Gas Tank, Gauge, Sending Unit WTF? - Team Camaro Tech
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post #1 of 9 (permalink) Old Jul 7th, 06, 12:19 PM Thread Starter
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Al
 
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Location: Vancouver, USA
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Gas Tank, Gauge, Sending Unit WTF?

Here's the scoop:

Bought a new (repro) gas tank for my 69. Installed it with a new sending unit, filled up the tank and it showed slightly less than 1/2 tank on the gauge, which has always worked fine. Removed tank and put in old sending unit, refilled tank, and the gauge still showed the same - just below 1/2. I'm thinking the tan wire is somehow partly grounded, so I drive the car until gauge reads empty, and based on miles traveled, think I still have 8 - 10 gallons in the tank. Soooooo, it runs out of gas! I get two gallons at the service station, drive it there and fill it up, and it only takes 8 gallons.

I know the tank will take a full 18 gallons, 'cause that's what I put in it initially. But for some reason I can only use 10 gallons before it quits drawing fuel. The only thing I can think is that the new tank is somehow a different shape(deeper?)than the original. (which I got rid of, so I can't compare them.)

Any idears or similar experiences?


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post #2 of 9 (permalink) Old Jul 7th, 06, 04:15 PM
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Jim
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

I doubt the new tank is deeper or else the old mounting straps would not work. I suspect a bent pickup tube. A bent pickup tube could also affect how the sending unit reads the fuel level in the tank. I know it will be a pain but if you have the old sending unit maybe lay it next to the new one and compare them. I would also look to see if both sending unit's float arms travel the same distance. If you don't have the old sender maybe you can do some measuring as to how much drop is on the pickup tube from the sealing rings surface and then stick a tape measure through the hole on the tank to see what room is in there.
To keep from filling the tank with gas only to maybe have to run it dry again, try and get an idea as to how the tank sits in the car with the car level and then shim it up on the garage floor and run some jumper wires to the factory wires on the car and then maybe fill it with water to get an idea how much fuel would give you a certain reading on the dash or console gauge. if there is a problem it's easier to access the sending unit with the tank on the ground.
Once you are done you can drain out the water and then run an air line into the tank while outside in the sun to get rid of any moisture in the tank and maybe let it sit out a day or so uncapped and then after it has gas in it again maybe use a can of heat for whatever water might still be in there.
I know it's hind sight but next time don't throw things away until you are sure the new parts work like they should. I too have made the mistake.
Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #3 of 9 (permalink) Old Jul 7th, 06, 05:02 PM Thread Starter
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Al
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

Thanks for the reply, SOA.

To clarify a little:

I DO still have the original sending unit - it's in the tank now. I put it back in because I thought the new one I bought was giving a false reading, but they both read the same. I am positive neither sending unit is bent.

I also replaced the straps when I installed the new tank - pitched the old ones. I know - bad idea, but it's done.

In a nutshell - I put 18 gallons in the tank, it reads 1/2 full, goes to empty and runs out of gas, and only takes 9 - 10 gallons to fill it back up. Wierd.


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post #4 of 9 (permalink) Old Jul 7th, 06, 09:17 PM
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Jim
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

If you want to you could try and troubleshoot each problem seperately (fuel reading first and then possibly why it's running out of gas with plenty in the tank).

I'm better with electrical so I'm going to start there.

There should be a ground wire off of a tab on the sending unit that goes to a bare metal spot under the car. At this point is the metal clean and making good contact ?. Maybe try cleaning that connection point ? and maybe adding a star washer between the connector and the surface it's bolted to so it digs into the metal on both pieces ?.
If that looks good then take a jumper wire with alligator clip ends and clip one end to the terminal on the sending unit's ground wire where it's bolted to the underside of the car and take the other end of the jumper wire and connect it to some bare metal spot on the car (maybe like where the trunk latch wears the paint off on the catch). If this makes the gauge read properly then the ground is not good and it might need better cleaning or run to a different spot (the spot it's at now might be a metal panel that is spot welded to something else and that is spot welded to something else also and could be giving you a poor ground).
If the work on the ground does not make it read correctly then somewhere there is a disconnect plug for the sending unit wire into the taillight loom of the car. For this test you are going to need that jumper wire and some resistors (a 90 ohm, a 45 ohm, and a 22.5 ohm). You now need to disconnect this connection and the end going towards the gauge you want to ground that wire out with the jumper wire with the alligator clip ends. The gauge after the ignition is turned on should then swing to empty. Now disconnect the jumper wire and the gauge should swing to above full. Now insert the 45 ohm resistor with one end going to a ground and the other end going to the wire going towards the gauge and the gauge should then read 1/2 tank. Do the same but with the 22.5 ohm and it should read 1/4 tank and with the 45 ohm and 22.5 ohm resistors in series the gauge should read 3/4 tank. If the 90 ohm is used then the gauge should read full.
Now if the gauge is reading fine and your sending unit ground is fine then it has to be something with the sending unit or the connections to it. I have seen it before and will again but just because something is new does not guarantee it will work properly.
If you have an ohm meter you can check the resistance of the disconnected wire to the sending unit and to ground and if it reads let's say 60 ohms the gauge should read about 5/8 full.


As far as the car running out of gas with plenty in it then I would look at the pickup tube location in the tank. If this is good then could there be something blocking the pickup when it gets to a certain level ?. Could it be a piece of rubber floating around in one of the fuel line to where it's a coincidence that it's happened at the same point of how much fuel is left in the tank ?. Like I said, I'm better with electrical but I'm out of idea's on that.

If the electrical doesn't make sense, let me know. I know a lot of Chevrolet forums have covered fuel sending unit and gauge tests before.

Jim

1974 Spirit Of America Nova (being restored), 1973 Nova Custom, 1968 Chevy II (Garage Find 2012)

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post #5 of 9 (permalink) Old Jul 10th, 06, 09:44 AM Thread Starter
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Al
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

Thanks for reply, Jim. I suppose I should have stated that I troubleshot the electrical end of things and all tested just fine - even bench checked both sending units and both would move the gauge as expected. I have tried a different/known good ground - I'm convinced the problem isn't in the sender or the gauge.

I guess I'll have to drop the tank AGAIN and try and figure out what the heck's goin' on. Thanks again!


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post #6 of 9 (permalink) Old Jul 11th, 06, 05:41 PM
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Daniel
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

BPOS,

I have the exact problem as you. I replaced the Sending Unit (old one was defective), Straps & Fuel Tank. I also got rid of my old tank!!!

After installing the new tank, I dropped the 5 Gallons of gas I had and the guage read around 1/4, which was Ok. After starting the car, it dropped to empty. I went to the gas station and once full the gauge only read 3/4.

I got another fuel sending unit (2nd new one) and got the exact same thing. When I took the 1st new sending unit out it read 90 ohms when the float would have been above the tank top, which explained the 3/4 reading.

The gauge seems to be in line with the ohms reading.

Between 3/4 (full) of a tank to empty, I can only add about 7 to 8 gallons.

The sending unit is a dii Corvex (3/8 Repro from Classic Industries) and the tank is Made in Taiwan P/N 642, CV6003.

I'm like you BPOS, I got new everything and it doesn't work anymore.

Daniel.

Daniel,
1969 SS-350 Camaro, Hugger Orange
2005 Black Mustang GT

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post #7 of 9 (permalink) Old Jul 12th, 06, 11:46 AM Thread Starter
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Al
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

Misery enjoys company,eh?!

Let me know if you figure anything out - I'll do the same.


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post #8 of 9 (permalink) Old Jul 12th, 06, 03:00 PM
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Daniel
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

We are not the only ones BPOS, here's a thread on the Chevelles.com forum:

http://www.chevelles.com/forums/showthread.php?t=135353

Like you, this guy has the problem since he put in his new tank.

Lets keep our ears to the ground !!!

I'm thinking about removing the fuel tank to bend the sending unit float rod in order to make it match the gauge.

Daniel.

Daniel,
1969 SS-350 Camaro, Hugger Orange
2005 Black Mustang GT

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post #9 of 9 (permalink) Old Jul 12th, 06, 03:44 PM Thread Starter
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Al
 
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Re: Gas Tank, Gauge, Sending Unit WTF?

Daniel -

Todd DeLaMuca (remember him?) is my evil twin - AKA me....so there's really only two of us....I posted it over there at brand "X" for the extra sets of eyes....I really want to figure this out. It just doesn't make sense!!

PS - I'm going to try and get this moved to "troubleshooting" - it might get more airplay over there.


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Last edited by BPOS; Jul 12th, 06 at 04:03 PM.
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