Fuel gauge travels around till driving a bit

bspurloc

Original poster
Member
Dec 27, 2012
295
I just noticed my daughters fuel gauge is having issues.
I started her car and noticed the Fuel Gauge was showing FULL sitting on F. I thought that was odd cuz she always has it near 1/4.
I sat there looking at her Mileage since I just changed her oil and the Fuel Gauge started moving towards Half tank. it sat there a bit then traveled back to F. then slowly moved to 3/4 and then back to F.
I drove the car back to her and on arrival I noticed it was solid at 1/4.
I turned off the car then started it again and the Gauge again moved to F sat there then started to slowly move to half and back. It kept slowly deciding a different fuel level. I turned it off.
She said its been doing that a few weeks and fixes itself after driving a few minutes.

This seems like a bad connection to me aka poor ground or something.
I replaced the whole fuel pump assembly in this car like 4 years ago so I highly doubt its bad.
Back then it was discovered the Fuel Motor wires loom was being crushed between the body of the car and the frame right at the driver side foot area causing the fuel motor wires to corrode and rot. aka no fuel pump spin, like a dummy I replaced the fuel pump then discovered this wiring harness crushed and the wire corroded. there was another wire corroded I cant remember if I spliced it or not. I hope so lol. (I did a post about it with pics)

So anyway... what can I check ASSuMing the fuel pump assembly is not at fault.
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
The stepper motor might be dying. Can you check what the reading is with an Bluetooth OBD adapter and Torque? If it follows what the dash gauge is giving, then either the sender or wiring is at fault.
 

Chickenhawk

Member
Dec 6, 2011
779
Fuel sender unit or bad stepper motor are the two most common problems. Bad stepper motors will wander around to random places, and can be diagnosed by where they end up when the key is off. If the gauge goes to the left pin on off, it is usually a bad sender. If the gauge stops at random places on off, it is usually a bad stepper motor.

Edit: Ah crap, my friend Moose beat me by two minutes. I am getting slow in my old age.
 

bspurloc

Original poster
Member
Dec 27, 2012
295
thanks.
I believe it goes to E left pin when turned off.
Bah. I cant believe the sender is dead after a few years.
I should check those wires again.
Any Idea what color wires are the sending unit going through the loom?
Where did Roadie go? he always loved linking wiring diagrams! lol
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
There are wiring diagrams in the manuals (link in my signature)

If it's a cheap fuel pump module with a cheap sender with the old version contact fingers, they can wear out quickly.

Roadie just dropped out, enjoying retirement and traveling.
 
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Sparky

Member
Dec 4, 2011
12,927
The sender on my 2.5 year old pump on my Silverado has been acting a little odd for about a year. At cold start it may read close to 1/4 tank lower than it really is but after driving a while it levels off. I know it is a the sender because the fuel range increases as I drive too.

Sometimes, you just get a flaky sender.
 

Dr.Fiero

Member
Dec 7, 2017
161
S. Alberta
Senders are getting worse everywhere I'm seeing. I blame the fuel. Senders are just wire wound variable resistors. Usually 90-270 ohm (or 0-90 if you've got something from the mid 90's and older).

I'm blaming the faults on the ethanol in fuel. It's quite corrosive, and is just screwing up the surface of the wire that the wiper drags across. This gives you erratic readings.

You could just unplug the connector, then stick a ~150 ohm resistor across the chassis side of the plug. See where your needle sits. Wait a bit. See if it moves (it shouldn't!). If it does - stepper issue.

Meter the other side - see what the reading is. Jiggle the truck. Should just barely change as the float 'bobs'. Here's where you wish you kept your old analog meter. :wink:
 

bspurloc

Original poster
Member
Dec 27, 2012
295
Thanks for the info. The car isnt near me anymore so it will be a slow process lol.
If it were my car I wouldnt care. but she is negligent with gas always leaving it at 1/4 or on E.
Where is this plug to test a resistor? behind the dash?
I'm definitely not dropping the tank again. last time I did it all alone with the car on the lawn. barely lifted if at all. took me 8 hours and crushed under the car the whole time lol. now that I think back I should have put wood under the tires to lift it as the lift kept sinking the wood it was on lol.
I still have a few analog meters... radio shack kits etc lol
The car suffers from heavy rust do to I think it used to be used to launch boats its first 80k miles. as all the severe rusting is in the rear area salt water food. so its possible connectors are corroded.
Isn't there a harness at the tank? where does it connect at the top middle? sigh
although my friend has a lift in his garage now. tank drop would be easier.

thanks for wiring, I'll look it over

....
You could just unplug the connector, then stick a ~150 ohm resistor across the chassis side of the plug. See where your needle sits. Wait a bit. See if it moves (it shouldn't!). If it does - stepper issue.

Meter the other side - see what the reading is. Jiggle the truck. Should just barely change as the float 'bobs'. Here's where you wish you kept your old analog meter. :wink:
 

bspurloc

Original poster
Member
Dec 27, 2012
295
for future reference and others searching this here is the color of wires from schematic.
Purple for sender unit and orange/black low ref? I know these run thru the loom driver side door next to frame and body of car. underneath look up at the frame driver door area.

What is this circuit saying? there are 2 wires for sender unit ground is not used?
Low Reference? is there another line to show fuel is LOW?
I mean sure I have an electronics degree and all lol but I dont see Ground referenced...
 

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Chickenhawk

Member
Dec 6, 2011
779
One thing to try is a good does of fuel additive in the tank. It has sometimes worked to clean the contact surface of the sender unit.

VERY IMPORTANT. Do not waste your money on any additive unless it contains a chemical call Polyetheramine or PEA. Nothing else works.

Follow the instructions, and maybe run a PEA fuel system cleaner in two tanks back to back. Read the label carefully. For example, here in Canada it is hard to get Techron or similar additives, but even Canadian Tire carries Gumout with Regane. (This is the only Gumout fuel system cleaner with PEA ... which is also why it is so much more expensive.)

No guarantee this is going to work, but at minimal cost and little effort, it is worth a try.
 

Dr.Fiero

Member
Dec 7, 2017
161
S. Alberta
Canadian Tire carries Gumout with Regane.

Hey, that's the stuff I've been using on the thinning patches up top and... oh... wait... that's rOgane. :crackup:

Yeah, might be worth a shot! Way easier than pulling things apart this time of year (dropping to -25 overnights). As with most additives, don't be tempted to 'up the dose'. I've seen people destroy injectors when they figured if a bottle to 50 gallons was good, so a bottle to 10 gallons must be better. It can melt the varnish right off the windings. I clean and flow test injectors as a hobby biz, so I've seen quite a few, and far too many dead shorted ones.

@bspurloc: the 2 wires coming of the sender - it's still a ground referenced setup. You'd see it if you tore open the BCM. They just don't really show it anymore. The 'fuel low' is determined by the ECM (something like "if X=< 110 ohm, set flag).

If you can find where those leads terminate somewhere up higher (vs the side of the tank where the connector is), that would be easier. But it won't be up at the dash like days gone by, since the gauge itself isn't just reading the sender. That'd be too easy. :wink:
 

Mooseman

Moderator
Dec 4, 2011
25,257
Ottawa, ON
The cleaner might help but like I said, it could have thin fingers like in this pic.

2009111021139_eh%20sedan%20tues%20007.jpg


You can see how they can wear out quickly. GM and the better parts makers later fixed this by using a thicker button at the ends of the contacts instead of those fingers. I look for that when getting replacements.

I hope you don't have to drop the tank. I can totally relate to working in -20c.
brrr-gif.82547
 
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bspurloc

Original poster
Member
Dec 27, 2012
295
you mentioned Techron? funny I have a case of Techron I was going to put some in her tank after changing her oil but put it back.
Techron doesnt say whats in there most dont bother to tell you cuz its a secret?
So does Techron have this additive?
Cases of Techron go on sale at Costco every once in awhile so I buy a case.
I wouldnt be surprised that her constantly having the car near E has harmed the Fuel Pump.
The bf just bought a used 2014 Acadia Denali 70k miles from a dealer. Seems like a nice car. cept for the torn engine mount, throwing engine codes of both banks lean and abs failure caused solely by a bad EGR. and a loose control arm. starter seems to be failing. things that shouldnt be bad on at 70k...
 

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