This thread belongs in the electrical section.
 
Just what Dave says,

Only one solution to be accurate: Go electronic.

Here a electronic sensor that also has a analoge output.

http://www.ssitechnologies.com/fuel.htm

Acu-trac® fuel level sensors are configured with two outputs, the first is an analog gauge drive output for driving the fuel gauge and the second is a digital J1708/J1587 serial data link for providing on demand remote fuel information.

The Acu-trac® fuel level sensors have no moving parts making it a reliable alternative to conventional float sensors. However, the Acu-trac® sensors are more than just another fuel level sensor choice, its J1708/J1587 Messaging coupled with its Off Vehicle Communications capability enable you to capture real time fuel data when you want it and where you want it. Such as:

Average MPG - The running average miles per gallon, MPG, achieved by the vehicle over the last 16 hours of operation.

Fuel Quantity Remaining - The quantity of fuel remaining on board in gallons.

Fuel Tank Capacity - The total capacity of the fuel tank(s) in gallons.

Idle Fuel - The cumulative idle fuel burned by the engine since the last time the fuel data was reset.

PTO Fuel - The cumulative PTO fuel burned by the engine since the last time the fuel data was reset.

Fuel Added - The cumulative fuel added to the tank since the last time the fuel data was reset.

Fuel Lost - The cumulative fuel lost from the tank since the last time the fuel data was reset.
Time and Date Stamp - and reason for the transmitted message.
Vehicle Position - i.e. - Latitude - Longitude and location relative to municipalities.
J1708 Bus or Sensor Tampering - When and amount of fuel used while the J1708 data link was disconnected.
 

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If you can figure out how to make your fuel sender accurate you will make millions. I would suggest an electronic solution too. A Lowrance LMF-400 or 200 is a neat gauge that will solve your problem. The more you use it the more accurate it gets too. It is a fuel flow sensor that will measure how much fuel you use. You can set your tank size and it will let you know how many gallons are remaining. This is just the tip of the iceburg of what it can do, but this should get you squared away for around $150.
 
Hi Gunn,

I have the same issue with my boat. Based on the comments above, it looks like you can get more accurate readings for a couple hundred bucks. That being said, after a season or two you'll probably become a master at "translating" your existing fuel gauge. Also, it's easy enough to simply open the engine hatch and take a peek at the tank. You should learn very quickly how much fuel you burn under your "normal usage" conditions. If you check the tank before you go out (which you should do anyway) you should already have a pretty good idea how much gas you'll have left when you get home. If there's a chance you'll be cutting it close, fill 'er up before you go.
 
Thanks for all the great suggestions and comments. Dave, thanks for the great description. I'm going to pop out the fuel sending unit and do as you said with my multimeter. $150.00 is not bad at all for piece of mind and accuracy.

I always fill up before each outing and generally only end up burning about 1/4-1/3 tank. But it seems with each outing last year (our first year), we kept venturing out further and further, finding new sights and places to visit. I'm sure it won't stop next year, and will eventually/quickly lead to a larger boat. :smt001

Now's a good time to get these things squared away!

Thanks again,
Tom
 
I have wanted a fuel flow sensor for my boat for some time. A few months ago I realized a fuel flow sensor cannot tell you how much fuel is in your tank any better than a stop watch can tell you what time it is. A fuel flow sensor does not look in your tank to see what is there. I know this is not news, and I understand that it can be quite accurate regarding how much fuel has been used, just like a stopwatch can be quite accurate about how much time has passed.

But here is the issue for me. I keep my boat in my shop. My normal day trip does not exceed 20 miles when I just go out putting around. I don't need, and don't want, 100 gallons of fuel on board to go take that trip. I would rather start out with 40 or 50 gallons, no reason to carry around an extra 300 lbs; other than my bad fuel gauge. And maybe when I have 40 gallons left, my gauge reads 1/8th tank. In that case, I can't even convince myself we are OK to keep going. let alone any passengers on board.

Secondly, I live on a few acres and have a tractor to feed, a farm truck, a riding mower, etc. So normally I have a few cans of gas sitting around with varying amounts of gas in them. I might decide I just want to put in a can or two of gas rather than going to a service station to get some. So I grab a can or two and dump some in. Not sure just exactly how much, just whatever was left after topping off the mower and the tiller.

So in this situation, what is the fuel flow gauge going to know about how much gas is in the tank? It can tell how much I used, but even I don't know exactly how much I put in. So it can't know how much is in the tank. Only a fuel level gauge can tell that.

Here is a link to an electronic sender made by Gaffrig. I think it would be accurate when calibrated, but I have never used one.

I have also found units made by Centroid. Here is a link on how to order. Note that a two terminal unit is available for 240/33 ohms for Teleflex and a few other manufacturers. Normally the unit would need an additional wire from what we now have, as the unit is electronic and needs power. For the gauges listed, though, it can draw power through the gauge, so does not need a power wire


Here is a description of how to install and calibrate the Centroid sender. A rectangular tank is assumed. Cost is way less than $100.
 
jhild said:
Hi Gunn,
That being said, after a season or two you'll probably become a master at "translating" your existing fuel gauge. Also, it's easy enough to simply open the engine hatch and take a peek at the tank.

Being able to visually check the fuel level in the tank would be nice, I can't do that.

But to help "translate" your fuel gauge, you can take fuel gauge readings while filling the tank, if you stop filling long enough to read the gauge. You can do this every tenth of a tank or so, and make yourself a fuel gauge curve. It might look something like this.

FuelAdded.png


This will help you interpret the gauge. You still have to figure out how much 'useable' fuel you are working with. I have a 100 gallon tank. I don't know that I can reliably run on the last 10 gallons. I really don't know where 'empty is in terms of gallons left in the tank when the engine quits.

Oh, also think about safety if you do this. You probably need to switch your ignition to 'on' to read the gauge. I suspect nobody is going to be happy if you do this multiple times during fueling while at a fuel dock. I would not do this unless the boat were on a trailer, and a nice breeze to disperse any stray fumes would be good too.

If you did have a fuel flow indicator, you could make such a curve as you used a tank of fuel.
 
Dave M,
Good idea about translation guide but one possible flaw on the filling on trailer theory. My previous boats fuel gauge read differently on the trailer than in the water. Likely becasue the boat sat a different angle. I think there are times during the day when nobody is at the fuel dock and with a helper you could get this done. Besides you could always tip the guy at the fuel pump!
 
Wish2fish, you are right. Ir might read different on the trailer. It depends on the shape of the tank and the location of the sender. If the tank is rectangular, and if the sender is located in the middle of the tank, then it should read about the same. Otherwise, all bets are off.

Another thing I unintentionally implied in comments above is that the response of the gauge itself is linear, and that if you get the sender to be perfectly linear, and calibrated at full and empty, your indication will be perfect. Probably not true!

If you tank the sender out of the tank and measure its resistance vs position, then you could also use it it make a plot of what resistance it takes to get various fuel gauge readings, say every 1/8th tank. You just move the float arm until you get the gauge reading you want to check, then disconnect the sending wire and measure the sender ohms at that point. I guarantee this is a lot easier said than done. If you had a decade box to use instead of the sender to vary the resistance your gauge is seeing, it would be much easier. I foolishly neglected to steal one from work before I retired. :smt021

So, if you did make this measurement, it might come out like this.

FuelGaugeCurve.png


Not exactly straight as an arrow. :grin:

Like Jim says above, getting a fuel gauge reading to really be accurate is difficult. If you learn to 'translate' your present gauge, yo get much closer. IF you start with a full tank, then a calibrated fuel flow gauge will be closer still. IMHO. AFAIK. YMMV. :wink:
 
Maybe I should be a bit less bothered about my unreliable fuel gauge by the sounds of this thread, thanks.
 
Hi everyone, my fuel gauge is saying full constantly so I took out the sender and replaced with a unit my local dealer recommend and it’s doing the same! I also put the two cables together that attaches to the sender to make a circuit and the Gauge goes up to full so it can’t be a wire issue.
 
Sounds like you have a broken ground wire. Put a known good ground wire to the signal wire at the sender. Gauge should read empty
 
And I leave it like that permanently?
 
Hell no, the gauge works from ohms values. So grounding would be 0 ohms which makes it go to empty. When the ohms are involved to get your full reading and everything in between .the sending wire is being partially grounded. So what I told you to do in post 15 is a test.for your gauge.
 
Ok thanks! I’ll report back shortly.
 

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