How to test the fuel gauge?

Bmachine

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I'm having some issues with my fuel gauge. When I fill the tank up, the needle stops before the top of the scale.

Then when it goes down towards the reserve (red), indicating almost empty, and I fill it up, I can only put about 7 gallons in there before the tank is full.

So... 1) When the tank is full, the needle does not go into the Full area and
2) When the needle is in the bottom red zone, it seems like there is still plenty of gas in there since I can only add 7 gallons.

I'd like to be able to confirm how much gas is still in the tank when the needle says "nearly empty" and that it is indeed full when the needle says "not quite full"...

Any ideas?

Thank you.
 

Stevehose

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Sounds like your sender is bad. With the ignition in the on position (idiot lights on), pull the brown ground wire off the sending unit base and touch it to the other 2 connections, the white "T" light should come on and the gauge should read full. If this checks out then your sending unit isn't sliding up and down freely and needs to be removed and carefully cleaned/repaired.
 

HB Chris

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Bo, Don’t you have an intank pump? If so the pickup had to be lengthened and a new sending unit installed which isn’t long enough. At 1/4 tank remaining I still have at least 7 gallons or so remaining.
 

Bmachine

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Excellent info Steve and Chris.

Chris, that sounds like a very likely culprit! Is there a different sending unit that is long enough? Or is that integral to the fuel pump and there is nothing we can do about it?
 

HB Chris

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No sending unit is available that is long enough, I used 318is pump and e28 sending unit. When showing half full it isn’t off as much but I have 10-11 g left. It should show full though. If you had a CSi tank you could use original sending unit as pump is separate opening.
 

HB Chris

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When it reaches the empty mark I know I have four gallons left. And I average 20mpg and 25mpg at reasonable highway speeds so I can calculate where I stand. You get used to it and don't worry.
 

Bmachine

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When it reaches the empty mark I know I have four gallons left. And I average 20mpg and 25mpg at reasonable highway speeds so I can calculate where I stand. You get used to it and don't worry.
Haha, ok then. That WILL take some getting used to! ;-) Might want to carry a spare can in the trunk just in case...
Have you ever thought of switching to a csi tank?
 

HB Chris

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Haha, ok then. That WILL take some getting used to! ;-) Might want to carry a spare can in the trunk just in case...
Have you ever thought of switching to a csi tank?
Hard to find and not worth it. Once you get some miles on your coupe you will know how much gas remains, not a big deal.
 

Bmachine

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Gotcha.
Of course, one critical component of that equation is... an odometer that actually works!
Off to find an iPhone based odometer since it will be a while before my analog one gets fixed.
 
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Bmachine

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I'll have to research it to refresh my memory but IIRC the speedo is not easy to remove, right? Don't you need to disassemble half of the dash?
 

Stevehose

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No you can get it out: remove the cable, loosen the set screws, then flip the speedo 180 degrees, and use your zen to work it down through the wires etc.
 
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