Why is my fuel tank getting smaller?

markr6

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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.
 

Ladd

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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.:shrug:
 
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HorizontalHunter

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^^^^ 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.:laughing:

Learn something new every day here. Thanks Ladd.

Bob
 

bykfixer

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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.
 

Pegaso

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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.
 

bykfixer

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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.
 

mcnair55

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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.
 

ronniepudding

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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..."
 
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Lynx_Arc

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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.
 
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Poppy

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I agree with all those who said it is most likely the sender or the gauge.
The sender is like a rheostat (a variable resistor that increases resistance as the float drops in the tank). The increased resistance, reduces the voltage to the gauge as the fuel level drops.

If you want to trouble shoot the sender, you can check its resistance values when full, and compare it to specs.

It is quite possible/probable that the sender has increased resistance over the years, it is wearing out.
You might also have a crappy (high resistance) connection to the sender. Either case will give you a lower gas level reading than what you actually have in the tank.

-------------------------------------------------
OK.... now time for a story, if you have time to read it, OR care to :)

I had a 32' A class motor home, that we used, to go cross country. That was our ONE big trip. We then used it each year to be a class A limousine, to take the family to Disneyland in Florida, where we didn't stay in the RV, but rather in a hotel.

So... one year I bring it home the day before we are ready to leave, to clean it up, and load it up. Sanitize the water holding tank, etc.

It has 3/4 tank. IIRC 70 gallon tank.

I try to start the generator, and it won't start... damn, that's important or we won't have roof air conditioning, and the cab air isn't enough on a hot day.
OK... so it starts on carb cleaner. Oh look... squirrels chewed on the fuel line to the generator! Got a new fuel line, but it still wasn't getting fuel. Maybe there is an in-tank fuel pump for the gennie. IDK, but the decision is made to repair it some-day while we are down there.

I figured... let me top off the tank, it should only take about 15-18 gallons, but let's do it and then go.

Well AT $2 a gallon, I started watching the pump at $30 ... $35 ... $40... $45!?? $50 ??? WTF??? $90
Sun of a gun! I was almost out of gas!
The generator won't drain the tank below 1/4 tank. That is why it wouldn't start.
With a full tank... the generator started right up.

I remember being really surprised, during the last stretch home during our last trip, that I was getting amazing mileage.

Yeah, what happened is that the wire from the sender was also chewed by those furry tailed rats, and snapped at 3/4 tank.

So here's the deal.
The gauge will typically stay where it was when you disconnect it/break the sender wire.
IF you short it, it will go to FULL tank.
I don't know what the resistance should be for EMPTY. It may vary with the gauge.
 
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Ladd

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I was wondering the same thing. Be nice if the OP could update......
 

Face

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Hi everyone,

Sorry for not getting back to you sooner, but I had forgotten that I had posted in this forum; I had posted in quite a few out of desperation :)

Since I posted the issue had persisted with the appearance of a smaller fuel tank.

However, with the last few refuellings, amazingly, everything seems to be back to normal and as it was before the mystery started.

I have no clue as to the answer to the puzzle, but I have noticed that I no longer get the "rushing" sound when I release the fuel cap on the last couple of fuellings.

Thanks once again to you all for taking the time to reply and please accept my apologies for the delay in replying to you all.

Enjoy the rest of your weekend.

Face
 

moldyoldy

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Hi everyone,

Sorry for not getting back to you sooner, but I had forgotten that I had posted in this forum; I had posted in quite a few out of desperation :)

Since I posted the issue had persisted with the appearance of a smaller fuel tank.

However, with the last few refuellings, amazingly, everything seems to be back to normal and as it was before the mystery started.

I have no clue as to the answer to the puzzle, but I have noticed that I no longer get the "rushing" sound when I release the fuel cap on the last couple of fuellings.

Thanks once again to you all for taking the time to reply and please accept my apologies for the delay in replying to you all.

Enjoy the rest of your weekend.

Face

As is usual, when a person does not understand problem, there are usually two or more problems...

1. The 'rushing sound' is an important clue. Actually, I had somewhat the same 'rushing sound' on my 1998 Honda Civic 1.6L manual xmsn, 50+ mpg highway, 40+ mixed, 220K miles at time of sale. still with original clutch.

In my case, the 'rushing' was an outflow of fuel vapors from the gas tank immediately upon opening the gas cap, soon after the engine had been switched off. meaning that the tank was pressurized even after the engine was switched off, at least overpressurized for a while. After being parked overnight, there was no over-pressure in the fuel tank. This overpressurization occurred most of the time when the ambient temp changed sharply during the course of driving the vehicle or the vehicle had been driven long distances. The excessive over-pressurization is not a correct condition.

The EVAP system is supposed to control excess fuel vapors by directing the fuel vapors from the gas tank to the ORVR canister in the engine compartment. The ORVR canister is filled with activated carbon. The trapped fuel vapors are then released into the intake manifold to be burned in the engine.

In the case of my 1998 Civic, the activated carbon in the filter was collapsing as well as the tubing between the gas tank and ORVR canister was partially blocked (unknown why). Which is why the overpressure slowly released over time w/o opening the gas cap.

Why did the OPs situation improve? another poster commented that old petrol will congeal on various surfaces. That would fit the situation with the activated carbon filter or tubing leading to the OR.

2. Ref fuel gauge questions: There are too many fuel gauge types to take a remote guess at this. Here is a short simple tutorial on fuel gauges.
 
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