SOLVED! Psychotic Fuel Gauge...

[Sqrly]

My fuel gauge is insane!

I'm still on my first tank of gas since buying this 04 Envoy XUV. We filled it up on the way home. I've only put 189 miles on it since I filled it and when I go out and start it the low fuel light comes on.

I do need to drop the tank to properly fix the crack where the rollever valve sticks through the tank, already have the parts for that, but it's not leaking enough to toss a 442 code and there's no gas smell so either I'm getting less then 10 mpg or the gauge is reading wrong.

Also, after driving a few minutes the gauge goes up to 1/4 and the low fuel light goes out. A few more minutes and the gauge is reading 1/2.

Just a little bit ago as I was coming home from a 2.5 mile drive the gauge was reading just over 1/2. When I stopped in front of the house I watched it for about 10 seconds before shutting off the engine. The gauge went up to full for a few seconds then dropped down to just above empty and the low fuel light came on, while sitting still on a flat surface!

I ordered the sending unit and I'll swap it when I drop the tank. I do have a 5 gallon gas can strapped in the back with 3 gallons in it.

If I unhook the fuel filter and turn the key on will it pump all the gas out of the tank?

 

[budwich]

for about two seconds.... as the pcm will drop the relay after that because the engine is not turning. You just jump the contacts at the fuel relay spot in the fuse box to get the pump to run. Remember, this is a PRESSURE line... expect things to happen.

 

[Mooseman]

That's the method I usually use unless the pump is dead.

For the fuel level gauge, since you're dropping the tank, I'd at least replace the level sensor. Since the low fuel light and the gauge are both indicating empty at the same time, sounds like it's the in-tank sensor or possibly the wiring. IIRC, if there is an open circuit, it will show empty and turn on the light. Might also throw a code.

Here's an idea, while pumping the fuel out by jumping the relay, have the key on and watch the fuel level go down. With nothing else moving, if it does squirrely, then it could very well be the sensor.

 

[TJBaker57]

Do I remember correctly that there was a change with the level sensor (and of course a matching change in the IPC calibration)? Some years an open circuit
reads full tank and some years an open circuit reads empty?

 

[budwich]

Do I remember correctly that there was a change with the level sensor (and of course a matching change in the IPC calibration)? Some years an open circuit
reads full tank and some years an open circuit reads empty?

here is a page out of a 2004 for what it is worth.

 

[Sqrly]

A little update and a new question.

The gauge only acts erratically when the tank is actually low on fuel. It starts just before the low fuel light comes on. This is however when you need the guage to be most accurate.

The light and the guage (when not malfunctioning) seem pretty close as I did run it out of gas once already. (I am carrying spare gas in a can the back)

The light came on again 2 days ago. (I drive this vehicle at most 5 miles a day) which is perfect because I'm dropping the tank this weekend to replace the cracked fuel inlet and fuel level sensor.

I'm glad I got the sensor too because it came with the big rubber ring that seals the pump to the tank. It hadn't occurred to me I would probably want to replace that too.

Q: Can one tell the difference between the 25.5 and 22 gal gas tanks visually? I noticed they don't sell the larger tank, only the smaller one, and I'm wondering if mines ever been replaced.
(I suppose I'll find out when I'm done if it takes over 22 gal to fill it.)

Either the tank is leaking while driving and I'm not smelling it (My grandson has complained of gas odor and he's got a nose like a dog so it probably is leaking) or I am getting horrible gas mileage. We filled it when we first bought it and it ran out of gas in only 200 miles. I did replace the spark plugs after that as they all had a nice round deposit on the ground electrode. I also noticed at that time that 2 of the coils are original and the other four bear markings from various manufacturers.

 

[Mooseman]

The larger tank is only on the long wheel base trucks ( EXT/XL/XUV) no matter the engine. Probably why they only sell the SWB tanks as those are the most predominant.

 

[Sqrly]

The larger tank is only on the long wheel base trucks ( EXT/XL/XUV) no matter the engine. Probably why they only sell the SWB tanks as those are the most predominant.

2004 XUV here so I should have the larger tank. We didn't pay attention to the quantity or cost the one time we filled it. After fixing the tank I will be documenting fuel usage for a bit to calculate my mileage.

I did get the original owners manual with it. Albeit GM's information is not always entirely accurate

 

[Mooseman]

I guess you're using a tank inlet repair kit? Dorman makes one.

Dorman Products - 577-106

Fuel Filler Neck Inlet Check Valve

www.dormanproducts.com

There were some ghetto repairs using plumbing fittings however, without the valve, the gas pump will not stop and overfill the tank, spewing fuel out the filler and destroying your charcoal canister.

 

[Sqrly]

I guess you're using a tank inlet repair kit? Dorman makes one.

Yes I'm using that one as well as a new filler neck and fuel sending unit and O-Ring, all dorman.

I'm following the video you posted. I even bought the Swimming Pool Return Line No. 9 Winterizing Plug 1.5 - 1.75 Inch to keep from getting plastic in the tank.

I have a long history and a lot of experience in many areas of the auto industry. I know when and where not to skimp on the quality of parts used.

"Gas explodes, you know?" - Bridget Moynahan. I, Robot

 

[Sqrly]

Is it normal for an old level sensor to look like it's been very hot?

 

[Mooseman]

I haven't seen it myself but it's a possibility. This would be especially true if the contact was worn and poor. Sure can be the source of your gauge issues.

 

[Sqrly]

It was definitely the source. The guage is working properly now.

Dropping the tank was pretty easy thanks to the video you shared. Knowing where the connectors you can't see are ahead of time really helps. (Having done this several times before on other vehicles helped too)

I did it without the Lock Ring Wrench. Getting the ring off was real easy. A tap here, a tap there with a flat nose chisel and it came right off. Getting it back on wasn't as easy.

While pressing down I clamped vice grips on two opposing lock tabs that kept enough pressure on the lock ring to partially compress the o-ring. I also applied a thin layer of silicone grease to the bottom of the lock ring.

Then it was a lot of (gentle) tapping, around and around and around but, it slowly went on. It definitely took a lot longer then it would have with the tool but I didn't want to buy a $70 tool I'll hopefully never need again.

Note: If you have a pickup, taking the bed off is both easier and faster as long as you have some friends to help move it.