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Thread: Why is my fuel tank getting smaller?

  1. #1

    Default Why is my fuel tank getting smaller?

    Hi everyone,

    I hope that someone who has some good car knowledge can help me find an answer to this puzzle please.

    I will explain in detail below but I just can't physically put as much fuel in to the fuel tank as I have been able to in the past. This isn't a MPG question; but a full tank capacity question.

    The car in Question is an N16 Nissan Almera 1.5 Petrol 53 plate which I have owned since January 2005 and has been very reliable indeed (the most reliable car I have ever owned).

    The car has a 60 litre fuel tank according to the manual and I can confirm it must be around this size because in the past when I have refuelled, I normally put in around 55/56 litres or so to brim it when I am just above the red mark.

    Historically, on a full tank I have been able to complete 11 round trips to my workplace and home (approx 50 miles round trip); this would leave me just above the red mark and this is when I would refuel with about 55/56 or so litres. Also, when full I would get 2 rounds trips to work before the needle moved down from "above" the Full mark to the Full mark - hope that makes sense.

    Now, over the last few months I began to notice that I was only getting 10 round trips to work and only 1 round trip before the needle moved down to the Full mark instead of the usual 2, which I put down to the cold weather etc etc.

    But over the last 3 or 4 refuels I have noticed that I am getting less fuel in to the tank from the pump. In fact, on Saturday I had run the car so low that the fuel light was on, I was almost at the bottom of the red mark and the fuel computer couldn't even work out how many miles I had remaining; in fact, this was the lowest I had ever run the fuel level down to. So I went and refuelled and completely filled it up to the click, and then a couple of little bits more and all that had gone in to the tank was 51.5 litres. Where has my other 8.5 litres gone? I would understand the odd litre or two, but we are talking nearly 2 gallons here!

    I looked back over the last couple of receipts and they were all in the 52-53 litres range as well. The MPG seems to be about right too.

    It just seems like something has made the fuel tank significantly smaller almost like putting a big dent in it would do or something along those lines. I have had a look under the car - not that I really know what I am looking for - and if what I think is the fuel tank is the fuel tank, then the bottom of it looks fine at least.

    Does anyone have any idea whatsoever that could be causing this?

    Any ideas, advice or tests would be greatly appreciated so that I can take it to the garage which some idea of what to tell them.

    Thanks for taking the time to read this.

    All the best,

    Face
    I am actively seeking employment in the Norfolk/Suffolk area of the UK. I have good IT and communication skills, with 18 years of print experience. If you can help me in any way, please send me a PM.

  2. #2

    Default Re: Why is my fuel tank getting smaller?

    Could be an electrical system problem causing your gauge to read differently as higher or lower voltages can reflect differing readings on analog gauges and a car battery that is getting weak can either register lower voltage or cause the charging system to be operating at higher voltage if it doesn't see the battery as charged up fully. My car has a battery that has developed high self discharge to the point that it is always charging it and charges it heavily upon startup causing my gauges to read differently.
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  3. #3
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by Lynx_Arc View Post
    Could be an electrical system problem causing your gauge to read differently as higher or lower voltages can reflect differing readings on analog gauges and a car battery that is getting weak can either register lower voltage or cause the charging system to be operating at higher voltage if it doesn't see the battery as charged up fully. My car has a battery that has developed high self discharge to the point that it is always charging it and charges it heavily upon startup causing my gauges to read differently.
    Your reply does not match the problem,the op says standing at the pump he is getting less fuel in,the only thing that I can think of could one of the baffles have come adrift and is stopping the normal amount of fuel to go in causing the forecourt pump to switch off(A UK thing this).Please bear in mind not all tanks are fitted with baffles.
    Last edited by mcnair55; 04-25-2016 at 05:00 PM.
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  4. #4

    Default Re: Why is my fuel tank getting smaller?

    Additionally I would consider that:

    The vent tube might have a blockage or restriction that is causing the pressure switch on the pump to trip.

    or

    The float in the tank might have cracked and taken on some fuel so it is reading low?

    Bob

  5. #5

    Default Re: Why is my fuel tank getting smaller?

    Could there be a vacuum created in the tank, that has somehow crushed it a little?
    Thanks, CPF!

  6. #6
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by HorizontalHuntr View Post
    Additionally I would consider that:

    The vent tube might have a blockage or restriction that is causing the pressure switch on the pump to trip.

    or

    The float in the tank might have cracked and taken on some fuel so it is reading low?

    Bob
    I think your vent tube idea could be the answer.
    Diagnosed with Grumpy Old Man Syndrome

  7. #7

    Default Re: Why is my fuel tank getting smaller?

    If you are basing your fuel calculations purely on the gauge and not the distance you get per tank, i would probably say the gauge might be giving you a false reading. The best way to check that would be to drive the distance you used to get per tank and see how far the gauge drops down (you may want to carry some extra gas/petrol in a Jerry can) and then fill it up and see how much petrol is required to fill her up. It is not unusual for the fuel sender unit to get worn out slowly due to normal use and give slightly false readings. The float (fuel sender unit) has electrical contacts on the other end which shows how much fuel you have on the gauge, and there's multiple little contact points so that the needle doesn't jump from full to half but rather gradually go down. If 1 or 2 of those contacts were to wear out, the needle could go down from 1/4 tank to almost empty and you might think you are almost out of fuel, where as in reality, you might have more than what you think you do. I hope that makes some kind of sense without actually showing you a fuel pump unit.

  8. #8
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by rishabharies View Post
    If you are basing your fuel calculations purely on the gauge and not the distance you get per tank, i would probably say the gauge might be giving you a false reading. The best way to check that would be to drive the distance you used to get per tank and see how far the gauge drops down (you may want to carry some extra gas/petrol in a Jerry can) and then fill it up and see how much petrol is required to fill her up. It is not unusual for the fuel sender unit to get worn out slowly due to normal use and give slightly false readings. The float (fuel sender unit) has electrical contacts on the other end which shows how much fuel you have on the gauge, and there's multiple little contact points so that the needle doesn't jump from full to half but rather gradually go down. If 1 or 2 of those contacts were to wear out, the needle could go down from 1/4 tank to almost empty and you might think you are almost out of fuel, where as in reality, you might have more than what you think you do. I hope that makes some kind of sense without actually showing you a fuel pump unit.
    He cannot get the same amount of fuel in the tank nothing to do with gauge and distance.
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  9. #9
    *Flashaholic* Str8stroke's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    I am going with fuel float too.

    If you have a vacuum seal issue, this should trigger the Check Engine light in most modern cars. IE: Loose fuel cap.

    Is the Check Engine light on??
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  10. #10
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by Str8stroke View Post
    I am going with fuel float too.

    If you have a vacuum seal issue, this should trigger the Check Engine light in most modern cars. IE: Loose fuel cap.

    Is the Check Engine light on??
    You have me confused,the op cannot get the same amount of fuel in his tank as normal,he is standing outside his vehicle so engine lights,fuel gauges makes no difference.Something is stopping the normal amount of fuel going in as if he has a smaller tank.
    Diagnosed with Grumpy Old Man Syndrome

  11. #11
    *Flashaholic* Str8stroke's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    mcnair55, lol Probably not you, maybe I didn't read it correctly? I got the impression he was gauging (what a pun) the amount of fuel the tank "needed" based off the reading from the fuel gauge. SO, if I interpreted him correctly (and likely didn't) then the gauge has a faulty reading.

    I said the vac part because usually a check engine light is present when there is a air leak.
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  12. #12
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    Default Re: Why is my fuel tank getting smaller?

    OP- Do you always use the same filling station?

  13. #13

    Default Re: Why is my fuel tank getting smaller?

    Only mentioning this because it is possible, but in this instance doesn't sound likely. Old gas solidifies, turns into a kind of gel and sticks to the bottom of the tank. You can't get rid of it, must replace the tank. When a car sits too long, this can happen, which is why it sounds unlikely in your case... doesn't sound as though you had old gas sitting long enough to turn to gel. But this could explain the symptoms of a shrinking gas tank.

  14. #14

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    Default Re: Why is my fuel tank getting smaller?

    Since you're going by the gauge to determine when the tank is empty, then that's very likely the problem. If you were to run it dry twice, I bet it'll require the same number of gallons to fill it both times.

    The electrical system or float are the two most likely causes.

  15. #15

    Default Re: Why is my fuel tank getting smaller?

    The symptoms lead me to believe the vent is plugged and the tank has started to collapse due to vacuum. Face, what does it sound like when you remove the filler cap, air rushing in?

    The area of the tank sustaining damage might be at the top, out of sight.

    ~ Chance
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  16. #16
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by just like me View Post
    Only mentioning this because it is possible, but in this instance doesn't sound likely. Old gas solidifies, turns into a kind of gel and sticks to the bottom of the tank. You can't get rid of it, must replace the tank. When a car sits too long, this can happen, which is why it sounds unlikely in your case... doesn't sound as though you had old gas sitting long enough to turn to gel. But this could explain the symptoms of a shrinking gas tank.
    It would need to sit a real long time for that to happen I can assure you,most tank bad running problems are caused by condensation in the tank which today can be sorted by an additive that turns the water based yellowish gooey mess into combustible fuel.
    Diagnosed with Grumpy Old Man Syndrome

  17. #17
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by Chauncey Gardiner View Post
    The symptoms lead me to believe the vent is plugged and the tank has started to collapse due to vacuum. Face, what does it sound like when you remove the filler cap, air rushing in?

    The area of the tank sustaining damage might be at the top, out of sight.

    ~ Chance
    I agree a venting problem like a previous poster wrote and many fuel tanks are plastic based these days.
    Diagnosed with Grumpy Old Man Syndrome

  18. #18

    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by Grizzman View Post
    Since you're going by the gauge to determine when the tank is empty, then that's very likely the problem. If you were to run it dry twice, I bet it'll require the same number of gallons to fill it both times.

    The electrical system or float are the two most likely causes.
    You hit the nail on the head. Most likely culprit is the gauge isn't reading right, and causing him to think he has less gas than he actually does, which is why he feels like he can't get as much gas in as he used to.

    I would run it dry (keeping a spare gallon in the trunk to get me to a gas station) and fill it up. Then, do it again. I bet OP can put 60 liters in it both times.

    edited to add: if it is a venting issue (which I've had with my GMC Yukon, but I doubt is the problem on OP's vehicle), try slowing down the rate at which you're pumping gas. This should allow the system to vent and prevent the pump from shutting off early.

  19. #19

    Default Re: Why is my fuel tank getting smaller?

    I can't see how a fuel tank in a modern auto could collapse and shrink in size so in my mind the size of the fuel tank remains the same. To me that means that the sending unit (containing the float) is under reporting the fuel level or the vent is obstructed to some degree causing the pressure switch in the fuel pump to trip prematurely.

    I would check the vent and if that is ok I would drop the tank and replace the sending unit.

    Bob

  20. #20

    Default Re: Why is my fuel tank getting smaller?

    ^^^^ it happens.

    Re "drop the tank": Try looking under the back seat for a access plate for the fuel pump/filter inside the tank.
    Thanks, CPF!

  21. #21

    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by cland72 View Post
    edited to add: if it is a venting issue (which I've had with my GMC Yukon, but I doubt is the problem on OP's vehicle), try slowing down the rate at which you're pumping gas. This should allow the system to vent and prevent the pump from shutting off early.
    Yes! I can get an "extra" 1-2 gallons in my Jeep Grand Cherokee by doing this. If I'm really wanting to stuff it, I back the nozzle out just 1" at the most and lock the trigger on the slowest setting. I overflowed it once doing this, so I rarely do it anymore. Slow is the key. No matter if the tanks is empty or 3/4 full, I can almost never pull the trigger all the way...it shuts off immediately.
    GOOD TINT!

  22. #22

    Default Re: Why is my fuel tank getting smaller?

    http://www.automoblog.net/2011/08/15/topping-off-learn-why-you-should-never-top-off-your-gas-tank/

    Please check your owner's manual, and look at the placards in the glove box or fuel filler area. You may find manufacturer recommendations re: topping off. Topping off most modern cars can result in damage to the evaporative system and it often results in not being able to fill the tank easily. It's illegal to top off in some states.

    Modern service station pumps do not allow "topping off".

    Note: none of this may apply to the case of "false clicks", probably what is happening to the Jeep above.

    Seems like life gets more complicated every day.
    Last edited by Ladd; 04-26-2016 at 10:29 AM.
    Thanks, CPF!

  23. #23

    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by Ladd View Post
    ^^^^ it happens.

    Re "drop the tank": Try looking under the back seat for a access plate for the fuel pump/filter inside the tank.
    Good to know. I have always owned pickups so it was never a thought. I bet that it would be a pisser to drop the tank only to look up and see and access plate.

    Learn something new every day here. Thanks Ladd.

    Bob

  24. #24

    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by scout24 View Post
    OP- Do you always use the same filling station?
    I wondered that sorta thing too.

    Here in the States there is a sticker on filling pumps showing the last time the metering system was checked by the government. Supposed to be checked once a year. Better filling stations check theirs every quarter year.

    If you want to do your own check take some small engine fuel cans that have been marked by you at the 1 gallon (or whatever similar metric size.

    I've got my weedeater (yard trimmer) tank marked at the point filling it with bottled water showed 1 gallon. I did it not as an accuracy check because I thought somebody was stealing, but because I wanted to ensure accurate mixture of oil and gas for the 2 cycle engine. But it doubles as an assurance the metering system of said filling station.
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  25. #25
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    Default Re: Why is my fuel tank getting smaller?

    The float in the tank may be cracked and some fuel have gotten into it, making the float not floating anymore.
    Will give you a false reading on the meter as it sits below the surface of the fuel and making you believe the tank is empty. But still high enough to show full tank since the top level in the tank normally is a bit above full on the meter.

    As said before, bring a gallon or two in the trunk and run it dry. Then fill it up and go from there.

  26. #26

    Default Re: Why is my fuel tank getting smaller?

    If you are going to run it dry a new fuel filter would be a wise move. All that stuff in the bottom of the tank that never reaches the fuel line because you never run it dry will get in your filter.

    May not be an issue. But it's probably time to change it anyway.
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  27. #27
    Flashaholic* mcnair55's Avatar
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    Default Re: Why is my fuel tank getting smaller?

    I had a word today with a mechanic customer of mine and his first thought is the tank vent is blocked not allowing the tank to breathe properly therefore creating vacuum and pressure problems.
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  28. #28
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    Default Why is my fuel tank getting smaller?

    This is a fun puzzle! The most likely answer is that there is a problem with the tank sender unit or (less likely) the gas gauge itself, such that the gauge is under-reporting the amount of fuel in the tank. When you go to fill the tank, thinking it's empty, less fuel can be added because there's actually more in there than you expect. In simple terms, the sender unit uses a resistor mechanism to vary the amount of voltage going to the gauge (moving the needle) based upon the position of the float in the tank. The sender unit can be degraded over time due to corrosion, gunk, etc. which slightly changes the resistance from factory spec, and thus affects the accuracy of the gas gauge. Since it's only off by a couple of gallons, and it's off in the conservative direction that means you have an extra reserve, I wouldn't go through the trouble of dropping the tank (messy, smelly, hard work) unless it gets worse. I certainly wouldn't run the tank to empty (edit: meaning actually DRY such that the car stops running), as you can damage the fuel pump that way, or at best clog up the fuel filter... Which on most cars is isn't fun to change on the side of the road.

    I base my answer above on years of shade-tree mechanic experience, but I've never owned your make and model. A Nissan specialist will likely be able to say, "yeah, I see that problem a lot on those, and replacing X almost always fixes it..."
    Last edited by ronniepudding; 04-26-2016 at 09:03 PM.

  29. #29

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    Default Re: Why is my fuel tank getting smaller?

    Don't forget that neither the engine nor the fuel pump need to be running in order to empty the tank.

  30. #30

    Default Re: Why is my fuel tank getting smaller?

    Quote Originally Posted by mcnair55 View Post
    Your reply does not match the problem,the op says standing at the pump he is getting less fuel in,the only thing that I can think of could one of the baffles have come adrift and is stopping the normal amount of fuel to go in causing the forecourt pump to switch off(A UK thing this).Please bear in mind not all tanks are fitted with baffles.
    Unless he is draining the tank completely dry (engine dies) then he is going by the fuel gauge to begin with and if the gauge is telling him different than it used to then he will be filling it based upon that. To put it short he may think the tank is almost empty 1/4 full when it may have more gas than that in it due to the gauge registering differently for many reasons. If 1/4 of a tank is actually 1/3 of a tank now then instead of putting 3/4 of a tank in gas in it you can only get 2/3 of a tank in.
    Last edited by Lynx_Arc; 04-27-2016 at 11:22 AM.
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