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Fuel Guage Inaccurate

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Hey folks -

Looking for ideas (hopefully the easiest) my sons truck is new to us and figuring out the this and that's for it - 79 F150 with 351, the fuel gauge is not accurate.

I did the math off the dimensions and seems to be about 17 gallon tank, it was reading less than a 1/4 tank; go to the gas station and it will only hold about 6 gallons - gauge will read full when you leave but through the week will give higher and lower readings depending on the day ending with the above scenario.

I am curious - there is often pressure released when you take the gas cap off, it is an old style cap - could this be as simple as a venting issue and needing a vented gas cap??????

Thanks for your ideas.
 

DNFXDLI

The Token Canadian
Staff member
As far as I know it is basically a rheostat (variable resistor) that will send a specific voltage as the float moves up and down. It might be a bad connection or the sender might be pooched.
 
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So - you're saying (as I feared) I have to chase all the wires just like if the sending unit or the gauge was bad? I was dreaming that with as much pressure was on there it might be impacting the floats ability to accurately travel up and down the slide
 
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I was hoping for the quick and easy since "Full" showed "Full" and use made it go down; come to think of it I do remember a time or two that when up to speed the needle kinda "wiggled" around a bit - guess that would lend itself to a ground not making a good connection huh
 
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No - only had it a couple of weeks or so, I was under there this morning measuring the tank to figure out the size and noticed it looks pretty new
 

DNFXDLI

The Token Canadian
Staff member
So - you're saying (as I feared) I have to chase all the wires just like if the sending unit or the gauge was bad? I was dreaming that with as much pressure was on there it might be impacting the floats ability to accurately travel up and down the slide

It's never simple...that Murphy guy is a bastard!
 

SuperCab

Moderator
Staff member
10,068
547
Montana
I was just thinking maybe the float arm is bent giving a premature empty reading. Could well be if the tank looks recently replaced.

It's not abnormal for the needle to move a little while driving.
 
In most cases the culprit is either the GROUND to the fuel sending unit.
All connection inside and outside of fuel tank/sending unit need to be as clean and secure as possible

the ICVR provides a 5 volts pulsing of power to the gauges so that can cause some movement left and right. This sis for both the Fuel and temp so if there is movement ,the ICVR would need replacing
or

The float has leaked in some fuel .

Having dual tanks allows for running one until empty... then switching
 
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I am thinking that I need to chase the wires- if the float had leaked in some fuel, in theory it would sit lower than the top of the tank / fuel level and when full the gauge would show less than full because it could not get to the top of the tank (right - or if not right at least make sense?)
 
As the float drops the gauge goes down.

It is the resistance in the 'TO GROUND' that changes the level.It operates on a 5volts pulsing of power to the SU sending a signal to the gauge.


FuelIndicatingSystemTest.JPG

FuelGaugeSystem.JPG


 
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