Fuel Gauge reading too high?

[67resto-coupe]

I've sputtered and run out of gas when my gauge is still reading 1/4 tank. Is this issue isolated to the sending unit? I reused my stock sending unit with a new tank. I was able to get a new basket filter, but everything else is still original equipment. Would a new sending unit remedy the error readings? Sucks to floor it on the freeway to merge only to sputter and die in rush hour traffic. Thanks.

 

[Horseplay]

It likely isn't the sender itself as much as how it is mechanically set-up. The float arm needs adjustment (bent) to work properly over the whole of it's motion in the tank. Electrical resistance (measured in ohms) is altered as the float arm moves which serves to tell the gauge what to display. Cmayna is the resident sender expert. He has a home-built device he uses to "calibrate" the set-up. Do a search and you will find lots of info from him on the subject.

 

[B67FSTB]

It likely isn't the sender itself as much as how it is mechanically set-up. The float arm needs adjustment (bent) to work properly over the whole of it's motion in the tank. Electrical resistance (measured in ohms) is altered as the float arm moves which serves to tell the gauge what to display. Cmayna is the resident sender expert. He has a home-built device he uses to "calibrate" the set-up. Do a search and you will find lots of info from him on the subject.

I'll second that.

 

[CUAviator]

Along the lines of the OP - how much does the average fuel gauge fluctuate? My gauges are (sorry, car came like this) digital and the fuel quantity will flux about 40% on any given drive...start out at 80%, somewhere along my drive it'll be down to 40. By the end, it's at 65. Start up again and it's at 78% could there be an issue of the signal being converted to a digital readout that makes it hyperactive, or is there another issue...or is that classic 65 mustang?


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[cmayna]

67resto-coupe,
What size tank do you now have? What was the size that you pulled out? Does your new tank have a drain plug? You mention a new basket filter. Are you referring to the filter on the end of the sending unit pickup tube? Depending on how it is constructed, is it possible that you might have shortened the length of the pickup tube when sliding the filter into place? If the new filter is similar to the old filter then odds are, this is not the problem.

If you are now reading 1/4 tank too high, it's time to calibrate the sending unit (bending the arm). If you have a drain plug then next time you have little gas in the tank, pull the plug and drain it all the way. Here's a pic of a sending unit. You need to bend the float's lower arm "B" upwards just a tad at the elbow "C". This will cause the upper arm "A" to rotate lower which will create a lower reading.

What I typically suggest to anyone who's putting in a new tank or sending unit is to first dump in only 1/4 tank of fuel. Read the fuel gauge. If off, drain the tank, remove the sending unit and adjust it accordingly. Repeat the process of putting the 1/4 tank worth of gas.

If you are reading too low, you would simply straighten out the float's arm a tad at the elbow "C".


CUAvaitor,
Once dialed it, the gauges will typically read spot on. They do not fluctuate. Sorry but I have no experience with digital gauges.

 

[Horseplay]

Along the lines of the OP - how much does the average fuel gauge fluctuate? My gauges are (sorry, car came like this) digital and the fuel quantity will flux about 40% on any given drive...start out at 80%, somewhere along my drive it'll be down to 40. By the end, it's at 65. Start up again and it's at 78% could there be an issue of the signal being converted to a digital readout that makes it hyperactive, or is there another issue...or is that classic 65 mustang?

The fuel sender causes a variance in ohms resistance as it makes its sweep up and down with fuel level. Given the situation as you describe, I would first check connections from front to back for both tightness and cleanliness as this is a likely cause of your problem.

 

[CUAviator]

Cool. Thanks. One thing is for sure - 0% actually means 0%. Found that out the first week I had it.


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