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Anyone had a weird fuel gauge issue like this?

863 Views 8 Replies 3 Participants Last post by  MrSurly
Car was fueled up and driven maybe 30-40 miles before it sat parked for over a week. Fired it up last night, fuel gauge read a little less than half. Didn't think anything of it. Hit the road for a 50 mile drive. At about 30 minutes in to the drive, I looked at the fuel gauge and it was a little over half. OK, I figured, it must have read low at startup and slowly drifted up to the actual fuel level. Except it continued to go up slowly. By the end of the drive it was well above 1/2 a tankful. Car sat for a few hours and then I drove it home. At start up it was at about the same place as it had been at shut down. Above 1/2 full. But by the time I got it home it was up to 3/4s of a tank again.

I drove about 100 miles and ended up with 1/4 of a tank more than when I started.

I know the fuel gauge is "damped" so it doesn't wiggle around as fuel sloshes around in the tank. And on my C5 Corvette, the tank was actually two separate tanks and the fuel pump could draw down the side with the float and you'd get a bad reading on the fuel gauge until the pump caught up. But the Caddy's tank isn't like that. I can't think of a way that I could get the behavior I got.

Anyone seen something like it? And where is the fucking float if I have to replace it? Should probably run some Techron through it...
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id guess at the time u got back into it and started up the fuel sender may have been at fault (may be wearing out over time) these usually read lower when they start to play up, and then as u drove the reading slowly worked its way back to normal in which the moving vehicle now prob keeps the reading out of the faulty spot or its not there long enough for the gauge to read it after dampening, if u always keep a full tank then it will wear out the full level first, if u use the tank each time then it can last longer as it slowly wears out the full span over time not just a single spot, we see that sometimes where people use 1/4 tank and fill up each time and the sender wears out the top 1/4 and the bottom 3/4 is brand new still,

if it becomes common maby log it on hpt will be a live reading before u start it up and see what it reads and if u have a full tank but its showing lower then shake the vehicle to get the fuel moving then if it changes and reads full u know sender is playing up
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Im having this weirdness from my fuel gauge but with the added sporadic dropping to zero and randomly regaining its place.
Because of the extensive work and mods I’ve done, I’ve attributed it to something I caused.
My gauge will rarely show the correct level at start up, usually wrong by a 1/4 to a half tank low, never high; it will slowly regain level while driving. However, it also will simply show zero randomly.
Now I also have a Banks iData gauge and have it displaying fuel level percentage.
This adds another layer of oddness, to wit: the Banks gauge will occasionally go blank, indicating a loss of signal when the oem gauge is showing empty (expected, right) but not always. When the Banks is not losing signal, it typically displays the correct percentage even while the oem gauge is reading way low.
Often, the Banks will be reading say, 72% and the gauge is at 1/4 and the gauge will slowing get up to 3/4.
The V tank has a saddle design with TWO sending units. I’m not sure if the two senders are reporting to the ecm as separate channels or if they are providing a cumulative reading.

I do need to sort out what is going on with this.
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both senders carry the same earth but each sender wire goes back to the bcm/ecu and its put together and then halved to get the average for the tank volume
both senders carry the same earth but each sender wire goes back to the bcm/ecu and its put together and then halved to get the average for the tank volume
That’s what I had assumed, two channels and the ecm does the math. But I wonder what the Banks gauge is reading? Possibly ONE sender? I have seen the Banks reading the correct percentage for what was in the tank while dash gauge is displaying either zero OR some lower, slowly rising level OR the correct level.
The Banks seems to be either zero (blank) or correct.
Very confusing. The only real constant is that neither system ever indicates MORE fuel than I actually have on board.
we can adjust both volumes within hpt but the only output i see is of the combined volume either for the tank or the gauge, id have to think the banks will be seeing prob the tank volume if it stays good but the gauge drops out, as the gauge reading drops out with a data issue where the tank reading shouldnt, would have to take the senders out to test properly use multimeter on ohms and wriggle the sender are around and it should stay good within the reading it should be at that part of the sender between 250 and 40 ohms, if it ever reads outside that or reading varies alot for a wriggle at the same sender point then there is prob a connection/sender issue
The issues with mine (of course) started after my build, during which virtually everything was altered.
Besides my clutch issue, three things come to mind as known issues:
Alternator is intermittent (new one going in next week)
Fuel gauge is wonky, as described herein.
Fuel pressure is now not reading in HPT (previously working)

Stock pressure sensor is in use but relocated to firewall location after FORE! filter. Extender harness from Fore! used to connect back to oem harness connector.
Are these all related? Don’t know.
I have visually inspected the fps extender harness.
I have inspected the sender harness at the fuel hat.
Nothing glaringly wrong.
I seriously wonder what effect the system voltage variance is having on the level sender system.
I’m not gonna lie…. I’m secretly hoping the new alternator will simply fix the other issues.
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could do if voltage is dropping they can start to turn some non vital systems off, id prob try that first fingers crossed, the pressure sensor is odd as the ecu needs it for fueling so im not sure if it defaults to a reading or what not just for the delta pressure, but just extending the wire and in a different spot should be fine pressure should be same within the system so thats an odd one, the voltage shouldnt affect the senders as they run a lower voltage anyway bout 5v i think but if its affecting the data then that will affect the reading, i hate all the little bugs after big mods PITA i usually leave it out of mind if i can and usually one day something pops into ur head to check or adjust and boom all good 🙏
Yeah, like a ground connection missed (had that a few weeks ago after a clutch r&r).
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