Why is my fuel gauge so inacurrate?

Factual Questions
1

I have a 2001 Honda Civic, which I love on all of those days of the month when I’m not actually mailing in a payment. The only bad thing about this car is that the fuel gauge is not accurate. It will stay on full for a very long time and then, the next time I start the car, I all of the sudden have a half tank of gas. Sometimes the fuel light will be on and the gauge reading empty when apparently gas will spontaneously regenerate, the fuel light will go off, and the needle climb a bit above the “E”. What’s the deal? I’ve heard of such things happening in older cars, but this car has been like that since day one.

Info from anyone with a greater automotive knowledge than I is greatly appreciated.

2

WAG:

I don’t think automotive engineers bust their humps going for very accurate gas gauges. My old car would get maybe 20 miles on the first 1/4 tank, around 200 miles on the next 1/2 tank and maybe 50-75 miles on the last 1/4. I just got used to it. Mostly the important thing to know is when you are running out of gas which the gauge will show fairly well.

Also realize that gas will slosh around the tank so if you’re parked on an incline that may affect what the gauge reads and once you move gas will seem to appear or disappear as the tank levels out.

My ‘trick’ (not really a trick) is to reset my trip odometer every time I fill-up. I happen to know from experience I can comfortably get 300 miles out of a tank of gas. Once I’m above 275 miles (or so) of driving I know it’s time to stop for more gas at my earliest convenience. This also allows me to track if my engine starts performing badly. If all of a sudden I’m only getting 250 miles to a tank I might want to have my car looked at (you have to allow for running the AC in summer which will lower your gas mileage somewhat).

3

Being that your car is so new, I would think that the dealer should replace the sending unit for you under warranty.

Gauges generally aren’t the most accurate thing but yours sounds worse than most.

4

The sudden full to half reading you describe goes way beyond lack of accuracy. It suggests a faulty fuel sending unit, fuel gauge, or wiring/connectors in the circuit. It will take some testing to pinpoint the cause, which could be frustrating if the problem is intermittent. It might be covered under warranty.

5

Ever since I had cars with electronic info displays, I find that I actually use the “miles left” setting in preference to the gas gauge. I usually reset the trip odometer when I fill up to, and between the miles left and the trip odometer, I can tell how I’m doing on a particular tankful. That display is usually based on the mileage for the last 20 miles or something like this, and you can watch it respond to the type of driving you are doing at the moment.

What I really wonder about on my current car’s guage is the habit the needle has of going back up nearly to the 3/4 mark from a cold position of 1/2 as the car warms up.

6

I’ve only owned one car–a Volkswagen Rabbit–which accurately displayed the fuel contents as checked against the odometer. All of the others tended to be highly optimistic for the first two thirds of the tank, then quickly drained toward the bottom. I have two WAGs:

  • A fuel tank is rarely a simple box, so its contents cannot be accurately measured by simply checking the level in the tank without adjustment. Yet I suspect that’s just what many fuel gauges do. This is bound to be inaccurate because a tank that is roughly half full by looking from the side has actually consumed more than half of its gasoline.

For example, most tanks are angled at the bottom to ensure that the fuel pools over the pump, which may effect the measurement. In some cars, such as the Jaguar XK-E, the fuel pump is located inside the tank, taking up further space at the bottom of the tank. Most modern tanks also have baffles to prevent sloshing, but those baffles don’t go all the way to the top of the inside of the tank. Other tanks have an internal coating of goop to ensure that it is self-sealing and won’t leak in an accident. That goop probably tends to be thicker at the bottom of the tank. In short, the second half of a gas tank has a lot of shit in it other than gasoline, and if a fuel gauge doesn’t take this into account, it would explain why the second “half” of a tank drains in a big hurry.

  • But it should be fairly easy to determine how much space is taken up within the lower half of the tank, and force the display to reflect that. Even if an engineer didn’t want to do the dang math s/he could still figure it out with a garden hose and a couple of graduated containers, right? So why don’t they?

I suspect that some, like the German manufacturers, probably do factor such things in and make the gauge reflect the level accurately. Others probably consider it an unnecessary expense. Still others might just be counting on that little psychological boost they get by having an inattentive owner say to him/herself, “gee, I’ve gone 200 miles and I still have half a tank,” and counting that the driver won’t notice when the fuel light comes on a hundred miles later.

It’s just a guess, but perhaps I can confirm it by calling around. I’ll let you know if I find out anything else.

7

Actually this is done on Purpose to make you think your car gets better gas mileage. Read This for More information: http://www.howstuffworks.com/fuel-gauge.htm Also it is aVERY good idea to never run your car below 1/8 a tank since the lines are cooled by the gasoline passing through them running your car very low can damage them. It should even say this in your owners manual.

8

It does sound like something might be wrong with your gauge, but if you can live with it, it isn’t going to hurt anything.

Here are normal reasons for large changes in gas gauges:

  1. You live in an area with many hills and inclines. Sometimes the gas is sloping away from the gauge, sometimes toward it.

  2. There are large temperature changes in your area. When gas gets warmer, it expands, and a gas gauge will read higher. For example, if one fills up on a frosty morning, then drives continuously until it reaches 85 degrees, the gauge will drop more slowly. (Then, when it’s cold, it’ll drop again.)

  3. The more accurate the gas gauge, the more expensive it’s liable to be. If I owned a Civic, I’d rather they skimped on the gas gauge than the brakes.

9

If it’s a 2001 it must still be under guarantee, so why don’t you have them look at it? It could just be a loose wire.

10

Ah, the fuel gauge. My pet peeve of the automobile industry.

For what it’s worth, here’s my knowledge of the problem…

As has been suggested before in this thread, the fuel sending unit is probably bad. Your car should be under warranty, so by all means take it to the dealership and get it documented. In some states, if they can’t solve the problem after 3 tries, lemon laws kick in and you could be looking at a new vehicle. (I know because I got a new vehicle from Chrysler for this very reason). If it’s not the sending units, then it could be the gauge itself or a bad electrical connection.

From what I understand, most German cars (and perhaps Japanese, I don’t know) use contacts in the gas tank to reflect the right amount of fuel in your car. The problem is, in the northeast at least, the additives in the fuel actually corrode these contacts. For BMW and Audi, they have solved this problem by using gold-plated sensors.

The sad fact of the matter is that the fuel gauge on my '67 Cougar works perfectly. It seems that the simple float mechanism works like a champ. The problem is that everything now is electronic and computerized. Newer isn’t necessarily better.

By the way… my Chrysler was a Cirrus, and what it did was take a reading every time the car started, and it would stay at that reading until I stopped and restarted the car. If I was on a hill, or an angle, the gauge would reflect a different amount. It drove everyone crazy, and the dealership replaced everything, including the car’s on-board computer with no change. After my 4th trip, I had had it, made a stink, and I had a new car within 2 weeks. Trust me, I had to make a big stink… but Chrysler and the dealership both knew I had them beat on the lemon law, so they figured it would cost them less to shut me up and not go to court. The new car had the same problem (apparently, it was an engineering flaw), so I learned to live with it.

11

I didn’t see in that article where carmakers purposely make their gauges inaccurate. Seems unlikely to me. Very unlikely.

**

Which lines? The fuel lines? Why would fuel lines need to be cooled? And if they did need to be cooled, why wouldn’t they be cooled by the gas flowing through them until the tank is completely empty?

12

Sofa King seems to know a lot about fuel tanks! I’ll add what little I know (which pertains to fuel tanks not made from plastic). First, fuels tanks have a “lot of shit” in them other than gasoline, but “goop” is a new one on me – maybe plastic tanks? Baffles are used with less and less frequency, because resevoirs are cheaper. Sloshing in and of itself isn’t a bad thing, but preventing sloshing keeps fuel at the fuel pump. I cup/resevoir is better in this respect. Also, all modern fuel injected cars have thier fuel pumps mounted to or inside the fuel tank. As we strive for zero emissions fuel delivery systems, you’ll find more and more items mounted inside the fuel tanks, instead of being mated to the tank somehow. The garden-hose and graduated cylinders would work, but on most cars the sending unit and indicator are very simple, non-computer controlled things. The sender is a variable resistor that changes value based on the movement of the arm from the float. It’s linear. Likewise, the gage is proportional to the voltage level (actually, probably a current level), so the inaccuracy is very easily explained.

Fancier cars, though, have their sender hooked to the computer with a analog-to-digital converter. This means that when the sender voltage is processed, the car can display any fuel level it wants for any given voltage/current level. The result is a more accurate gauge. A side benefit is you can use this same information to display “miles to empty” and so on and so forth.

But there is even great variation in this. My previous car would say “low fuel level” at 60 miles remaining, with NO ABILITY to show less than that. Great help. My current vehicle will warn me, but continue showing the miles to empty. I’ve had it as low as 9 miles to empty! (made me a little nervous). It’s also a lot more accurate. The first car would always say 451 miles to empty after a top-off, but only really deliver 350, per the trip odometer. The current car always says 422 miles to empty, and really, really delivers that. I think it’s adaptive to my driving style, whereas the first one wasn’t.

13

I own a 1999 Honda Civic, and while the fuel gauge isn’t as bad as that in the OP, it is unreliable enough for me to use the trip odometer method to tell how much gas I have left in the tank. I fill up the tank every 300-350 miles and don’t even bother looking at the tank gauge anymore.

14

Heh. Thanks for the clarifications, Balthisar. Whatever experience I’ve had with gas tanks is all annoying and bad, and you’ll notice I still managed to confuse the gauge with the sending unit.

Oddly enough, I’ve actually seen the goop thing twice, on the aforementioned XK-E and in a '72 Buick GS. Both vehicles were old enough that I wouldn’t be surprised if they were some sort of aftermarket attempt at corrosion control.

15

You got company:
" Having made a positive statement on the Civic, I will now mentions some negatives from a long time owner. First, it is a noted common problem that some Civics have fuel gauge problem, where the low fuel light comes on too soon (still about 3 gallons left in the tank!) And I will make note again on the bouncy rear suspension, which you should watch out going over the bumps if you have tall passenger sitting in the back. "
http://www.epinions.com/content_68723510916

16

???

That’s strange.

I’ve always heard that low fuel lights are SUPPOSED to come on with approximately three gallons left in the tank. I ask rhetorically: how is that a problem?

17

Ok, here’s a bit of info too. I had a professor when I was in college, and one of the students he had advised through their masters thesis had done a study on gas tanks as part of his masters in physics. He came to an interesting conclusion. The “float” style gas guages actually accelerate as they pass from full to empty.

He set up an experiment where he had a tank full of water and a bob attached to a swingarm, just like the float style gas guages described in the howstuffworks link. He attached a spigot to the tank and drained the water at a known rate. On the other end of the pendulum was a little device which had a solonoid which fired at regular intervals, forcing a marker attached to it’s plunger to contact a piece of paper and would make a mark on the paper. As the tank drained, at a constant rate, you would expect the spacing of the dots made by the firing of the solonoid, also a regular event, to be evenly spaced in an arc.

They were not regularly spaced. They were closer together at the beginning of the draining of the tank and further apart towards the end when almost all the water was gone.

The full thesis had lots of physics and mathematics explaining why this behavior existed, but basically it was the result of the nature of a pendulum.

For a layman, this is his conclusion. A pendulum-float based gas guage will move slower when moving from full to 1/2 than it will from 1/2 to Empty. If you’ve ever wondered why it seems to take forever to get from Full to 1/2 and almost no time to go from 1/2 to Empty, well you’re not just imagining things. Even in a completely regular, cylindrical tank, this is true of pendulum-float guages. It really DOES move faster the less gas you have. It’s a physics thing.

This does not apply to the types of guage which have a pole with sensors at various levels instead of a float. If one of them is inaccurate, it is probably because of the shape of the tank, the “other stuff” Sofa King mentioned, or a malfunction.

As for the OP? Sounds like you have a defective guage, your problem goes beyond the physics-based inaccuracies.

Enjoy,
Steven

18

I’m always uncomfortable with explanations like “it’s a physics thing,” but here’s my best guess at the reasons behind the effect reported by Mtgman.

If the float in the tank is actually mounted on a pivot near the top of the tank (like this one), then what amounts to be a cosine (or is it sine?) error will be injected in the process. I say “if” because I don’t know if that schematic really depicts a typical float setup or not.

In that schematic, it appears that the resistance change in the potentiometer is proportional to the change of angle of the float rod - call this angle q. Say q = 0 when the tank is full and increases as the level drops.

Let’s assume for now that then tank is a perfectly uniform box, and thus that the surface height above the bottom exactly reflects the volume of fuel remaining. Suppose for our purposes that the depth of the tank and the length of the float rod are both H. h starts out a zero, and increases to H as the tank empties.

We don’t really want to know q - what we want is h, the distance from the top of the tank to the surface of the fuel. However, q is much easier to measure so we’ll measure that and relate it to h.

A little trig shows that h = Hsinq. Here lies the source of the problem.

The rate at which q and h change with fuel consumption are different: dh = Hcosq dq. In other words, a given change in h will cause a change in q that is proportional to cosq.

When h (and q) are both about zero, the relationship is close to linear. However, as the fuel level drops and q goes up, a given change in h will result in a much larger change in q than it did before. IOW, the float rod angle will start dropping faster as the tank goes down.

Unless this was compensated for (which they probably don’t bother to do), this would result in the first “half” of the tank as indicated to last much longer than the second “half.” The gage would read half-full when q = 45[sup]o[/sup]. However, that would occur when h = 0.707H = far lower than half-full. You’d use 70% of your fuel getting to the half-way mark, and have only 30% left at that point.

I don’t know for sure if this is what’s going on, but it sounds reasonable to me. Also, I did work with perhaps overly-simplified geometry, which may be why these numbers sound quite a bit worse than one commonly experiences.

19

Interesting thought about the sine/cosine error. But it would be a simple (and inexpensive) matter to vary the resistance of the substrate the arm wipes against to counter-act any such non-linear tracking.

Maybe someone already mentioned this and I missed it. But for most cars the fuel gauge you see on the dashboard is merely a Voltmeter and the fuel guage sending unit is a variable resistor. Thus, when you first start your car in the morning the fuel gauge will read lower because the battery has a lower Voltage. As you drive the the battery Voltage goes up as it gets recharged and thus you may see the needle creep up. For a little more money the car maker could include a circuit that would produce a constant Voltage for the fuel gauge or even use a digital sender but every penny counts when you are manufacturing cars.

20

Many designs do just that.

And many cars, going back decades, do that. The part is usually called a voltage stabilizer.

While it seems some manufacturers are more concerned than others with designing relatively accurated fuel gauges, it’s just not affordable to make precision fuel gauges for automobiles. Nevertheless, the gauge in any given car should be reasonably consistent.