Theory on fuel fills

[acdii]

As I was driving with the fuel gauge in the red, DTE 0 for 10 miles, and the trip showing used 10.3 gallons, it dawned on me, wait, if this is a 13 gallon tank, and 10 gallons is empty, maybe the fuel gauge isn't what we are used to. Being that nearly everything in this car is based on computer programming, it is entirely possible that so is the fuel gauge. It could be that with each and every fill, the gauge is trained for the amount of fuel used when reset, so if you use 10 gallons every tank, which is about where mine is, that is where the empty mark will be. If you drive until it is really empty and pump in 12.5 gallons over several tanks, it could very well reset the fuel gauge empty point by 2 more gallons.

 

We already know that MPG and fuel used are all averaged out over time, so maybe, just maybe, so is the fuel gauge. It may be using the mechanical float as reference only. If it wasn't winter time, I would carry a 2 gallon can full and experiment with it to test that theory, but not when its cold, I dont want to be fiddlin with that pour spout in the cold.

[jsolan]

I don't think any manufacturer would ever do that. What if you don't fill up every time, what if you only put a few bucks in here and there? I wouldn't say I know a lot of people that do that, but I do know some. I can't imagine the software would have anyway to account for that.

 

I just think the inaccuracies in the capture method of fuel burned as well as the high reserve is why most people see the fuel light come on at about 10 gallons used.

[hybridbear]

I just think the inaccuracies in the capture method of fuel burned as well as the high reserve is why most people see the fuel light come on at about 10 gallons used.

I still don't buy the 2+ gallon reserve theory because C-Max Hybrid owners can consistently drive to where the trip meter shows 13 gallons used before the DTE says 0 miles. My theory is still that the design of the FFH tank leads to us never actually getting the tank full. I think the pump clicks off at 10 gallons with 3.5 gallons of air space in the tank. Maybe there is inadequate ventilation of the tank so the air can't escape when putting gas in which leads to erroneously early pump shut offs.

 

One way to test would be to completely run out of gas and then see how much you could put in the tank. If you drive to the point of running out you should be able to put in 13.5 gallons before the pump shuts off.

Edited November 14, 2013 by hybridbear

[GrySql]

What about those of us that have filled the tank to it's max rated capacity with DTE miles left, around that magical 13.5 gal mark?

One time after a slow fill-up I put in an additional 3 gallons after the first click off - that is the venting system's job, and I think it does a poor job at times.

There is something about the venting system, shape of the tank and computer interface that is not quite right.

Those of you with Fuelly know the pump and computer numbers don't always match.

 

There is not much one can do to change this issue so I basically just do my 3-click fill and ignore it.

[acdii]

 

 

One way to test would be to completely run out of gas and then see how much you could put in the tank. If you drive to the point of running out you should be able to put in 13.5 gallons before the pump shuts off.

This is what I was referring to with carrying a full 2 gallon can with, its just too cold now to try it.

[acdii]

Hmm, according the the hybrid docs(MOST excellent Find too), looks like I may be correct. 100% Credit to airbusguy for this find, http://fordfusionhybridforum.com/topic/7913-general-info-for-ffh-techies/?p=70255

 

 

"Fuel consumed" is continuously calculated based on

PCM fuel pulsewidth summation as a percent of fuel tank capacity. (Fuel consumed and fuel gauge reading range

are both stored in KAM and reset after a refueling event or DTC storage.)

Edited November 14, 2013 by acdii

[Waldo]

I think you're mis-interpreting that. The fuel gauge reading range seems like the range that the sender moved since the last fill up. That's compared to the fuel consumed calculation just to determine if it seems like the sender is stuck. It has nothing to do with what's actually displayed on the fuel gauge.

The fuel gauge itself is based on a fixed percentage of the fuel tank size, it does not change over time.

Edited November 14, 2013 by Waldo

[kuzzi]

I dont want to be fiddlin with that pour spout in the cold.

Damn straight.

[acdii]

There is only one way to find out for sure, drive til you can drive no more then fill er back up and do it again a few more times. Just hope you are close to a station when it does run out.

[Texasota]

On many cars the fuel pump inside the gas tank can easily be damaged when running out of gas as the pump depends on the fuel for cooling and lubrication. This can result in costly repairs. I don't know if this would be true for the FFH but it might not be a good idea to intentionally run out of gas.

[Hewlettk]

Interesting the fuel lubrication/cooling theory. I worry about this too, however, curiosity gets the best of me!

On my Prius I have run the tank dry about 4 times with 2 gallons to refuel.

No problems with the fuel pump, and figured out the car had about 2.3 gallons remaining (120mi) after the range went to "0"

and the Fuel gauge flashed empty, nice to know!

 

Now I have the Energi :-) With a full battery (approx. 20 mile range worst case) I go to "EV only mode" when the range = "0". When the ICE finally dries the tank I have had between .46 and .51 gal of fuel past the "0" range mark.

 

Ford plans on this happening. At the moment the fuel is completely exhausted,

1. The A/C compressor shuts off.

2. The map shows all the gas stations.

 

So, they 'plan' on people to run out of gas, on purpose, or by mistake, and do everything they can to help get you to the station.

 

Clearly the fuel pump shuts off after the last drop gets pumped, so I am not so concerned about the fuel pump burning up...