macdady

Has anyone heard that topping off ur gas tank is bad for the car? Two Gas stations here in NJ told me they won't top off my tank b/c it some how effects the fuel injectors. I say its a bunch of BS. I cant see how thats possible.

Also, is it bad to let the tank run low, say a 1/2 a gallon before filling up? I've heard people say its not good. Other then running out of gas in the middle of the road, can it lead to problems?

artinmoTTion

I've heard the same thing about topping off from my Audi techs at the dealer. From what they said, I gathered that it was a sensor issue, and that regular topping off could foul the sensor in time. Not sure about the injectors.

Depending on the design of the gas tank and fuel pump, I have heard that it's the gas in the tank that keeps the fuel pump cool, and so if there isn't gas around it to help keep it cool, the pump runs hotter and may be at risk of premature failure from that. Not knowing much about our "fuel sending units" in the TT, I can't tell you if their design is predisposed to the same problem.

Still, probably wise not to top off, and not to chronically let it run low...

Rich

blackfnTTruck

Letting it run too low, I would think could bring in any junk at the bottom of the tank. It also leaves more surface area for condensation during those temperature changing months.

kodeTT

and overfilled the tank. to make a long story short...i suppose there wasn't enough breathing room in the tank, which made the allroad sputter and die on me a few times. i drove it, it died. i drove it, it sputtered, then died. did it a few times and eventually, it cleared up and the car was running fine to get back on the interstate. never had that happen to me.

TTschwing

There is apparently a certain amout of air space necessary for the proper operation of the tank emissions capture and ventilation system. My daughter always has a hard time starting her Mazda 3 when she squeezes the last drop into it. Triggered a CEL twice now. Dealer relayed this cause and affect...

artinmoTTion

That somehow affected a sensor in my B5 S4's gas tank, requiring REPLACEMENT of the gas tank, and thus removal of the whole rear axle!! Good thing AoA picked up the tab on that one...all for another Audi sensor.

I don't top off my tanks anymore....

Rich

OldSchool AG

the "running low" thing is definitely a myth.... the pickup is *always* at the bottom of the tank

msauve

Think about it - the gas pickup is at the bottom of the tank, so it's contantly picking up any "junk" at the bottom of the tank, regardless of the gas level. Unless we're talking about _floating_ junk, but what's that, and how did it get in your car, unless you also filled up from a gas station which had floating junk and had an almost empty storage tank! I don't buy it. Additionally, there's a coarse screen and the fuel filter to contend with - when's the last time someone here reported a clogged fuel filter?

The surface area (I assume you're talking about the surface area of the tank which gas is not touching) isn't going to change a great deal once you've used a couple of gallons - most of the area is the "top," not the sides, because our tanks are long/wide and not very high.

I've only ever had problems with water in gas once - and it was the station's fault, not condensation. Modern pressure sealed gas tanks (required for emissions) don't "breathe" like the ancient ones, so there's not much water vapor entering the tank to begin with. I suspect what little does get into the gas just flows through mixed with the gas and makes a bit of steam during combustion.

As far as topping off, don't do it. The evaporative emissions system has a hose which runs from near the top of the filler neck to a canister filled with activated carbon which is responsible for storing gasoline fumes, those fumes are then burned when the engine is next run. Topping up can allow liquid gasoline to enter the canister, causing problems.

blackfnTTruck

<center><img src="http://images17.fotki.com/v308/photos/6/645139/2489087/pic1-vi.jpg"></center><p>I know I had bad gas in my Subaru, when they opened the tank you could see two lower troughs, portions that were lower than most of the tank. The dirt and gunk was all settled at the bottom in these troughs, the pickup didn't seem to go that low. Maybe the TT is different. But you can see at the bottom of this pic there are some lower sections that are lower than the main tank, just thought that was a settling area.

OldSchool AG

ok, but if the pickup doesn't go that low,the dirt and gunk that settles there will never be inhaled