Fuel reserve

[DeeKay17]

Might be a silly one, but is there anything wrong with waiting till you get to the end of your reserve tank in a 997 before filling up?

I tend to wait till I'm almost out of fuel and then fill it up all the way again.

Some have said to me the last few litres of fuel contain sludge, etc from the bottom if the tank and are bad for the engine.

Any advice appreciated.

[mollytherocker]

This used be true years ago but I'm not sure it's relevant now. There's a lot more protection against contaminants and fuel is better now.

[Angelus]

The less fuel in the tank, the more air you have. Changes in temperature ( day/night ) can cause the moisture in the air to condensate and you'll end up with water in the tank eventually. The more fuel, the less air and less chance if this happening.

[mollytherocker]

Angelus said:

The less fuel in the tank, the more air you have. Changes in temperature ( day/night ) can cause the moisture in the air to condensate and you'll end up with water in the tank eventually. The more fuel, the less air and less chance if this happening.

Does that really still happen with modern tank design?

[Angelus]

Yes, it's hard to engineer out unless you can pressurise the tank with oxygen free nitrogen.

[benjj]

Angelus said:

The less fuel in the tank, the more air you have. Changes in temperature ( day/night ) can cause the moisture in the air to condensate and you'll end up with water in the tank eventually. The more fuel, the less air and less chance if this happening.

This is absolutely correct, even taking into consideration modern tech.

I don't think it matters much with a regularly used car but I always keep my stored cars brimmed with fuel.

That being said if I'm storing (without using) for more than around 6 months I'd drain the coolant and leave rad/expansion cap and drain hole open.

[mrdemon]

car is faster with less fuel :-) a full tank weighs a lot.

not that I like to go below 40 miles range left, why get caught out with no fuel !!!

And is there any such thing as a "reserve tank " I doubt there is 2 tanks in a car.

[Rockster]

DeeKay17 said:

Might be a silly one, but is there anything wrong with waiting till you get to the end of your reserve tank in a 997 before filling up?

I tend to wait till I'm almost out of fuel and then fill it up all the way again.

Some have said to me the last few litres of fuel contain sludge, etc from the bottom if the tank and are bad for the engine.

Any advice appreciated.

Not sure what you mean by waiting until you get to the end of the reserve tank? There's no reserve tank. The low fuel light comes on at some point when there is just a few gallons of fuel left. Not sure how many gallons but it is less than a quarter of a tank so call it 3 maybe or even 2 gallons of reserve.

It is not good practice to run the fuel level so low. Not because of any sludge. There is no sludge on the bottom of the tank unless you put it there. (When my 02 Boxster's fuel pump quit at around 200K miles the tech reported no sign of any water or anything else on the bottom of the fuel tank other than a tiny amount of particulate matter and not much of that. 200K miles. Or around 8K gallons of gasoline.)

There is no water because these modern tanks are not vented to atmosphere. The fuel pump keeps the fuel stirred up when running so any water gets mixed in with the fuel and burned. It helps in this case if there is ethanol in the gasoline as this absorbs the water and helps carry it into the chambers to be burned, well, gotten rid of in any case.

Why it is not a good practice to run the fuel tank so low is the fuel pump is cooled by the fuel. It is immersed in fuel and in fact part of the pump's fuel output is redirected out and into a housing around the pump proper (which is sealed aluminum can about the size of a soda can -- at least the pump in my car) which circulates around the pump and helps cool it.

When you run the tank real low the fuel gets hotter and the pump runs hotter than it would otherwise. Not lots but hotter. I note my fuel pump lasted 200K miles and part of that I like to believe is that I didn't make it a habit of running the tank much below 1/4 tank. The lowest I ever ran it was not too long ago and it took 15.625 gallons to fill up. The tank's capacity is 16.9 gallons so it was quite low on fuel. But most of the time I fill up at 1/4 tank if not a bit before.

BTW, the new cars with their DFI engines have two fuel pumps and one of them is the high pressure fuel pump and it is quite expensive to replace. Were I to own a car with such a fuel pump (and I have more on this below) I'd treat it real nice.

Now I owned a VW Golf TDI which had a high pressure fuel pump and I treated it real nice. Filled up the fuel tank at 1/4 tank and changed the filter when called for and drained the water when called for and just well treated that very expensive fuel pump like the it cost the dickens to replace, because it would have cost the dickens to replace.

Price a new high pressure fuel pump for your car and then decide if you want to continue to run the fuel level way way down...

[Rockster]

anonymous said:

[redacted]

It might not matter. But I was always told with in-take fuel pumps to avoid running the tank real low on fuel and it is a habit I've always kept.