# What would make a gas gauge do this? **URL:** \ **Category:** Factual Questions\ **Created:** [May 20, 2010, 5:11pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147 "2010-05-20T17:11:37Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![batsto](https://avatars.discourse-cdn.com/v4/letter/b/b2d939/32.png) [@batsto](https://boards.straightdope.com/u/batsto)\ **Post date:** [May 20, 2010, 5:11pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/1 "2010-05-20T17:11:37Z")
I recently acquired a used car, and the gas gauge behaves oddly. Whenever it gets down to below 1/4 tank, the needle begins bouncing all over the place…from the E all the way up to significantly past the F. It will do this repeatedly until I put gas in it, making it pretty much impossible to know when I’m really near the E. Anyone have an idea what could be going on with this? ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [May 20, 2010, 5:14pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/2 "2010-05-20T17:14:55Z")
What kind of car are we talking about? Does it do it constantly or just when the car is moving(or not)? ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [May 20, 2010, 5:20pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/3 "2010-05-20T17:20:36Z")
This is a known issue with GM vehicles. Usually this is the result of either a bad connection in the wiring, the gauge unit itself is bad, or something’s wrong with the float in the tank. ---
**Author:** ![batsto](https://avatars.discourse-cdn.com/v4/letter/b/b2d939/32.png) [@batsto](https://boards.straightdope.com/u/batsto)\ **Post date:** [May 20, 2010, 5:28pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/4 "2010-05-20T17:28:57Z")
It’s a 1993 Pontiac Sunbird. I’ve only noticed it while the car is moving, but I’ve only had it for a week and when I noticed it I headed for the nearest gas station. ---
**Author:** ![DCnDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dcndc/32/2842_2.png) [@DCnDC](https://boards.straightdope.com/u/DCnDC)\ **Post date:** [May 20, 2010, 5:34pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/5 "2010-05-20T17:34:03Z")
I would run this by a mechanic first, but usually the fix for this is to replace the fuel level sending unit. If you’re mechanically inclined you can do it yourself; it’s relatively simple and the part is cheap. ---
**Author:** ![batsto](https://avatars.discourse-cdn.com/v4/letter/b/b2d939/32.png) [@batsto](https://boards.straightdope.com/u/batsto)\ **Post date:** [May 20, 2010, 5:56pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/6 "2010-05-20T17:56:16Z")
My only real concern was whether this is likely to be indicative of some other serious problem. I can live with an inaccurate gauge for a while, as I’m hoping to for once go a whole month without having to repair anything on my car. I’ve had a string of bad luck with cars lately and I’m tired of trips to the mechanic… ---
**Author:** ![Dallas\_Jones](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dallas_jones/32/3277_2.png) [@Dallas\_Jones](https://boards.straightdope.com/u/Dallas_Jones)\ **Post date:** [May 20, 2010, 6:57pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/7 "2010-05-20T18:57:48Z")
> [@batsto](#): > > It’s a 1993 Pontiac Sunbird. I’ve only noticed it while the car is moving, but I’ve only had it for a week and when I noticed it I headed for the nearest gas station. I was just about to ask you if it was a Pontiac. I’ve had a series of Pontiacs with this problem, the ones in the '90’s were the worst. Just fill it back up when you get down to half a tank because at that point you can almost visually watch the needle drop. They will run for many miles on the top of the gauge but once it hits half, find a gas station. The later years were much better and had fuel gauges that actually reflect the amount left in the tank. ---
**Author:** ![Philster](https://avatars.discourse-cdn.com/v4/letter/p/13edae/32.png) [@Philster](https://boards.straightdope.com/u/Philster)\ **Post date:** [May 20, 2010, 7:00pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/8 "2010-05-20T19:00:27Z")
If you can ground the gauge wires, the gauge should peg at ‘full’. If this happens, the gauge is fine and you may need to replace the sending unit. ---
**Author:** ![Hail\_Ants](https://avatars.discourse-cdn.com/v4/letter/h/dc4da7/32.png) [@Hail\_Ants](https://boards.straightdope.com/u/Hail_Ants)\ **Post date:** [May 20, 2010, 9:00pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/9 "2010-05-20T21:00:34Z")
> [@DCnDC](#): > > I would run this by a mechanic first, but usually the fix for this is to replace the fuel level sending unit. If you’re mechanically inclined you can do it yourself; it’s relatively simple and the part is cheap. I wouldn’t call it “relatively simple” as it involves removing the gas tank. Especially if you live in a wet climate as everything will be very rusted. Not so sure the part will be that cheap either as sometimes the sending unit and fuel pump are all one part… ---
**Author:** ![kayaker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/kayaker/32/441_2.png) [@kayaker](https://boards.straightdope.com/u/kayaker)\ **Post date:** [May 21, 2010, 2:45pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/10 "2010-05-21T14:45:18Z")
I’ve owned 3 Jeep Wranglers over the years. At some point I had the sender unit go bad in each. I had one replaced, but just kept 1/4 tank at all times in the others. ---
**Author:** ![DesertDog](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/desertdog/32/11503_2.png) [@DesertDog](https://boards.straightdope.com/u/DesertDog)\ **Post date:** [May 21, 2010, 5:32pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/11 "2010-05-21T17:32:29Z")
> [@ghardester](#): > > I was just about to ask you if it was a Pontiac. I’ve had a series of Pontiacs with this problem, the ones in the '90’s were the worst. Just fill it back up when you get down to half a tank because at that point you can almost visually watch the needle drop. They will run for many miles on the top of the gauge but once it hits half, find a gas station. Every single vehicle I’ve owned had a gas gauge like this, slow to the mid-point then almost visibly dropping thereafter. The mid-point was at best a third of capacity, at worst less than a quarter. Even my present CNG Honda is like this (more towards the 1/3 point). Presumably it is measuring pressure rather than using a float sensor but the mind boggles. Can it be that hard to design a system a little closer to linear? ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [May 21, 2010, 6:02pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/12 "2010-05-21T18:02:14Z")
> [@DesertDog](#): > > Every single vehicle I’ve owned had a gas gauge like this, slow to the mid-point then almost visibly dropping thereafter. The mid-point was at best a third of capacity, at worst less than a quarter. Even my present CNG Honda is like this (more towards the 1/3 point). Presumably it is measuring pressure rather than using a float sensor but the mind boggles. Can it be that hard to design a system a little closer to linear? I have a 2010 Hyundai Elantra, and it’s the first car I’ve ever owned that has a linear fuel gauge; it moves off the full mark right away and keeps on moving at the same rate till empty. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 21, 2010, 8:10pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/13 "2010-05-21T20:10:27Z")
> [@DesertDog](#): > > Presumably it is measuring pressure rather than using a float sensor but the mind boggles. Nope. It’s a float. ---
**Author:** ![Uncommon\_Sense](https://avatars.discourse-cdn.com/v4/letter/u/f9ae1b/32.png) [@Uncommon\_Sense](https://boards.straightdope.com/u/Uncommon_Sense)\ **Post date:** [May 23, 2010, 5:45am UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/14 "2010-05-23T05:45:44Z")
> [@Gary\_T](#): > > Nope. It’s a float. Will STP or similar fuel/injector system cleaner have any affect on the float? Or is replacement the only solution. ---
**Author:** ![jasonh300](https://avatars.discourse-cdn.com/v4/letter/j/46a35a/32.png) [@jasonh300](https://boards.straightdope.com/u/jasonh300)\ **Post date:** [May 25, 2010, 1:49am UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/15 "2010-05-25T01:49:41Z")
> [@Uncommon\_Sense](#): > > Will STP or similar fuel/injector system cleaner have any affect on the float? Or is replacement the only solution. Old floats tend to develop leaks and fill up with gasoline, causing them to sink to the bottom of the tank. This causes the gauge to read Empty all the time. The problem with the GMs and the gauge going crazy is probably electrical. It could be the electrical part of the sending unit or the wiring/circuitry/computer between the sending unit and the gauge. ---
**Author:** ![Gary\_T](https://avatars.discourse-cdn.com/v4/letter/g/3ec8ea/32.png) [@Gary\_T](https://boards.straightdope.com/u/Gary_T)\ **Post date:** [May 25, 2010, 2:45am UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/16 "2010-05-25T02:45:09Z")
> [@Uncommon\_Sense](#): > > Will STP or similar fuel/injector system cleaner have any affect on the float? Or is replacement the only solution. If the sender doesn’t work properly, replacement is really the only fix. Note that most fuel sender problems are not with the float itself, but with the rheostat that the float arm connects to. [This description](http://auto.howstuffworks.com/fuel-gauge1.htm) answers many of the questions raised in this thread. ---
**Author:** ![digs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/digs/32/14889_2.png) [@digs](https://boards.straightdope.com/u/digs)\ **Post date:** [May 25, 2010, 4:28pm UTC](https://boards.straightdope.com/t/what-would-make-a-gas-gauge-do-this/540147/17 "2010-05-25T16:28:43Z")
> [@Gary\_T](#): > > If the sender doesn’t work properly, replacement is really the only fix… Or you could de what we do. Ignore the gauge. We just zero the trip odometer after every fillup (so we can figure out gas mileage). But it means we can judge our gas level by mileage instead of some pisspoorlydesignedbyfrickinclowns needle.