Fuel Tank Problems - Vintage Mustang Forums

 
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post #1 of 7 (permalink) Old 05-18-2017, 09:10 PM Thread Starter
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Fuel Tank Problems

Hello everyone,

I just had a lot of help tracking down shorts in my car, and now I'm hoping for a little help with my gas tank. I have a '67 coupe that only gets out about once or twice a year, but lately I've been on a tear trying to fix all the small issues to make it a little more streetable. Next on my list is the gas tank. I have two problems:

-Gas gauge doesn't work
-Leak at sending unit.

First, the gas gauge not working:
-Grounded the sending unit wire to the tank, gas gauge went all the way to full. Wiring and gauge are good.
-Tested the resistance on the sending unit. Hooked up the meter and the ohms ranged from 10-75 from empty to full. Sending unit works.
-Attached the sending unit to the wire that leads to the gauge, grounded the sending unit to the tank and had my wife watch the gauge. Slowly moved the float up and down and the gauge reflected accurately.
-Cleaned up the area around where the sending unit fits in, and have as much bare metal showing as possible where the retaining ring will sit.

With all of that together the gas gauge doesn't work. I'm thinking that the ground is still the weak point, as I can verify everything else works. I'm ready to rip the tank out and start over with a new one, but I'm hoping someone else has gotten this far and found a solution.

Second problem, the leak:
-The o ring is new(er) but it still seems to have a tendency to get squeezed out as the retaining ring is tightened down due to it being square. Used some petroleum jelly to get it to stick, no luck, last time I had it pulled apart I had gasket sealer around the o-ring to seal it up, still didn't work. The sending unit appears to be sitting flush, with the two tabs angled correctly in the tank.

The one odd thing is that the gasket does not match the size of the indent around the hole where you would expect it to sit. It is a bit bigger (and it was when I put it in a couple years ago) than the indented ring which makes it difficult to hold in place. Is this correct? Did the parts store give me the wrong size before?

An entire new tank, sending unit and hardware is under $200. If I mess with this much longer I'm going to replace everything. Just hoping someone has dealt with these issues in the past with any luck

Thanks!
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post #2 of 7 (permalink) Old 05-19-2017, 03:22 AM
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Try touching a jumper wire from the tank to the back of the sending unit while it is installed. Based on everything you said, I'd expect it to work. Worth testing.


Regarding the o-ring, sounds like it has swelled. Time for a new one.
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post #3 of 7 (permalink) Old 05-19-2017, 10:49 AM Thread Starter
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Originally Posted by Rufus68 View Post
Try touching a jumper wire from the tank to the back of the sending unit while it is installed. Based on everything you said, I'd expect it to work. Worth testing.


Regarding the o-ring, sounds like it has swelled. Time for a new one.
Thanks.

It's hard to test the ground with the wire method as you pointed out with it leaking. So I need to figure the leak out first and then I can fill the tank back up again and test.

I hoped to figure out the grounding issue first so if I put it back together and it doesn't leak, I don't have to pull it back apart again and risk it leaking the next time it goes together. It's a catch 22 as there's no good way to do this without risking pulling something back apart that it finally working. I've owned this car for 15 years and it's never had a working gas gauge and has always leaked gas.

As for the swelling gasket - it was always a little bigger than the indent on the tank, even when it was new. That's why I'm curious if it's wrong. I can't find if it is supposed to match that indent or not. It wouldn't hurt to get a new one, that's for sure. But if I can't get the leak and/or grounding issue figured out I'll just start with a new 22 gallon tank, which is the goal once I have the body fixed up.
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post #4 of 7 (permalink) Old 05-19-2017, 12:16 PM
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personally i think your best bet is to ante up for a new tank, that 22 gallon tank is a good idea.

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post #5 of 7 (permalink) Old 05-19-2017, 12:23 PM
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It sounds like the tank isn't grounding to the body. Try running a ground to one of the tank bolts in the trunk. If that works maybe the tank bolts are rusty at the underside of the head and not grounding the tank to the body. When I installed a new sender in my '68 I noticed the sender would move as I tightened the ring. Try to keep the sender centered as you tighten the ring.
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post #6 of 7 (permalink) Old 05-19-2017, 12:46 PM Thread Starter
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Originally Posted by rbohm View Post
personally i think your best bet is to ante up for a new tank, that 22 gallon tank is a good idea.
That's kind of what I'm thinking. I was hoping to delay this for a while and do in once the new floor is put in, as I just ordered a steering wheel, turn signal harness, and other various parts. But that's just part of owning these things I guess...

Quote:
Originally Posted by Oldkid67 View Post
It sounds like the tank isn't grounding to the body. Try running a ground to one of the tank bolts in the trunk. If that works maybe the tank bolts are rusty at the underside of the head and not grounding the tank to the body. When I installed a new sender in my '68 I noticed the sender would move as I tightened the ring. Try to keep the sender centered as you tighten the ring.
I believe the gas tank is grounded to the body. Correct me if I'm wrong, but if I grounded the wire from the gauge to the tank and it caused the tank to show full, that's a good ground. I was also able to ground the sender while holding it outside the tank, and it read from E to F through the whole range, while grounded to the tank. So I think that's good too. I think it's purely an issue with the grounding at the sender itself. Maybe I can't get enough clean metal with it in the car. But If I'm going to go through the work to pull the tank out to try and clean it more, there's no reason to put it back it. I may as well just upgrade it at the same time.
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post #7 of 7 (permalink) Old 05-19-2017, 01:51 PM
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Is it possible the float has a leak, causing it to fill up with fuel and stay sunk in the down position? It would read empty all the time in that situation.
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