How low can you go on fuel?

[StaceyS]

Another dumb question to add to this thread:
How does one get the water out of the filter? Is it just done by emptying the filter housing and changing the filter? Or is there some built in process that lets the car deal with "reasonable" amounts of water?

 

[bruce846]

Unfortunately on the newer cars late 09 and up, I think it's left to combustion?

 

[MayorDJQ]

Unfortunately on the newer cars late 09 and up, I think it's left to combustion?

No! Water sinks in fuel. First it sinks to the bottom of the tank. If it gets drawn into to the fuel pickup, it then collects in the bottom of the fuel filter. I suppose some water at the molecular level may get passes the filter and get injected, but it's an insignificant amount.

My understanding is that this is a cold weather problem and the more air space you have in the tank the better the chance of condensation with the warmed fuel... any one have a better explanation, don't hesitate to chime in. Last winter I ran my tank always more than half full and did not experience some of the hesitation problems listed on this board FWIW.

As I mentioned, I've been running my tanks down to 1/4 or 1/8 tank for years. I've never had any hesitation issues. The most water I've ever found in any of my Diesel's fuel filters has been the size of a sesame seed.

Another dumb question to add to this thread:
How does one get the water out of the filter? Is it just done by emptying the filter housing and changing the filter? Or is there some built in process that lets the car deal with "reasonable" amounts of water?

The process to get rid of water is simple: every 20k miles you remove the filter, drain the canister, and install a new filter. Water collects in the base of the filter filter canister, aka the "water seperator".

 

[bruce846]

I suppose some water at the molecular level may get passes the filter and get injected, but it's an insignificant amount. [This is by combustion, no?]

In your case it was an insignificant amount, but that may not be the case for other owners.

 

[mysql]

i've gone 650 miles on a tank before the low fuel light came on. I think I could do 700 miles, but given that I was on the highway and didn't know when the next diesel station would be, it wasn't worth trying.

 

[JRide]

I've also let my car go to where it was reading 0. I drove about 5 miles with it reading 0. I'm picky where I get my fuel and wanted to wait until I got back out to the highway. Unfortunately it was a few more miles than I thought. Luckily I didn't run out of fuel. I try and fill up as soon as I can once the low fuel light comes on.

 

[bsalbrig]

I have gone 780 on one tank and filled 14.5gal; I was about 15mi past zero to empty. I am about convinced there is another gallon once I reach zero. In my 02 I took it over 100mi past the empty light, that time I put 16.5gal in it (specd at 15gal tank) I have not run out of fuel yet but wont say it will not ever happen.

 

[tedkidd]

I am a bit confused about warm fuel causing condensation in tanks. Typically condensation occurs when dew point is reached - which means declining temperatures, not increasing temperatures.

 

[dwpc]

Stretching it isn't worth the risk

If you were cruising on the interstate getting 45+mpg, you might encounter slow traffic conditions and be stuck under 30 mpg in a heartbeat, and the 50 mile range reading you had at 30 miles from home can drop to 20 over the next hill. If you run out of fuel while trying to get 20 mi. more the MFI, you'll reeaalllly wish you had listened to your common sense.

I generally fill up before I get into the red. The last thing I want in the TDI is an air bubble in the HP pump.

 

[steelmb]

You can run till the engine sputters and dies.

Believe it or not, the answer to your dumb question is that simple.

Ok, I know this is a dumb question, but I gotta ask.
I did 540 miles on my very first tank of diesel. We did a 340 mile road trip yesterday, and I was aiming to sneak back into town and fill up before heading home. My MFI was telling me I had 40 miles to go when I past the last gas station before Bend, with about 20 miles left to drive. I then spent 20 minutes worrying that I was going to get air in my fuel line and have the car crap out on me because I had to start, stop or go around a turn too fast. The MFI said I could drive another 25 miles when I actually got to the station.
Anyone run their car lower than that and actually have it quit because you got air in the line?
I don't think I'll run it that far down again. The fuel light was on and I was half-way through the red portion!

 

[flylow2]

Isn't there already enough stress in life than to push the limits of fuel capacity? The only time I pushed the limits a little bit was my first long road trip in the car. The light had not come on (I have no idea if it even works but no reason to suspect it doesn't) and I was down to less than 1/4 tank. The first place I stopped didn't have a car nozzle and then off I go wondering about. Finally had to chose Exxon (the car ran like crap on this stuff).

I would much rather have the luxury to chose my fuel when things aren't close to critical. I never ever want to run out of fuel. My normal commute puts on about 300 miles per week. Add a bit of weekend driving and I am about 400 miles per tank so I fuel once a week. That way, I never need to take extra time during my commute to fuel.

Running very low all the time exposes your fuel to more turnover and high temperature exposure which potentially thermally degrades the fuels and potentially adversely affects the fuel's lubricity. Who knows how significant this potential issue is but with the HPFP problems, why push the limits for the sport of it? Fuel when convenient per your optimal balance of range and convenience.

Two more comments:

The fuel gauge is not linear. I see that some here realize that. I know that when mine reads half, it takes about 8.5 gallons or 41% remaining. This is another reason to fuel at different points on the fuel gauge to better characterize the gauge.

When fueling to the auto shut off and stopping, you are leaving behind approx. 0.5 gallon of capacity versus filling until the fuel is all the way up into the neck.

 

[bsalbrig]

Running very low all the time exposes your fuel to more turnover and high temperature exposure which potentially thermally degrades the fuels and potentially adversely affects the fuel's lubricity. Who knows how significant this potential issue is but with the HPFP problems, why push the limits for the sport of it? Fuel when convenient per your optimal balance of range and convenience.

Do you have any evidence of "thermally degrading" your fuel by running it low or are you making up a theory? I figured my comment would provoke a response but thermally degrading fuel is not what I figured I would get. The reason I bought the TDI is for cruising range defiantly not reduced operating costs. I drive over 1000miles a week and want to minimize fueling stops. I have been stretching these cars out for quite some time. I would run my 02 ALH far lower and it didn't have a fuel cooler like these cars have.

 

[flylow2]

No I have no evidence of thermal degradation but I did not make this up. I have read that concern in this forum and in other articles about diesel fuel outside of this forum. If you read aftermarket additive bottles you will also see that they claim to enhance thermal stability.

If you think about the HPFP, they generate very high pressures. In other words, a lot of energy is put into the fuel by the pumping action. The injectors use what is needed and the remaining fuel is returned to the tank. Energy equals heat so the fuel heats up. When the level is low, the fuel will reach higher temperatures and will see more time in the high shear environment of the HPFP.

The issue is real but how significant is it? I couldn't answer. It is probably more of a factor in hot weather when you are running long and hard. It is also probably dependent on the composition of the fuel as well. Some fuels will have better thermal stability than others and this is not brand specific but more specific to the crude that the fuel was distilled from.

I wouldn't let this issue rule me but it is just one factor in the decision of when to fuel. It may not be a factor at all for your car but for the CR cars, where the jury is still out on HPFP longevity, why push it?

 

[ultranaut]

I've taken it down to 0 a few times, the furthest I've driven after it said I had 0 miles left was around 2.5 miles. Yes, I was extremely nervous the whole way!
Normally, as soon as I have less than 75 miles left to go I start thinking about filling it up ASAP. It's hard to be sure how much you really have left because it seems like the amount of fuel it takes to fill the tank doesn't correspond to the fuel gauge, I figure once you are nearing the red it's better to err on the safe side.

The most I've ever put in was 14.379 gallons.

 

[MayorDJQ]

In your case it was an insignificant amount, but that may not be the case for other owners.

The majority of Diesel drivers will never find more than insignificant amounts of water in the fuel filter, especially if they're buying fuel from high volume stations and using an additive that helps control water buildup.

In my case, I'm not talking about a few years of driving TDIs. While only 33, I think I've owned 9 Diesels, all of which I did my own service on. Using a very rough estimate, I'd say I've probably driven 600K+ on Diesels in my short driving history. A fuel filter every 20k means I've examined the drainage of ~30 fuel filters. At the very most, I may have seen a few "sesame seed" size drops of water, but never anything worse.

No I have no evidence of thermal degradation but I did not make this up. I have read that concern in this forum and in other articles about diesel fuel outside of this forum. If you read aftermarket additive bottles you will also see that they claim to enhance thermal stability.

There's a simple explanation: because they're trying to SELL you the additive!

If you think about the HPFP, they generate very high pressures. In other words, a lot of energy is put into the fuel by the pumping action. The injectors use what is needed and the remaining fuel is returned to the tank. Energy equals heat so the fuel heats up. When the level is low, the fuel will reach higher temperatures and will see more time in the high shear environment of the HPFP.

The issue is real but how significant is it? I couldn't answer. It is probably more of a factor in hot weather when you are running long and hard. It is also probably dependent on the composition of the fuel as well. Some fuels will have better thermal stability than others and this is not brand specific but more specific to the crude that the fuel was distilled from.

I wouldn't let this issue rule me but it is just one factor in the decision of when to fuel. It may not be a factor at all for your car but for the CR cars, where the jury is still out on HPFP longevity, why push it?

Personally, I think you're worrying to much about this.

The way to test this would be to log readings from the Fuel Temp Sensor with VCDS over the course a few tanks. If in similar driving conditions and weather the fuel readings are higher when the tank level is low, maybe there's some validity to this theory.

 

[flylow2]

The majority of Diesel drivers will never find more than insignificant amounts of water in the fuel filter, especially if they're buying fuel from high volume stations and using an additive that helps control water buildup.

In my case, I'm not talking about a few years of driving TDIs. While only 33, I think I've owned 9 Diesels, all of which I did my own service on. Using a very rough estimate, I'd say I've probably driven 600K+ on Diesels in my short driving history. A fuel filter every 20k means I've examined the drainage of ~30 fuel filters. At the very most, I may have seen a few "sesame seed" size drops of water, but never anything worse.




There's a simple explanation: because they're trying to SELL you the additive!




Personally, I think you're worrying to much about this.

The way to test this would be to log readings from the Fuel Temp Sensor with VCDS over the course a few tanks. If in similar driving conditions and weather the fuel readings are higher when the tank level is low, maybe there's some validity to this theory.

Worry is what I do. You are right though which is why I do not obsess over it. I mostly don't like the idea of stressing out on where to buy fuel when low and chose to fuel at my convenience versus absolute need. I borrow from an aviation axiom: Nothing is more useless than fuel left behind, runway behind you, and altitude above you. I chose to be conservative rather than roll the dice except in the decision to buy this car for which I have no regrets. At least not yet.

 

[back2vw]

Worry is what I do. You are right though which is why I do not obsess over it. I mostly don't like the idea of stressing out on where to buy fuel when low and chose to fuel at my convenience versus absolute need. I borrow from an aviation axiom: Nothing is more useless than fuel left behind, runway behind you, and altitude above you. I chose to be conservative rather than roll the dice except in the decision to buy this car for which I have no regrets. At least not yet.

Don't worry, I'm with you. I fly helicopters and I'm often limited in how much fuel I can carry based on my pax load, but I will always take more fuel when I can carry it.

As for the car, with a 550-600 mile range it still blows me away that people will run the car to empty. I just don't see what could possibly be so important that a person can't stop, pour in a few gallons and leave that concern behind. Typically we fuel the car up after 350-450 miles. If I'm traveling a long distance I'll stop at around 450 miles as that is the longest I want to sit in the car anyway.

My truck burns a ton of gas, but I fill it up between 1/3 and 1/2 a tank anyway typically. I've been called out too many times at random occasions to want my truck sitting in the garage low on gas.

To each his own, but you'll never see me walking down the side of the road with a fuel container.