OK, listen up all you Chevy owners. I keep seeing a thread about a "wandering" fuel gauge when the tank gets down to about a 1/4 tank. It just about drove me crazy trying to figure out what was wrong and how to fix it. I checked a bunch of stuff, then found it.
The tank sending unit (located on the side of the electric fuel pump inside the gas tank) is usually the source of the problem. Basically the way it works is that there is a potentiometer attached to a float by means of a lever. The float rises and falls as gas is added or consumed in the tank. The potentiometer translates the mechanical motion of the float arm into a varying electrical resistance which is displayed in on the gauge.
After 75K to 100K miles or so, the float has moved up and down enough times to wear down the contacts or brushes on the potentiometer. At a low level in the tank, the contacts no longer touch the resistive area on the potentiometer reliably, and you get a gas gauge that doesn't read reliably when there is low fuel.
To "repair" the problem is quite an undertaking. If you are not pretty experienced with automotive repairs, don't take this on as a DIY project.
WARNING: This is very dangerous. Do not try this at home if you are not willing to take a HUGE RISK because it involves removing the gas tank, lowering it, taking out the fuel pump assembly, making a complicated repair, and re-installing everything. Every attempt should be made to be as safety conscious as possible. You must disconnect the battery as the FIRST STEP in this procedure.
Procedure:
1) Disconnect battery cables. 2) Remove and lower gas tank. (See shop manual.) 3) Remove fuel pump assembly from fuel tank. (again, shop manual.) 4) Repair sending unit. See below.Details: Once you have the fuel pump assembly in hand, drain the excess gasoline from the sending unit into a clean gasoline proof plastic container. It holds a bunch of extra gas and will drain for a while. Just set the pump unit in the container and then let it drain for a few minutes. Then locate the sending unit on the fuel pump unit.
To verify the problem is indeed the sending unit, you will first need to carefully inspect the sending unit brushes on the side of the fuel pump assembly to confirm that indeed the brushes are not making contact when the float is low in the tank. Do this by moving the float arm up and down and looking at the tiny brushes on the sending unit. You can use an ohm meter to confirm this as well.
Next remove the float and float arm. There is a clip holding the arm onto the sending unit. After you have removed the float and float arm, remove the sending unit from the pump assembly.(very tricky but it can be done.) Once you have the sending unit in hand, you will need to take the wiring loose from the fuel pump assembly and then also the sending unit. It takes some understanding of how connectors work. If you aren't completely comfortable with these steps, you can skip them, but it makes the repair more tedious.
Using a supporting surface (I used a slightly open bench vise)lay the sending unit face down and CAREFULLY drive the center pivoting module out of the main unit by gently tapping it with a punch using a small hammer. (If you can't fix something with a hammer, well...you just aren't doing it right or you have the wrong hammer.)
This will give you access to the brushes on the assembly. Look at them very carefully. You should see wear on the ends of the brushes.
The next part is VERY tricky and should only be performed by qualified individuals like: brain surgeons, pilots, marine snipers, and other over-confident, egotistical, maniacs such as myself. Or just be really, really careful.
Carefully note the attitude of the brushes. You are going to take a small needle nose pliers and bend the brushes in such a way that they make better contact when reassembled to the resistor that is in the other half of the sending unit. This is important. They must make firm contact, but they are extremely small and very fragile. You have to do it right the first time, or you'll be explaining to your wife why you have to buy a $400 fuel pump assembly because your gas gauge was a little unreliable. By the way, if you read this far and you are feeling panicky or beginning to sweat a little just thinking about doing this, then you probably ought to start using your trip odometer as an indicator of when to buy gas.
OK, we adjusted the brushes. Now it's time to reconnect and reassemble everything in reverse order.
As soon as you get the pivot assembly installed back onto the sending unit, do yourself a favor and check to see that it's working using an ohm meter. You don't want to put everything back together only to find you didn't get the brushes JUST RIGHT.
Once you verify everything with the meter, continue with the reassembly in reverse order.
Note: Be very careful handling the fuel tank. It's very heavy all by itself. I waited to do the job until I had very little gas in the tank. The problem with that is, that's when gas tanks are the most dangerous. The more air in the tank, the bigger the explosion if you make a mistake. And you will only have the one mistake...
That's all there is to it. Remember to hook up the battery cables LAST.
I spent about 2 or 3 hours doing this job last year, and I haven't had any problems since then.
I don't know when to expect the fuel pump to fail but when it does, at least I know what to do to change it out, and how much it will cost.
Again, this could be dangerous. Please be extremely careful when working around gasoline and tanks.
Good luck!!!