Why is my fuel tank getting smaller?

[Moleyman69]

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,

Moley

 

[problematiq]

Might be your gauge/sensor thats going bad. Also, This is a site for Computer hardware.

 

[smorizio]

from age your going to get slug/water on the bottom of tanks. at some point you should have the fuel filter replaced. also put in a few bottles of dry gas to get any water out of the system. also have you replaced the timing belt/chain on the car a car of that age if the belt starts to go the timing will be off and it wont be as fuel effecent. also make sure you do a full tune up. wire/spark plugs air filters. cap and rotter. also have the shop do a tune up and read engine codes if the car has a computer to read them. also make sure your breaks are not dragging.

 

[Martell1977]

I know that there is sediment in the gas and I've heard of it collecting at the bottom of the tank, but never heard of it taking that much room. Even at 11 years old, you would have had to been buying some extremely dirty gas for it to take that much space.

Is it possible you ticked someone off and they put dirt or something in your tank?

 

[InvalidError]

Might be your gauge/sensor thats going bad. Also, This is a site for Computer hardware.

OP is letting the gas station's pump automatically fill his tank to full and his fuel quantity is based on his receipts, not the car computer. That takes the car computer and sensors out of the equation.

I'd guess the "tank shrinkage" could be due to residue, sludge, rust, etc. in the tank, though 8L worth of junk in there sounds like a bit much. Changing the fuel filter (if it has not been changed recently) and looking into the old one to get an idea of what is going through the pump might be a good idea.

 

[bigpinkdragon286]

If you're not consistently using the same fill station, the pump handle's auto shutoff may be calibrated differently. Heck, you can see variations even at the same gas station.

Are you physically verifying the level of the gas in your tank, or just going by the automatic shutoff and your gas gauge? Warm temperatures are enough to affect that.

I suspect 2 gallons worth of sediment would manifest itself as clogged fuel filters. I doubt your sediment in the tank is quite this bad.

Also, while gas pumps are required to be tested, that doesn't preclude going to a station that is cheating on the numbers between tests. I assume it's rare, but it certainly has happened before.

 

[smorizio]

if your gas cap is not a locking one...you can have someone be taking gas out of the car at night or when your not watching it. another trick is someone has another set of keys and is driving the car when your not looking. eay check is use the trip meter and odomter..see if there moved when you get into your car.

 

[problematiq]

Might be your gauge/sensor thats going bad. Also, This is a site for Computer hardware.

OP is letting the gas station's pump automatically fill his tank to full and his fuel quantity is based on his receipts, not the car computer. That takes the car computer and sensors out of the equation.

I'd guess the "tank shrinkage" could be due to residue, sludge, rust, etc. in the tank, though 8L worth of junk in there sounds like a bit much. Changing the fuel filter (if it has not been changed recently) and looking into the old one to get an idea of what is going through the pump might be a good idea.

I understand. But if his sensor is wack it might be telling him he's empty 5 litres sooner than normal.

 

[smorizio]

also check to see if the car has rubber or metal fuel line...could have a small break in the line.

 

[CelicaGT]

It's probable that your fuel vent valve or charcoal canister are plugged or getting plugged, though you should have a code (P0440 or the like) for those IF your daily commute meets the conditions for your vehicle to perform an evap system diagnostic. Though possible I would highly doubt you have collected 8 litres of sludge, this amount would have surely covered the fuel pump/sending unit assembly and not only would you see drivability issues you would never reach "empty" due to the level sensor arm not reaching bottom. In addition if your tank is of the flatter variety even a small dent could definately make 8 litres disappear.

 

[Moleyman69]

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,

Moley

 

[gumbob3]

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,

Moley

A few tips:
1. Driving with your fuel anywhere below 1/4 of a tank left can be hard on your car's fuel pump.
2. It's not good to continue putting gas in once the pump automatically stops it, it can damage your vehicle.

 

[InvalidError]

I have had this whooshing noise happen on all the cars I have had though from memory.

There shouldn't be any "wooshing" in or out on any car since the evaporation vent valve is supposed to be open whenever the engine is off.

If you are filling your tank "to the brim" though, you may be flooding your coal canister and that's preventing it from working correctly: the coal canister is supposed to provide a buffer zone between the tank and outside air to reduce evaporation loss but flooded coal won't let air or gasoline vapors through as easily. The extra flow resistance may also cause the gas station to detect "full" prematurely.

 

[mymutter]

I was also going to say check your breather and gas cap as well. If your car has a plastic tank, easy to see, if it's metal, it still might have a bladder inside. My first thoughts were sender, but it does not sound like that.