I've run into this before.

I did a quick little 30 mile trip, having just topped the tank off, and then filling it again from the same pump and same station (I needed to turn in a gas receipt), and I got 24.5MPG out of my 00 XJ.

Granted, this trip was all two-lane highway with me doing 55-60 on it.

 

I'll give it a try and post back here in a few weeks

 

I have notices My Mpg going up as well when I fill up after driving under 80 Miles

 

You're telling me... My '71 VW Beetle gets around 26 mpg. Although it does only put down something like 40 hp

 

can anyone attest to increased mpg on the on board computer while maintaining a more full tank? Even if the OBC doesn't give exact real world mileage, you should see the value indicated increase...

 

I heard the computer determines MPG by the pulse width of the injectors? so its not all that accurate.

 

I always thought that it was figured using time, speed, miles traveled, and fuel burned as indicated by the gauge. Some systems do use inputs like MAP and/or MAF and pulse width times but I think (can't recall what it was) it was on a newer BMW.

 

Older BMW's just had a vacuum gauge and a scale to 'show' mileage. Easily turned into a FUN gauge! lol

 

Just had a thought on this. How long does it take to empty the tank? I'm certainly not an expert on the evap system but it occurs to me that with a lower volume of fuel in the tank, it is more likely to evaporate while it's sitting there. I recall reading that federal standards for summer gas require that it is less prone to evaporation (compared to winter gas) for toxic smog controlling reasons. Obviously the fuel system isn't hermetically sealed lol.

Could it be that with less fuel in the tank, it takes less time to heat up and just evaporates away? Whereas a larger amount would absorb more heat before getting to that stage

Thoughts?

 

Could it be that what does evaporate gets caught in the canister, then goes into the system while it's running, enriching it a little, so the 02 sensor can tell the puter' to shorten up the pulse a tad?

 

Well I assume that's how it's supposed to work, but no XJ is new anymore and I don't really read of people working on the evap system unless a code shows up. After so many years its not like everything works 100% as originally intended Now that I'm sorta awake it does sound a bit silly that so much gas would just evaporate away. I'm not a mechanic lol, I can't tell if you're trying to be facetious on purpose or not.

 

No! Salad, not at all, (here anyway. ). I wanted to explain my take on how the thing works, and point out that whatever fuel IS absorbed ends up getting used by the engine, although not via the injectors. I think if there is a leak it will set a code Btw, for instance the famous "gas cap" did it......

With my years of a real pain in the butt with fuel separating and fouling up, sometimes my LIFE, I'm wondering if there is anything to my idea that as the pump at the station runs longer is sucks up more of that crap out of the main tank. WEAK, defiantly unlikely. After all, the last guy just used it. It separates pretty quickly in a jug though

 

Heh alright then, tone doesn't convey well over the interwebs

That's a possibility! I had forgotten about those pictures of your moonshine :P (grats on staying dry btw). If this whole keep-the-tank-full business goes somewhere for me I'll fill up a few jugs too and post the results.

 

......Partly I put it as a question so someone could chine in easy, "Well Don your close but...." I will re double my efforts!

When I run my gas pump to fill my 1000 gal tank, (about every 10 days since 1987), I don't go down in the gully to turn it off, I measure the gas so it runs out in 1hr , 45min. Since the system is shared with another house, I measure, (about 10in.) into 1/2 dozen clear jugs so when I leave town "others", can run the pump to have water. They seal pretty tight, crumple in the cold sometimes. I'll come home in a month or so and find 1 to 3 inches of brown...."additives". Not always brown right away, I'm not sure if it needs sun.

Bites in a carb bowl. Some things almost run on choke. Sloshing around in our Jeeps it seems to work better at least.