Fuel gauge not accurate? 3 gal mystery

Greetings,

In trying to determine why my TDI only takes 10-10.5 gal of fuel when empty (i.e. refuel light ON, gauge on the "0" mark, after MFD indicates range of 5mi left). And no, I don't normally drive the car down to the empty mark, this is purely because I suspect there is something wrong with the fuel gauge.

My fuel gauge gets to the 7/8 mark (between 3/4 and full) only 50mi after refueling, and I have tried topping it off (little to no difference). In comparison, my old MK6, which I realize had a bigger 14.5 gal tank) would take 13gal consistently when I would hit 1/4 and the fuel light would come on. The difference of 1.5gal (the MK7 has a 13.2 gal tank) does not seems to account for this large delta, particularly since my MK7 would only take 9 gal is I refuel at 1/4 when the light comes ON.

Two questions:

1- How much fuel do you usually put on your MK7 when refueling before the gauge gets to 1/4 left (i.e. when the light comes on)? Is anyone experiencing the above possible issues?

2- I found this setting under Instruments -> Adaptation:
- (1) Offset for tank calibration values - Offset sensor 1
The value it is set to is "-12.8 l"

It looks very suspicious to me. I interpreted that as -3.3814 gal, which seems close to the 3 gal of diesel I seem to be missing during refuel. But I could be wrong. Does anyone know what this setting does? and the units? For those of you with VCDS, could you check what this is set to in your car? (whether MK7 or even MK6).

Thanks in advance for you help.

When my MKVI low fuel light comes on I have 2.5 gallons in the tank.
Never like running it that low. Makes the in tank pump work extra hard
and remember the fuel acts as a coolant for the pump.

I believe the last three gallons are intended to reduce wear on the pump and provide coolant like vwmk4 said.

No experience with the Mk7 Golf. I am driving a '15 Passat TDI loaner and noticed that when I fill the fuel tank, the needle moves off of full within 10-20 miles.

Both of my Mk6 cars take at least 100 miles to move the needle off of full.

When my range says zero and I fill to the brim the most i can put in is 11.5 gallons

Fuel light usually comes on right at 1/8. Usually fill 10 - 11 gallons.
As I understand it the remaining fuel capacity past zero is to prevent the fuel from getting too hot and causing premature pump wear. I think of it as emergency reserve capacity. If necessary I would drive 50 miles without fear of running out.

When I had 20 miles left on the range, mine took 10.8 gals. Yeah, it would be nice to easily drive over 600 miles and empty the tank, but is 450-500 that bad? Since I don't drive a ton, I kinda like topping it off anyway, so the diesel isn't getting 'stale' and I don't get as much condensation.

vwmk4 said:

Makes the in tank pump work extra hard
and remember the fuel acts as a coolant for the pump.

Click to expand...

Please explain why less fuel would make the pump work "harder"?

Thanks everyone. It seems from your responses that this was done by design and/or hence my car is not alone on this. A closer inspection on fuelly also seems to corroborate that, at least for vehicles on this side of the pond. I do wonder whether people in Europe see this too, and I still question the reason for leaving 2-3gal of gas on the tank after the gauge gets to Empty. But yes I could live with that. Mostly I was just curious and disappointed I get 200mi less range compared to my old MK6 while averaging the same MPG. Perhaps I'll do more research and experiment with the above vcds setting, maybe bring it up to the dealer first as a question to VWoA. Does anyone know from the service manual what's the security access code for unlocking the Instruments->Adaptation setting? It seems to be different from the Central Electronics module code, at least for the above setting.

I took a picture of my last fuel up, I was 10 miles to empty when I refueled, and I put in 12.290 gallons to the filler neck. Do you stop fueling when the nozzle clicks off? My needle doesn't move off of the full line until 90 miles.

Interesting. Thanks for that data point, drew. Maybe there is something wrong with my gauge after all. For my last refuel yesterday I drove 5mi past the 0mi range on the computer, and then made sure I slowly pumped fuel all the way up to the neck, past the nozzle clicking off. It took 11.3gal of fuel, so in theory 2 gal were still left in the tank. Yours seems much more reasonable to me, particularly not seeing the fuel needle drop from full like mine. I'll check with the dealer. Thanks again!

I drove 10 miles past zero and put 11.85 gallons in yesterday. 50.6 MPG per fuelly.

I've only been able to get 11 to 11.5 gallons in mine too. And I've driven about 15 miles when the guage was at 0. I did some research on the forum and apparently the diesel foams when your filling the tank so the foam prevents you from getting an exact 13.2 gallons in there. Some people on here have adapted a thing called venting or filling the tank at a trickled rate to reduce foaming. I tried this and ended up spilling fuel all over the place. It wasn't worth the hassle.

Ronnie

psd1 said:

Please explain why less fuel would make the pump work "harder"?

Click to expand...

With an empty tank, the pump doesn't have the weight of the fuel working in its favor. Who's to say if this actually makes a difference to pump longevity.

Several people, myself included, have remarked that their fuel pumps are louder when the tank is low on fuel. Louder may or may not mean it is working harder... it's possible that the extra fuel is merely absorbing/damping more of the pump's sound.

Either way, that's where the idea came from.

VeeDubTDI said:

With an empty tank, the pump doesn't have the weight of the fuel working in its favor. Who's to say if this actually makes a difference to pump longevity.
Several people, myself included, have remarked that their fuel pumps are louder when the tank is low on fuel. Louder may or may not mean it is working harder... it's possible that the extra fuel is merely absorbing/damping more of the pump's sound.
Either way, that's where the idea came from.

Click to expand...

And to the second point, does the pump really create so much heat as to need 2-3 gallons to "cool" it? nah!

Mine does the same thing my biggest disappointment with this car is the 10 gallon range on the fuel tank 400 to 450 mile range vs 650 to To 700 on my old 2002 ALH jetta

psd1 said:

And to the second point, does the pump really create so much heat as to need 2-3 gallons to "cool" it? nah!

Click to expand...

The high pressure fuel pump does indeed create a lot of heat. You can monitor your fuel temperature with VCDS and see for yourself.

The white text "nah!" is a nice touch.

I'd like to put this myth of pump heat to rest. The in tank pump is a low pressure pump and although they get warm, the only damage would be excessive wear from no fuel running through when dry. The fuel running through them is plenty to cool them. Always has been. I've had 15 cars in my driving lifetime so far, and all of them I've run down to the "last drop" tank after tank. I don't like leaving any amount of fuel in there too long as fuel goes bad without additives causing scale and other deposits. On all 15 cars, I've never had a single pump issue. Changed in tank filters on many and have never really had to since I never had deposits form.

It is a myth that pumps overheat and burn out if there is no fuel around them. and let me illustrate this, with words. A pump is about three to five inches, it doesn't lay flat. A tank is about 1 foot deep. The pump then has a downward inlet that is usually covered by a sock style filter. This stands straight up, from filter to top of tank. So for the quarter tank theory to be true, you'd have to have a 2 inch wide pump laying flat to be fully submerged. Standing tall in the tank, it starts to get into open air at half a tank and by one third left, there is no fuel around that motor of the pump.

You people need to stop perpetuating the myth of pump overheat and just drive your car.

The HPFP, that is a different animal and is not an intank low pressure inline pump.

Well that's great and all, but I'm mostly talking about the high pressure fuel pump and how fuel temperature can affect it. We also know running out of fuel can cause significant wear due to lack of lubrication within the pump.

There is a reason why VW leaves two gallons of fuel in the tank when the gauge displays empty.

VeeDubTDI said:

Well that's great and all, but I'm mostly talking about the high pressure fuel pump and how fuel temperature can affect it.

Click to expand...

The high pressure pump is NOT IN TANK... it has nothing to do with that "extra" three gallons in the tank... you are talking something completely different on this thread.

And that HPFP will get hot anyways. no matter how much you try to cool it. You are compressing liquids which is damn near impossible to begin with and you are creating heat in the process.

Tell me again where you get your theories? I live in FL, the car will run miles past the low fuel light, and I've not had a single issue. The pump gets hot, and I mean really hot. Yet I get better fuel mileage with a really hot pump and hot air temps than I get in cold weather. You might want to crack open a book on how fuel is actually burnt. It isn't the fuel but the vapor. Throwing cold fuel into a combustion chamber makes it harder to work.

Now back to the IN TANK PUMP. OP, take it to the dealer and find out why your gauge is off. As for the 1/4 tank fill method... it isn't harmful, just don't run it dry.

There is the low pressure pump in the tank that makes more noise when the tank is low on fuel (speculated to be working harder... I'm just passing that notion along since it has come up several times... I'm not claiming that it's a fact).

There is the high pressure pump on the engine that requires cool fuel for lubrication.

VeeDubTDI said:

There is the low pressure pump in the tank that makes more noise when the tank is low on fuel (speculated to be working harder... I'm just passing that notion along since it has come up several times... I'm not claiming that it's a fact).

There is the high pressure pump on the engine that requires cool fuel for lubrication.

Click to expand...

It makes sense that the pump will be noisier with less fuel to dampen the noise, no?

Yes - that is one possibility that I mentioned earlier in the thread, although in my case, low fuel doesn't not always cause increased pump noise. So who knows...

85Steve said:

The high pressure pump is NOT IN TANK... it has nothing to do with that "extra" three gallons in the tank... you are talking something completely different on this thread.
And that HPFP will get hot anyways. no matter how much you try to cool it. You are compressing liquids which is damn near impossible to begin with and you are creating heat in the process.
Tell me again where you get your theories? I live in FL, the car will run miles past the low fuel light, and I've not had a single issue. The pump gets hot, and I mean really hot. Yet I get better fuel mileage with a really hot pump and hot air temps than I get in cold weather. You might want to crack open a book on how fuel is actually burnt. It isn't the fuel but the vapor. Throwing cold fuel into a combustion chamber makes it harder to work.
Now back to the IN TANK PUMP. OP, take it to the dealer and find out why your gauge is off. As for the 1/4 tank fill method... it isn't harmful, just don't run it dry.

Click to expand...

The 2-3 gallon reserve can be though of as a heat sink. The HPFP returns hot unburnt fuel to fuel tank. The in-tank lift pump imparts minimal heat to the fuel so the fuel entering the HPFP is mostly heated by the returning. By keeping 2-3 gallons of fuel in the tank, the hot fuel is sufficiently cooled before entering the HPFP. Without the reserve you are running less dense, warmer fuel through the HPFP. This causes the lubricity of the fuel to be lower (due to being closer to vaporization) so you will get better MPG but you will be causing higher wear on the HPFP. The amount of wear will be minor but will be cumulative, so the more often you run the pump with minimal lubrication the "quicker" it will wear and potentially fail. And by quicker I mean somewhere along the lines of 175k-200k miles if you don't have an HPFP issue not related to the lubricity issue that is.

Now as to why this matters in this thread is that the OP is questioning his fuel gauge (really a meter but thats semantics). Because VW engineers need to account for the cooling reservoir, the fuel light comes on with that reserve remaining to discourage drivers from removing the heat sink and operating the car outside a specified temperature range to help ensure that the pump is sufficiently lubricated to minimize the chance of failure. (This could be a lesson learned from the previous generation of CR engines, but this is just conjecture).

Added: I get my theories from my understanding of thermodynamics learned through years of study and application in the nuclear navy.

OP, Given my above argument I would still say go ahead and bring your issue up with your dealer, but I would not hold my breath in trying to find a solution.

85Steve said:

The high pressure pump is NOT IN TANK... it has nothing to do with that "extra" three gallons in the tank... you are talking something completely different on this thread.
And that HPFP will get hot anyways. no matter how much you try to cool it. You are compressing liquids which is damn near impossible to begin with and you are creating heat in the process.
Tell me again where you get your theories? I live in FL, the car will run miles past the low fuel light, and I've not had a single issue. The pump gets hot, and I mean really hot. Yet I get better fuel mileage with a really hot pump and hot air temps than I get in cold weather. You might want to crack open a book on how fuel is actually burnt. It isn't the fuel but the vapor. Throwing cold fuel into a combustion chamber makes it harder to work.
Now back to the IN TANK PUMP. OP, take it to the dealer and find out why your gauge is off. As for the 1/4 tank fill method... it isn't harmful, just don't run it dry.

Click to expand...

The fuel in the tank is cycled through and cools the HPFP.

Done. Damn, that was simple.

Bill

Thanks for all the additional responses and interesting theories. I have 4000mi to go before my next service, at which point I'll ask the dealer about the fuel gauge behavior.

40X40 said:

The fuel in the tank (that isn't used) is cycled through and cools the HPFP.
Done. Damn, that was simple.
Bill

Click to expand...

There, fixed it for ya!

except the fuel that is used still serves to remove heat from the HPFP.

it does not, however, return to the tank.

psd1 said:

There, fixed it for ya!

Click to expand...

Thank you!

Bill