Gas gauge goes UP when I press the pedal???

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Righty Tighty

Blame it on the dog
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My fuel gauge has been inconsistent ever since I bought the car. When the ignition is turned on, the needle will rise to the correct fuel level, but will "stutter" on its way, and sometimes randomly quit working during the drive. I installed a new sending unit when I swapped 5/16" for 3/8" lines, and the gauge's performance improved. I haven't gotten under the dash to look at the wiring, because quite frankly, I hate working under the dash and I'm procrastinating.

However, I noticed something odd yesterday while driving down the interstate.

I noticed that when I let off the gas pedal, the gauge would go down. Almost to the point of disappearing past "E". Then when I press the accelerator, the gauge goes back up. I could repeat this quite consistently. What is happening here? How on earth is this even possible? I made a video of this, and I'll share it once I can get it uploaded on the Tube.

'74 Barracuda with a rallye cluster.
 
That is far better then most. Mine always goes down when on the gas and it don't come back up until I put gas in. Stay on the throttle and mileage will be unlimited.
 
Not sure about E Body, but on my Dart there is a metal clamp on ground strap from the metal fuel line on the sending unit to the fuel line that is on the body, it basically jumpers the rubber line that connects the two. Do you have this?
 
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Gas is sloshing to the front of the tank when you let off the gas (G-force), letting the gas sending float down.

Gas sloshes to the back of the tank on acceleration causing the gas sending unit float to go up.
 
Gas sloshes to the back of the tank on acceleration causing the gas sending unit float to go up.

Im sure this is the case, but arent they supposed to be buffered somehow, so they display slowly?? Resistor of some sort at the gauge end???
 
There is a 5 volt regulator attached the back of the fuel gauge. It supplies the power to all of the gauges. The OEM version has points and can act up. That combined with a grounding problem can make strange things happen like this. It's easy to check the grounds under the hood first. The engine can rock under throttle and pull on a grounding strap. Put a volt meter on the power wire going through the fire wall connector and try revving the motor. Cleaning up all of the ground connections is a first step.
 
As your RPMs increase, voltage and amperage output from the alternator also increase.

Two things to check:
First, does it happen while parked (free revving in neutral or park), and does it stay in the elevated position if you maintain the higher RPM or does it drop back down once you stop actively increasing engine speed?
 
Not sure about E Body, but on my Dart there is a metal clamp on ground strap from the metal fuel line on the sending unit to the fuel line that is on the body, it basically jumpers the rubber line that connects the two. Do you have this?
Yes, I've actually recently replaced the old ground strap with a new one right when I updated the fuel lines and made sure it's making good contact.

Gas is sloshing to the front of the tank when you let off the gas (G-force), letting the gas sending float down.

Gas sloshes to the back of the tank on acceleration causing the gas sending unit float to go up.
I thought about this possibility, but 2 things: when the fuel level goes down upon decelerating, it will sometimes go all the way down to below "E" even when I have a full tank. I wouldn’t think sloshing would cause it to go down that much. Also, the way the sender is oriented, sloshing towards the front would move the float up and cause a "full" reading, while sloshing towards the back would make the float go down and cause an "empty" reading. So decelerating would cause the gas to slosh to the front and read full, but I'm experiencing the opposite.

I'm interested in testing the voltage theory, and I agree I should confirm I have good grounds.
 
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Here's a video. I can't figure out how to enlarge the image so it's easier to see the gauge, hopefully you guys can see it okay.

https://youtube.com/shorts/PxPg2buz_-0?feature=share

I don't know enough about your year's grounding and will defer to someone else, but gauges work by resistance. Lower resistance makes it go higher. When you floor it, your resistance is going to zero. If it is not the sending unit, I would look to see if you have a problem with a voltage regulator or some melted wire somewhere. The speed at which it changes is pretty interesting though.

On an aside, you can buy a resistance box on Amazon for under $20. Excellent tool for verifying the gauges are reading where they are supposed to.


EDIT - Then again, if it was just an alternator issue then other gauges would also have similar reactions. If it were me, I would start by checking the resistance reaction of the gauge against a known chart, then use a meter to get the resistance at the tank and then work my way forward.
 
There is a voltage limiter located on the back of the instrument panel. It is a rectangular box with 3 prongs that pushes into the plastic 'circuit board'. Ground, 12v, and 5v. The 5v goes to the temp, oil pressure, and fuel gauges. I'd swap out the voltage limiter first.
 
I don't know enough about your year's grounding and will defer to someone else, but gauges work by resistance. Lower resistance makes it go higher. When you floor it, your resistance is going to zero. If it is not the sending unit, I would look to see if you have a problem with a voltage regulator or some melted wire somewhere. The speed at which it changes is pretty interesting though.

On an aside, you can buy a resistance box on Amazon for under $20. Excellent tool for verifying the gauges are reading where they are supposed to.


EDIT - Then again, if it was just an alternator issue then other gauges would also have similar reactions. If it were me, I would start by checking the resistance reaction of the gauge against a known chart, then use a meter to get the resistance at the tank and then work my way forward.
My feeling is that it’s not the sender, because the gauge has been acting weird since I had the OEM sender. I wish I could find an OEM or U.S. made 3/8” sender, but they’re nearly impossible to find.

There is a voltage limiter located on the back of the instrument panel. It is a rectangular box with 3 prongs that pushes into the plastic 'circuit board'. Ground, 12v, and 5v. The 5v goes to the temp, oil pressure, and fuel gauges. I'd swap out the voltage limiter first.
I’ll look into getting to the back of the board. I swapped in an IVR on the Scamp with good results, might not be a bad idea for this car.
 
what a deal! Keep you foot in it and you might be able to drive forever!!!
 
My fuel gauge has been inconsistent ever since I bought the car. When the ignition is turned on, the needle will rise to the correct fuel level, but will "stutter" on its way, and sometimes randomly quit working during the drive. I installed a new sending unit when I swapped 5/16" for 3/8" lines, and the gauge's performance improved. I haven't gotten under the dash to look at the wiring, because quite frankly, I hate working under the dash and I'm procrastinating.

However, I noticed something odd yesterday while driving down the interstate.

I noticed that when I let off the gas pedal, the gauge would go down. Almost to the point of disappearing past "E". Then when I press the accelerator, the gauge goes back up. I could repeat this quite consistently. What is happening here? How on earth is this even possible? I made a video of this, and I'll share it once I can get it uploaded on the Tube.

'74 Barracuda with a rallye cluster.

-----LOL , MINE HAS ALWAYS GONE DOWN WHEN i HIT THE LOUD PEDAL !!
 
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