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Fuel gauge reads low on highway
#1
I've never seen this with my other Mustangs...

72 Mach with Sports Interior.  All of the gauges work perfectly except...
Fuel gauge reads 3/4 full.  First time on highway with the car today.  At 55 fuel gauge drops to 1/2 full.  65 mph, 1/4 full. By the time I was up to 70 mph, the gauge read 1/8 full.  Got off highway to turn around toward home.  Gauge starts climbing.  Back up to highway speed and the gauge drops to under 1/4 full.  Off highway back up to my house, and the gauge is back up to 3/4 full.
Seems the faster the car goes, the lower the fuel gauge reads.

Fuel Sending units change a fuel gauge reading on hills,  but not at highway speeds...

Any thoughts?
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#2
Are those readings at a steady speed, or when accelerating? All the fuel goes to the back of the tank when your foot is down.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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#3
Something is causing higher resistance only on the fuel sending unit line at speed. I suspect wind is catching the wire where someone put in a bad splice...just thinking out loud.

Let me check your shorts!
http://midlifeharness.com

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#4
It happens at steady speed.  I only went down the freeway about 3 miles.  It read low the whole time.
I just finished a complete restoration of the car so I know every inch of the wiring.  The gauge reads fine around town.

Was hoping that this was an issue that someone else has had.  All of my previous mustangs were 66-67's.
My first 71-73.
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#5
Voltage Stablizer on back of the instrument cluster??? (usually when bad it reads HIGH - but I guess it can go bad the other way)
Mark
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#6
If the problem was the constant voltage regulator, the oil pressure and water temperature gauges would react the same as the fuel gauge.

Let me check your shorts!
http://midlifeharness.com

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#7
Yes if faulted on a the HIGH side (right?) I read this as reading low - I agree more likely the WRONG OHM (cheap senders available universally) IMO but it seems WE have to do all the digging/work and get answers (read more info) to help.

If I have learned anything with internet diagnosis.....I won't even assume motor has the right amount of oil or pressure/car OR has the water temp hooked up LOL

Mark
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#8
Guys, I'm a little embarrassed.
I just took the car on another drive and the problem is at ALL speeds.  Even around town.
I've only had the car on the road for a couple weeks and I was wrong.
It does seem though to go lower the faster I go still, weird.

In my 5 year resto of the car I did not replace the sending unit.  The previous owner had put in a new tank and I'm sure he reused the original sending unit because the car was a flip. 
I wonder if it could be as simple as replacing the sending unit?
It could be giving erroneous resistance readings as the car moves and the gas sloshes around?
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#9
Yes, the sending unit is a likely suspect. The higher the resistance, the lower the gauge will read. The wiper on the inside end of the float arm may be worn out and not making good contact with the resistor strip or the resistor strip is worn out, or the connections inside the tank are corroded.



“If you can't explain it simply, you don't understand it well enough.”
--Albert Einstein
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