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Why is my car's fuel tank getting smaller?
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Matthew Attoe
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Apr 25, 2016, 03:57 PM
 
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,

Matthew
Early 2008 Mac Pro (8 x 2.8), original Core Duo 2.0GHz MacBook Pro
     
andi*pandi
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Apr 25, 2016, 04:17 PM
 
the car guys will be along shortly... but I'm wondering if you have other stuff in your tank, ie, water, rocks, rust... ever any issue starting up?
     
osiris
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Apr 25, 2016, 04:22 PM
 
Interesting situation - but sorry I don't believe I can really help.
Is there a chance that something is in the tank occupying the space? Perhaps debris, plastic, or a dead animal....
I am puzzled as well, losing physical space is quite odd.

edit: andi*pandi beat me to it lol
"Faster, faster! 'Till the thrill of speed overcomes the fear of death." - HST
     
reader50
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Apr 25, 2016, 04:37 PM
 
It could be missing space, although 2 gallons of missing space is a stretch. That'd be one major dent. The drug boys may have placed a package in there, but they'd have retrieved it after you crossed the border.

Most likely explanation: the float which measures your fuel level has logged down a bit. Making it register lower than usual. Test this by taking a filled gas can along with you, and let the car go empty for real, until the engine dies. The 3-4 liters in the gas can will be enough to reach a fuel station.

If it is an under-registering float, you can have that changed without too much trouble.
     
turtle777
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Apr 25, 2016, 04:42 PM
 
^^^^

What he said. The float seems to register the tank empty with more gas in it than before.
That's why it fits less pumped gas, and the car gets less trips out of it before it goes red.

It's just measuring differently.

-t
     
Matthew Attoe  (op)
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Apr 25, 2016, 05:12 PM
 
Thank you all for taking the time to reply.

I don't think I have anything extra in the tank. I'm not sure how it would get in. In the part of England where I live we don't have too much of a drug problem :-)

Would the float trigger the fuel low warning light on the dash or is the fuel light triggered by another sensor? Also, is the float used by the Fuel computer to calculate how many miles are left or does that use another method?

I would love it to be something simple like the float if it is easily replaced by a mechanic.

Many thanks again,

Matthew
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Laminar
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Apr 25, 2016, 05:15 PM
 
A typical fuel level sensor has an arm that wipes a series of contacts and the resulting resistance tells the computer how much fuel is left.



Each of those little hash marks is a different resistance value. This assembly is submerged in gasoline in the tank. If something happens to the circuit that affects its resistance, like degrading component quality or poor a electrical connection, the computer sees an incorrect resistance value and reports an incorrect fuel level.
     
reader50
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Apr 25, 2016, 05:29 PM
 
The above pic shows a fuel level module. The float arm is the metal rod going off-screen at the bottom. If the pic were zoomed back, you could see the float.

All fuel-level readings depend on this module. So yes, a float going bad will throw off all those readings. It could also be the pickup contacts shown in the pic. Since the module is replaced as a unit, it doesn't matter if the problem is the float or the pickup contacts.

The module inserts into the tank through an access hole. Most likely they'll have to drain and lower the tank to change the module. So changing it won't be trivial, but it's not a major job either.

note: the fuel pump is sometimes part of the same module. If so, it will cost more to replace. Still not a major job though.
     
MisterEqual
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Apr 26, 2016, 03:01 AM
 
Possible that your gas tank is not venting as it should and therefore you're unable to fill with liquid as too much gasoline vapor is stuck in the tank.
     
P
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Apr 26, 2016, 04:26 AM
 
Originally Posted by reader50 View Post
All fuel-level readings depend on this module. So yes, a float going bad will throw off all those readings.
This. Extra sensors are expensive. Another light on the dash is cheap and more software in the trip computer is effectively free - they all go off the same sensor. Either you replace that sensor, or you just learn that you can go a little further before refueling.
The new Mac Pro has up to 30 MB of cache inside the processor itself. That's more than the HD in my first Mac. Somehow I'm still running out of space.
     
Matthew Attoe  (op)
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Apr 26, 2016, 03:31 PM
 
Hi again, everyone.

Thank you all so much for taking the time to reply with your ideas and observations.

The most common ideas so far seems to be that I may have a blocked breather pipe which has caused the fuel tank to inwardly buckle and therefore reduce its size. How would I check if this was the case? Sometimes (not always, but more often than not), when I take off the filler cap I will get a whooshing noise but I don't know if this is air rushing in to the tank or pressure from inside the tank venting outwards - when it happens though there is a strong smell of petrol which may mean it is pressure from inside coming out bringing petrol fumes with it, maybe? I have had this whooshing noise happen on all the cars I have had though from memory. The fuel filler cap is the same on that has been on the car since I have had it (from 9 months old) and I cannot see any holes or vents on the cap itself which would allow it to breathe. However on the rubber seal that fits around the outside of the fuel filler "hole" where the fuel nozzle goes there is a very small hole (maybe 1-2mm across) which was blocked but I cleared it a couple of weeks ago when I noticed it. I have still had the whooshing noise though since this hole has been cleared (but I have also not had it since clearing it as well, so don't know if this small hole is even relevant or not).

The other common thought is the fuel gauge is now worn out or out of whack.

Thank again everyone - I really appreciate it.

All the best,

Matthew
Early 2008 Mac Pro (8 x 2.8), original Core Duo 2.0GHz MacBook Pro
     
turtle777
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Apr 26, 2016, 09:49 PM
 
Originally Posted by Matthew Attoe View Post
Hi again, everyone.

Thank you all so much for taking the time to reply with your ideas and observations.

The most common ideas so far seems to be that I may have a blocked breather pipe which has caused the fuel tank to inwardly buckle and therefore reduce its size.
Seriously ? Did you just ignore 90% of the posts in this thread ?
This was NOT a common 'idea'.

-t
     
Matthew Attoe  (op)
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Apr 27, 2016, 01:35 AM
 
hi Turtle,

Thanks for replying.

Sorry, I didn't mean to give that impression; I had also posted in half a dozen other motoring related sub forums with the same question, as I had no idea that I would receive such an overwhelming response here. Between all the replies I had received it was pretty much an even split split between fuel sensor and blocked breather pipe. With hindsight, perhaps I should have listed the fuel sensor first and then the breather pipe.

Sorry for the confusion and apologies for any offence I may have inadvertently caused you; that was not my intention at all.

Thanks again,

Matthew
Early 2008 Mac Pro (8 x 2.8), original Core Duo 2.0GHz MacBook Pro
     
turtle777
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Apr 27, 2016, 08:30 AM
 
Ah, ok, makes sense.

-t
     
ghporter
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May 5, 2016, 06:37 PM
 
Your fuel gauge sensor, not the gauge itself. The car is 11 years old, so it's not a huge stretch of the imagination for the sensor to have a problem.

Now, is the sensor borked, or is it blocked? Most likely it's borked, or you'd get the opposite indication: the gauge would say 1/4 full and you'd run out of fuel.

Either way, the repair isn't complex, but it's not something most people can do easily at home. And it's probably worth your time to go to a shop with a good reputation and tell them what you're seeing - not OUR conclusions - so they can troubleshoot it professionally. There could be other factors that we non-experts in your car haven't considered. Just don't be surprised when they say "it's the floaty gizmo that tells the dashboard how much petrol is in the tank."

Glenn -----OTR/L, MOT, Tx
     
   
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