more powerful with a full gas tank

[Smiling]

I found that after I fill up the car with a full tank of gas it drives (for a quarter of a tank then it gets usual) a bit more responsively - accelerates faster, feels like it gets more power somehow.

I also noticed that when I remove the gas cap I would hear a sound of air rushing in, and a bit of googling get me that it could be the issue with mass air flow sensor.

Any ideas of what it could be?

[DJKOR]

I found that after I fill up the car with a full tank of gas it drives (for a quarter of a tank then it gets usual) a bit more responsively - accelerates faster, feels like it gets more power somehow.

I would hate to say, but if you are feeling this immediately after filling up, what you are feeling is most likely the placebo effect. Unless you are noticing a difference in feel due to slightly more weight being over the rear wheels causing less squat on acceleration, thus making it feel like something is different.

I also noticed that when I remove the gas cap I would hear a sound of air rushing in, and a bit of googling get me that it could be the issue with mass air flow sensor.

That is actually air rushing out. Petrol is volatile. For example, (hypothetically) if you were to pour petrol onto the ground in one spot, then pour an equal amount of water on another, the petrol will evaporate faster. I would hope that you would know that already because if not, I'm not sure what they taught you in school, and that may be concerning since I'm sure most school kids go through the metho on the back of the hand lesson on evaporation.

Anyways, when you have fuel in your tank, the lower boiling point of fuel means that part of it will become a gas and pressurise itself in your fuel tank. I won't go into the dynamics of that one because there is no need to. If you want more information, research "vapour pressure".

When you open your fuel tank, you are releasing this evaporated fuel into the atmosphere, hence the hissing sound. It is nothing wrong with your car, it is just basic physics.

[Silver-Aurion]

Every time I open my fuel cap mine always hissing loudly. So I always open it kind of slowly to let the pressure out. I'm surprised people don't know why this happens and how. I remember when I was kid and every time dad would fill up this noise would happen.

Also the car feeling faster, talk about placebo effect. I pretend that the car feels faster too as soon as I drive out of the petrol station.

[Hiro]

Probably the reason why the car feels faster is that you're no longer driving around like a granny hawking the fuel gauge to get every last kilometre out of the tank before you fill up....then, once you've filled up, you're worry-free and can floor it with glee.

[Kenpo]

Probably the reason why the car feels faster is that you're no longer driving around like a granny hawking the fuel gauge to get every last kilometre out of the tank before you fill up....then, once you've filled up, you're worry-free and can floor it with glee.

Man this is so true..

I drove 53km last week from when the fuel warning light turned on till I filled up, somehow only managed to get in 61L

Edit: Forgot to mention that I drive like a granny when the light comes on.. and like a P plater after I fill up ;)

[Topdog]

All modern cars have a fully sealed fuel system to prevent fumes from entering the atmosphere, the hissing and bubbling when you open the cap the hissing and bubbling is a consequence of this, especially if you overfill the tank(like I do :)). If you overfill I think it causes an airlock in the fuel system. It is not dangerous or detrimental to the. cars operation, although I have a problem with one car I was driving(belonged to someone else)where is was coughing and spluttering. Cured it by removing and replacing the fuel cap..Some sort of airlock I assume and once again most likely caused when I refueled the car and filled it to the top of the filler.

I still overfill but when I get home I just loosen the cap and let the air in..

As to your first comment re "more powerful with a full tank", I think you're standing too close to the filler when you fill up and sniffing a little too many fumes...:) :)

[Jordo2]

Just a thought.... Mabey the weight of a full tank of fuel pushes harder against an allready blocked fuel filter, thus allowing the motor to get the correct amount of fuel until the weight drops off as the fuel burns away.

[Hiro]

Just a thought.... Mabey the weight of a full tank of fuel pushes harder against an allready blocked fuel filter, thus allowing the motor to get the correct amount of fuel until the weight drops off as the fuel burns away.

Except that cars don't have gravity-feed fuel systems, and the fuel pump provides much more suction than a full tank would provide pressure.

[Smiling]

I found that after I fill up the car with a full tank of gas it drives (for a quarter of a tank then it gets usual) a bit more responsively - accelerates faster, feels like it gets more power somehow.

I would hate to say, but if you are feeling this immediately after filling up, what you are feeling is most likely the placebo effect. Unless you are noticing a difference in feel due to slightly more weight being over the rear wheels causing less squat on acceleration, thus making it feel like something is different.

You could be right about placebo effect. Went to my usual petrol station yesterday with a full tank, stopped there for a couple of minutes, and drove off the same way I usually drive from the gas station. The car was accelerating a bit quicker than just a minutes before... or so it felt on a long empty highway (just off the station) compared to the usual 40k limit neighborhood streets.

I also noticed that when I remove the gas cap I would hear a sound of air rushing in, and a bit of googling get me that it could be the issue with mass air flow sensor.

That is actually air rushing out. Petrol is volatile. For example, (hypothetically) if you were to pour petrol onto the ground in one spot, then pour an equal amount of water on another, the petrol will evaporate faster. I would hope that you would know that already because if not, I'm not sure what they taught you in school, and that may be concerning since I'm sure most school kids go through the metho on the back of the hand lesson on evaporation.

Anyways, when you have fuel in your tank, the lower boiling point of fuel means that part of it will become a gas and pressurise itself in your fuel tank. I won't go into the dynamics of that one because there is no need to. If you want more information, research "vapour pressure".

When you open your fuel tank, you are releasing this evaporated fuel into the atmosphere, hence the hissing sound. It is nothing wrong with your car, it is just basic physics.

My physics is a bit rusty and I don't know much about how the car works (a time to start learning perhaps) - but I though if the fuel somehow goes out of the tank into the engine through the fuel system, that should be creating a bit of vacuum in the tank.

[riversandlakes]

OP has a good point. Could very well be vacuum causing air to rush in.

But the next time you open that cap feel for the air rushing out. Hence it's as above posters said - evaporated petrol...

Yet to go on full tank since taking delivery about 18 days ago :-S Fuel card still on its way...

[DJKOR]

OP has a good point. Could very well be vacuum causing air to rush in.

I would more inclined to say that it isn't caused by negative pressure inside the tank because in the sealed system of the fuel tank, the vapour pressure will maintain either an equilibrium or positive pressure.

[Hiro]

OP has a good point. Could very well be vacuum causing air to rush in.

I would more inclined to say that it isn't caused by negative pressure inside the tank because in the sealed system of the fuel tank, the vapour pressure will maintain either an equilibrium or positive pressure.

Fuel caps normally have a one-way valve in them anyway, so they draw in air as fuel is consumed to stop vacuum forming in the tank. Positive pressure is more likely to occur due to heating of the tank (either due to proximity to exhaust pipe or simply ambient conditions) which causes the gases in the tank (either vapours or air) to expand, pressurising the tank - this pressure is relieved when you crack the fuel cap (the one-way valve is designed so that it doesn't release vapours into the atmosphere when the cap is closed).

[Martinj]

According to US magazine "New Scientist" July 2010 edition: "Myth or fact? Cars go zoom after gas refills. Scientists at the Group of 3 consortium (including Chrysler, Ford and GM) have concluded that car performance increases by on average 2-3% on a full tank of gas compared to a quarter tank or less. The reported cause is increased fuel feed and reduced vacuum effect in gas tanks".

As for the whooshing sound? I have no idea what you are talking about there.

[Hiro]

According to US magazine "New Scientist" July 2010 edition: "Myth or fact? Cars go zoom after gas refills. Scientists at the Group of 3 consortium (including Chrysler, Ford and GM) have concluded that car performance increases by on average 2-3% on a full tank of gas compared to a quarter tank or less. The reported cause is increased fuel feed and reduced vacuum effect in gas tanks".

As for the whooshing sound? I have no idea what you are talking about there.

I choose not to believe that statement for several reasons:

1) New Scientist is British, not American (my dad has been a loyal subscriber for over 10 years)

2) There is no "July 2010" edition, New Scientist is a weekly publication (see above)

3) You can search New Scientist articles on their website. No results come up for the above topic, especially not in July 2010

4) If they actually happened, the tests were conducted by the manufacturers, not an independent third party, thus results could be biased

5) The only increase to fuel feed you could see would be due to hydrostatic pressure....except that the hydrostatic pressure you'd get at the bottom of a 50cm-deep fuel tank (most tanks are flat these days) would be about 0.5PSI (water is about 0.433 psi/foot of height, petrol is about 0.75 relative density). Typical 4AGE fuel pump puts out about 75 psi, regulated at the rail to about 35psi. Thus, 0.5psi of hydrostatic pressure does **** all to a pump which produces almost 200% of required pressure.

[DJKOR]

^ Go Hiro. I can always count on quality replies from you.

[Martinj]

Hiro, you can choose to believe (or not believe) whatever you want. That's what makes Australia such an appealing place for people to come to live in. I choose to believe that the Toyota Aurion is not a boring, front wheel drive fridge on wheels.

[Hiro]

Hiro, you can choose to believe (or not believe) whatever you want. That's what makes Australia such an appealing place for people to come to live in. I choose to believe that the Toyota Aurion is not a boring, front wheel drive fridge on wheels.

And you choose to believe something that looks like it came from a chain email? To me, individual research beats blind "it was posted on the internetz" hands down.