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Car runs better on full tank of fuel?

So after i fill my car up i notice an increase in performance. Everything just feels smoother.

I have only had a car for 6 months ish but I have never heard of this effect that i am experiencing.

Does anyone know anything about this?

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you car was hungry

My car is a 1.6 diesel and yes I feel the same thing too. Not sure why this is the case.

Reply 3

Bern Herkins

Original post by shawtyb

you car was hungry

I see

Reply 4

Bern Herkins

Original post by BullViagra

My car is a 1.6 diesel and yes I feel the same thing too. Not sure why this is the case.

Mines only a 1.0 Petrol. It doesnt make sense to me. Surely as long as the engine has fuel it should run the same? idk its weird. Hopefully someone can explain

Reply 5

Prince_fancybum

If you're running it low it might be sucking old fuel through, with a full tank it's getting lovely fresh fuel. That's the best I can think of, probably *******s though.

Reply 6

Bern Herkins

I've done a little bit of research and some people suggested that it could all be mental. Perhaps.

I fill up at the last station on my way home and from there on its open country roads. Perhaps when im on a full tank I drive a little different knowing I have an abundance of fuel.
Sometimes when the car goes to the mechanics, when i get it back i feel as if it drives better? I recently had my handbrake and brakes replaced/tinkered with and i thought my car performed better.

Others suggested the fuel filter needs replacing. When i bought the car i had my filter replaced with one the guy gave me for free, it may not have been the best quality.

So a mental effect or a mechanical explanation. I dont know yet

Reply 7

mphysical

Original post by Bern Herkins

So after i fill my car up i notice an increase in performance. Everything just feels smoother..............

In an older car sediment will have built up in the bottom of the tank. if the fuel gets low this sediment can be picked up with the fuel causing misfires.
A fuel filter should stop this if there is one fitted. Some are inline which can be replaced. Some are in the tank.
I would say below a quarter of a tank in an old car can cause this.

Reply 8

XMaramena

....wonder if it's something to do with the increased pressure on the injector. More likely, it's impurities towards the bottom of the tank. You car should actually run better WITHOUT a full tank, as you don't have as much weight to pull.

Reply 9

CurlyBen

It's possible (but unlikely) that you're getting a little more fuel pressure with a full tank, which in turn would suggest your fuel pump is weak.

Reply 10

JC.

Original post by CurlyBen

It's possible (but unlikely) that you're getting a little more fuel pressure with a full tank, which in turn would suggest your fuel pump is weak.

I can't see the additional weight of the fuel having any effect on an injection engine as I don't think gravity will play much of a part.

Two theories...
The far fetched one:

Low fuel level might give rise to a bit more (for want of a better word) cavitation in the swirl pot area of the tank leading to the pump scavengin air rich fuel.
When the fuel dead heads against a closed injector you might see a miniscule drop in pressure as the air is compressable.

More likely theory:

Small crack in the pump body. Pump is drawing a small amount of air when tank is half full but this won't matter with a full tank as the pump will be totally submerged?

Not sure. Seem's like an odd one doesn't it?

Reply 11

Shiv is Light

Original post by Bern Herkins

The way fuel meters works causes it to over estimate when it's full. It's more of a mental thing like someone mentioned earlier.

Reply 12

Jammy Duel

It probably is largely mental, you would expect it to be a fair bit worse after all given there is a load more weight to carry

Reply 13

CurlyBen

Original post by JC.

I can't see the additional weight of the fuel having any effect on an injection engine as I don't think gravity will play much of a part.

If I remember correctly static head for petrol should be in the region of 10psi per metre, so full to empty on an average tank could give you a 5psi variation, if some part of the fuel pressure regulation system wasn't working. That's not insignificant in petrol injection pressure, but I agree it's not too likely you'd see that variation at the fuel rail after going through the pump and the fuel pressure regulator.

Obvious thing to do would be to check the fuel pressure before and after filling the tank and find out if there is actually any difference.

Reply 14

JC.

Original post by CurlyBen

Yes I think that's sound advice. Although, testing for pressure differential won't help located a problem it will at least prove or disprove that one exists or not!

Reply 15

Pegasus2

Just an fyi, petrol does have a shelf life and its octane rating does decrese slowly over time. Whether or not this is enough to be noticible or is placbo i'm not sure. I've noticed my car seems to run better for a short time after new fuel. I'm not sure about diesel but i'd assume it's similar.

If the fuel has come fresh from the refinary, then it's possble you lose a small amount of rating over the following days as it mixes with older fuel.

Very, very far down the line it eventually turns to jelly.

(edited 8 years ago)

Reply 16

Bry061

The more space in a fuel tank as the fuel level drops allows volatile solvent to evaporate out of the liquid fuel.The fuel becomes less volatile as a result.

Reply 17

PTMalewski

Original post by JC.

Original post by Pegasus2

Just an fyi, petrol does have a shelf life and its octane rating does decrese slowly over time.

Actually the low octane rating alone doesn't decrease engine power, unless the engine has knocking combustion sensor and ECU reduces power to protect the engine.
In the old times of carburettors, rally drivers who wanted to boost powers of their engines without illegal modifications, used low-octane fuel. It was killing engines designed for high-octane fuel very quickly, but provided significantly higher power output.
Old gasoline might provide less power, but it happens because of other changes that occur in chemical composition.

Reply 18

IWMTom

Original post by PTMalewski

A fuel pump gives much more pressure that is actually necessary for injectors. The additional pressure is released in the injection system by pressure regulator, and the additional fuel is returned to the tank. Low fuel level should have no effect, unless there's some heavy cornering involved- then engine simply dies down, or it might the fuel itself.

Actually the low octane rating alone doesn't decrease engine power, unless the engine has knocking combustion sensor and ECU reduces power to protect the engine.
In the old times of carburettors, rally drivers who wanted to boost powers of their engines without illegal modifications, used low-octane fuel. It was killing engines designed for high-octane fuel very quickly, but provided significantly higher power output.
Old gasoline might provide less power, but it happens because of other changes that occur in chemical composition.

4 year old thread.

Reply 19

PTMalewski

Original post by IWMTom

4 year old thread.

It's his fault!

Original post by Bry061

The more space in a fuel tank as the fuel level drops allows volatile solvent to evaporate out of the liquid fuel.The fuel becomes less volatile as a result.