# Mystery Mileage on my useta be vehicle



## pdentrem (Dec 29, 2011)

I heard the same thing just recently. Tank not venting properly was suggested. I just do not understand this as the pump in the tank is suppling a larger amount of fuel than the engine requires and the overage returns to the tank via the return line. Thus there is no shortage of fuel at the injectors but who knows. Worked in a GM dealership fixing cars for over 10 yrs prior to current career and never had experienced this problem.

I do know that if the tank does not vent there is a vacumm and the engine will be fuel starved prior to stalling and after a few minutes the problem is gone the engine starts up fine and you can drive again until the vacumm builds back up again.

My 07 Cobalt is different as it does not have a return line. It has been suggested that to improve power etc there should be a return installed for the power adders. Also on an episode of 2 Gus Garage on SpeedTV they installed a return line on a newish Mustang (I think, I can not remember the car to be sure) to also help in making better power with the power adders they had installed on the car.


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## 75Plus (Jan 1, 2012)

Just for sake of discussion, have you considered the possibility that the gas is much cooler when you fill up. As it warms up it expands a bit and you get a slight increase in the volume of fuel in the tank. That extra volume did not show up on the pump but did so at the injectors.

I have heard for years that it is better to fill up in the morning, when the fuel would be the coolest, to obtain maximum mileage. 


Joe


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## Hawkeye (Jan 1, 2012)

Since the fuel at the gas station is stored underground, there will be little difference in temperature throughout the day. In the days of the glass-top fuel pumps, shifty gas station owners would leave the fuel in the reservoir to heat up so they would give the customer less gas for their dollar. Smart motorists would hit the dump valve to send the warmed fuel back to  the u/g tank, and then get cooler fuel pumped up to the reservoir  before filling up.

With computerized engines, there shouldn't be any mileage effect from cooler or warmer gas in the tank. The fuel/air mixture is set according to how much oxygen is detected at the oxygen sensor.

An interesting mystery.


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## ScrapMetal (Jan 1, 2012)

I've seen this with a couple vehicles I've owned but at least in my case I don't think that there is any mystery.  The cause, in my cases, was that the fuel level sensor worked on a linear scale but the tank itself was an odd shape.  For an example think of an upside-down pyramid (Not an accurate depiction but it will help to visualize what I'm referring to).  If you put a linear "depth" scale on the pyramid and assign a value to each equally spaced level (1,2,3,4, etc.) your readout of the actual fuel used would be inconsistent.  Going from top ("bottom" of upside-down pyramid) to bottom the first level would hold much more fuel than the second level and so on.  While your output would be saying level 1, level 2 (let's call them "gallons" now) the actual volume of fuel being used would be deceptive.  The first "gallon" would last much longer than say the third.

The other thing that can also happen is that you have filled the tank above the level that the fuel sensor can read or the scale starts.  This will result in your first couple of "gallons" to provide "better mileage" but only in that you've actually used more fuel than you have registered.

Just a couple of things to consider :biggrin:

-Ron


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## ScrapMetal (Jan 2, 2012)

Whyemier,

I had meant to ask if you had already taken that into account but was going to go launch into a dissertation on how to accurately track mileage and fuel use anyway but I got tired of typing. :biggrin:  Any chance that it's your driving conditions?  Do you commute down out of the hills for work (first couple of gallons) and return up hill to go home?  Any certain patterns of picking up a stiff tailwind when driving? :biggrin:  Heck, all kinds of factors will have an affect but it does seem rather odd that you can't quite put a finger on the source for the discrepancy.

-Ron


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## ScrapMetal (Jan 3, 2012)

Whyemier said:


> The still trying to understand,
> 
> Whyemier



Agreed.  Very, very odd.

-Ron


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