Odd gas gage problem

[G-749]

Hello all , Happy Thanksgiving,
New tank, gaskets, sending unit from Classic Industries
Finally got the Longhorn powered up n running , driving !!!
11 18 put 50.00 $ gas / 14 gals, gage goes up to 3/4 full moves slightly back n forth while driving ,put 66 miles on new cam, valve job, new Edelbrock intake , quadrajet that came with truck
11-25 , drive to gas station, put 10.3 gals in to top off tank ,get in cab , gage is reading 3/8 tank , WTF , drive home gage goes to 1/2 and has been stuck there since, driven 4 times for 24 miles and still stuck on 1/2 tank , doesn't move even when driving , did the search thing and no clear answer . Real bummer, upside is its under power, spins both rear tires , 700R4 shifts flawlessly, new tilt column with Lokar shift adjustable shift rod works .
Anyone's thoughts, experiences, input welcome
Good luck on all your projects!!!
I

[1970cstblazer]

I would focus on the sending unit, since the replacements are generally lower quality than original OEM GM/Delco Remy.

To verify that the gauge is ok, unplug the tan wire from the sending unit. The gauge should easily go to the 3 o'clock position, way past full. Then take the tan wire and touch the terminal to a good ground. It should sweep just past empty. If that checks out, this verifies the wiring and gauge as good. The sending unit can be tested for resistance. GM sending units in this period were 0-90 ohm impedance.

[old51sedan]

You didn't say as to whether you had an original tank or one in the rear, not that it should make a difference. A few years ago when I did my truck I installed a suburban tank in it. I had a similar problem and mine was the float, it had filled up with gas apparently from a bad soldering job. I replaced it with a float from a jeep and that cured my problem. Not saying that is your problem, as ,most problems I have found is a grounding issue, that's the first place you should look. Good luck with your search.

[Rich69shortfleet]

Just recently I had an experience with a new sending unit which worked fine to begin with but soon failed to move and sat at empty. Turns out the float sprang a leak and thus wouldn't allow things to read correctly. Apparently this is not an entirely uncommon thing. A new float fixed me right up.

[G-749]

Thans to all who replied, in cab tank , not that it matters , still have to do seat recover thing. Will wait until that to access tank, non folding seat , hope it has to do with sending unit , don't want to pull cluster for gage ,will post about it when I get to seat cover operation.

[Rich69shortfleet]

I have a non-folding bench seat too. If you slide the seat to the full forward position you can reach the sending unit and remove and replace it. It IS a bit cramped to reach, but not really that difficult. I've done it three times now in the past few months.

[G-749]

Thanks for your input about the seat /fuel tank thing . Now that it's running I will wait to do the sending unit thing test when I do the seat cover operation. 3 times ,did u get lucky on the third time or did you strike out ?

['68OrangeSunshine]

I went with the Plastic sender float. It's a Jeep part, but identical in size and fit to the OEM GM brass one. Newer manufacturers of the brass floats are not using the hot flame soldering technique of over 50 years ago. A cold solder process is used which saves expense but does not always seal -- letting fuel leak into the float, which reads lower and lower until it sinks completely.
Got it on Amazon.

[Rich69shortfleet]

First time was to pull the original sending unit out to test it. Found it to be bad. Second time was to install the new sending unit. Third time was to pull the new sending unit to install a new float.

['68OrangeSunshine]

I'm lucky. I put a '69 T-Bird seat in my '68 C/10. It's got a single bench seat with dual Bucket backs and an Armrest.
I only have to pull the Armrest down to access the Fuel Tank Sender.

[K10-Kansas]

My LMC sending unit was good when I first put it in and then it got more and more wonky after a few months. I tested the gauge with a potentiometers and tested the sending unit with a multimeter and saw both were good. I checked the float and sure enough, it was full of gas. I bought the plastic Jeep float (OMIX-ADA 17729.01 - OMIX-ADA Fuel Tank Sending Unit Replacement Parts) and it fit perfect!

The brass floats filling with gas is such a common issue, manufacturers are surly aware right? Why does every sending unit still come with a brass float? What a PIA.

Here's the one I bought https://www.summitracing.com/parts/OMX-1772901

[jocko]

with the years and years of problems reported on those floats, you'd think LMC (or any vendor for that matter) would actually address the issue. I have one in my truck, but probably don't have enough gas in the tank to fill the float, yet. And my gauge isn't working anyway, so still have some troubleshooting to go. Lots of good info on the site when the time comes. I've got worse issues to fix first once things thaw out up here.

[Steeveedee]

Quality had dropped. Many millions of vehicles had soldered brass floats that lasted. I have a plastic float in my truck's tank for that very reason.

[K10-Kansas]

I read that from the 1940s to 1970s there was little to no ethanol in gasoline but that changed in the 70s when lead was phased out. The enthanol corrodes and creates pinholes in the brass floats. I gave mine a slight squeeze with some pliers and gas shot out of about five different pinholes.

[Steeveedee]

Ethanol combines readily with any moisture, which is dispersed throughout the fuel. Moisture would generally not mix and basically settle at the bottom of the tank, so yes, ethanol's a problem...even though there are other chemicals that are supposed to buffer its corrosive effects, and that help the water combine with the fuel. We're probably going to find out in 50 years that those chemicals are just as big a problem as tetraethyl lead was, back in the day.

Off topic, but related- I don't use sugar on any food. Many of the sweeteners developed to make food taste sweet are being found to be carcinogenic. You can't win.