Why is my car fuel tank not filling up properly?

November 12, 2007 10:46 AM

Why is my car fuel tank not filling up properly?

I have a 2004 Smart Fortwo and I live in the UK. I get pretty consistent mpg, and I know how many miles it takes before the fuel indicator goes from 5/5 to 4/5 (the readout is 5 little blobs that disappear one by one as fuel is used). However, the last two times I've filled up with petrol, it's taken significantly less (~50-60 miles) distance to use up the first 1/5 of the tank and the overall mileage between fill-ups has been down by the same amount.

The first time this happened I wondered if I'd inadvertently being driving faster than usual, or if there was fuel leak somewhere, but I filled up again the other day and it used up the first 1/5 after 60 miles rather than the standard 120 or so. I also noticed that despite my tank being almostempty (in the region of 3.5 litres left), it only took about 23 litres of petrol until the petrol pump did the auto-cutoff thing (where it clicks and won't dispense any more). Now, my tank is 33 litres, so I would have expected to get at least another 5 litres in before it was "full". I always try an extra couple of squeezes on the pump handle after it cuts off the first time to make sure the tank is full but this didn't seem to change anything.

So, it seems that rather than a leak or a drop in fuel efficiency, the tank is not filling all the way up. What would cause this? Is it possible for an air bubble or something to cause the pump's auto-cutoff to think the tank is full? Is this something I can fix myself? If not, is it something that can wait a while before being fixed?

posted by EndsOfInvention to travel & transportation (10 comments total)

You are missing another possibility: the gauge has gotten off somehow. Given that you're unable to put as many L in as you expect that seems most likely to me.

You need to compute your kilometers per liter not by that little gauge - which uses no real world distances or amounts, only 5 little blobs - but by how many kilometers you have driven since you last filled up divided by the number of liters you add.
posted by phearlez at 11:30 AM on November 12, 2007

Have you switched gas stations?

The gas pump shuts itself down by feeling backpressure, but different gas pumps can be calibrated slightly differently, which would mean that if you've switched to a new station, the pumps may be shutting down earlier than you're used to. The gas gauge would still show "full" as you pulled out of the gas station, but there would be less gas to burn before reaching the 4-blob measurement threshold.
posted by Steven C. Den Beste at 11:59 AM on November 12, 2007

I'm not sure if it has anything to do with your problem, but some fuel indicators are not linear. I drive a small VW, and my fuel indicator has four "quarters" that look the same size on the dashboard, but actually are different in capacity. The first two "quarters" always go down quite slowly, and the latter two go down a lot faster. I think that what is really measured is not the amount of petrol left in the tank, but the height of it. If the tank doesn't have the same cross section over all, the height doesn't go down evenly.

Re: your gauge being off: It sounds like the five blobs are just little lights, so maybe you can get the gauge reset or recalibrated or something like that.
posted by amf at 12:08 PM on November 12, 2007

Do you compute your actually mileage? Next time you fill up reset the trip meter. If your car lacks that write the miles down from the odometer. Then the time after that when you fill up you'll know how many miles you drove on X amount of gas, X being the amount of fuel you added. Fuel gauges themselves aren't always that accurate. My car does the same thing amf's does, where the first "half" of the tank on the gauge is actually more 2/3 of the volume. Anyway, I always calculate my mileage every time I fill up; if my mileage is unexpected I'll check my tires to make sure they're properly inflated and think back on my recent driving to see if I've been doing something different, like excessive short trips.
posted by 6550 at 12:17 PM on November 12, 2007

If you are in the habit of "topping off" your tank, and have a gasoline engine version of your car, you could have caused a problem, by saturating your Evaporative Emission Control Canister. Gasoline tanks shouldn't be filled past the automatic cutoff point of modern gas station pumps. Filling the tank as much as you possibly can leaves insufficient room for expansion of the gasoline liquid as it warms, and the overflow can fill the EECC, which should normally only be tasked with storing gasoline vapor from the tank, when the car is shut off, and there is no engine vacuum to purge it. This kind of problem due to repeated overfill can be exacerbated if you only use the car for short trips, as the engine may never run long enough to fully purge a saturated cannister. With a saturated canister, and a freshly filled tank on a warm day, it might be possible that excessive fuel system pressure has damaged a hose or fuel fitting, or even the fuel pump, simply due to gasoline in your tank heating up from 60° F ground tank storage temp, to 90° F air temp, which would need about an additional 2 liters of volume in your tank. Thus, the warning in most gasoline vehicle owner's manuals since 1970, about not overfilling the fuel tank.

If your EECC is saturated, you might be able to drain it by taking the car for a long drive (100+ miles) at highway speeds, but some kinds of canisters never fully recover if they are fully saturated, as the physical media within them deteriorates if left constantly soaking in liquid gasoline. Your only option then is to replace the EECC, and have the fuel system carefully checked for damage and leaks.

However, as diesel fuel is far less volatile than gasoline, diesel engine versions of the car do not have the same kinds of evaporative emission controls. If your vehicle is diesel, ignore this comment as a possible cause.
posted by paulsc at 12:20 PM on November 12, 2007

Has the temperature dropped off considerably about the same time you noticed this? If so, the pumps could be shutting off at a slightly different point. Also, the actual fuel-gage sending unit might be affected by cold.

Really, though, two time filling up does not a trend make. I used to make a ~250 mile drive every week, in the same vehicle, so I experimented on how much speed, tire pressure, and tailgate up/down (pickup truck) affected my fuel consumption. I'd factor out wind speed and temperature, even using the exact same pumps. Despite all these safeguards, I still had to make a ton of experimental runs to get a good trend - the amount of fuel used varied plus/minus half a gallon, at the extremes. Also, the tank level which made the pump shut off would vary according to how many other people were filling up at the same time, and the fuel level in the underground tanks at the station.

In other words, until this occurs five or six times without fail, I'd not be worried. At the very least, you might have a slightly wonky gage. Ignore the gauge, and fill up at a reasonable mileage interval to make sure you have at least a quarter tank.
posted by notsnot at 12:21 PM on November 12, 2007

You might have a blocked pressure release valve. As you're pumping petrol into the tank, there has to be some way for the air in the tank to escape. If the pressure release valve isn't working, pressure builds up in the tank, causing the pump to stop pumping.

You can test this by dribbling petrol into the tank. That'll allow air to escape through the filler pipe.
posted by veedubya at 12:51 PM on November 12, 2007

I'm with Phearlez: Just because it shows empty on your gauge doesn't mean the tank is actually empty. This would explain why your mileage between full to "empty" is down and why it's taking less fuel at the pump.

Unless there's something wrong with your fuel pump I would bet that if you kept driving your car after it shows empty, you would still be able to drive the remaining distance that you used to be able to between fill-ups. (If you're game, take a can of fuel with you and drive around til you run out of fuel to test this.)

I would get your sender unit and fuel gauge checked out. Tanks don't mysteriously lose x litres capacity unless the tank itself is warped or you have an issue with you fuel pump not being able to reach fuel at the bottom of the tank.

In fact I had a similar problem recently after buying a used car. My tank was 50L in capacity but would only take 40L at the pump even though my gauge was showing empty (not only showing empty but the car would actually run out of fuel if I kept driving it at this point.) Also, when I filled it up, the gauge would only show 3/4 full.

The problem was two-fold: Firstly my fuel pump was not set correctly and it couldn't reach the last 10L. Secondly, the float arm on the sender unit was bent and therefore was not accurately gauging the fuel level.

I doubt it's anything to do with the gas station pump or the act of filling up itself.
posted by Tsar Pushka at 1:59 PM on November 12, 2007

Have you switched gas stations?

No, same one I normally use.

I'm not sure if it has anything to do with your problem, but some fuel indicators are not linear.

Yeah mine's def not linear - when I first got the car I logged the mileage it took to use up each blob and it was very different (but consistent between fill-ups).

Next time you fill up reset the trip meter.

I always reset the trip meter when I fill up and I know that usually, when it gets to 320 miles (ish) the gauge will start it's 5 litre countdown*.

*when the fuel tank gets down to 5 litres the trip meter starts displaying litres left in the tank (in 0.5l increments) instead of the trip meter reading (although you can switch it back to that).

This kind of problem due to repeated overfill can be exacerbated if you only use the car for short trips, as the engine may never run long enough to fully purge a saturated cannister.

Hmm, yeah my car is "gasoline" (petrol) rather than diesel. Not sure if what I do counts as "topping off", but I squeeze the pump a couple more times to make sure it really is full - e.g. if I just squeezed the pump really hard the fuel would come out too fast and it would cut off anyway, even if the tank was empty. So I give it an extra try or two at the end to check that's not what made it stop. It's fairly chilly here at the moment though so I'm not sure the positive temp change you mention would be happening.
My weekday commute is 22 miles each way, and the odd shorter journey at the weekends.

Has the temperature dropped off considerably about the same time you noticed this? If so, the pumps could be shutting off at a slightly different point.

It has got a couple of degrees colder this week.

Someone mentioned tire pressure, I'll check that the next time I fill up. I'll also try the dribbling method in case there's air that needs to escape. Finally, I'll borrow a petrol can from somewhere and try and drive around when the tank's nearly empty to see if the gauge is lying!

Finally:
You need to compute your kilometres per litre not by that little gauge - which uses no real world distances or amounts, only 5 little blobs - but by how many kilometres you have driven since you last filled up divided by the number of liters you add.

I'll make a note of this as well - miles driven before the gauge says "nearly empty" & litres added, then compute the mpg from that.

I guess I didn't think of the gauge being off because the car is a lot newer & more modern than I'm used to (ooh flashy digital fuel gauge!) so I expected a bit more from it :) My first car used to show negative fuel after it reached half-empty...
posted by EndsOfInvention at 2:39 PM on November 12, 2007

If you always reset the trip meter than you can determine your kpl (is that how you folk across the pond abbreviate kilometers per liter?) at any point by filling up. However many K show on the trip meter at that point divided by how many L went in and ta-da. So long as you've driven a reasonable distance more than 3 or 4 K) you'll get a decent measurement from that.

Really, if there was one thing I could snap my fingers and magically cause every person in the world to do every time they fill up, it wouldn't be that - it would be check their tire pressure.

But 2nd or 3rd on the list would be keeping a log of distance traveled and gas added to the tank. Seeing how your mileage is going is one of the easiest ways to keep an eye on your car's condition and it doesn't take anything other than a pen. An alteration from normal is almost always an indicator that something is off, like tire pressure, or broken, like a thermometer.
posted by phearlez at 2:09 PM on November 13, 2007