ffcbairn

This is the second time this happened to me, so I figure it's worth posing the question to see if there is an answer.

I have been keeping meticulous records of my fuel economy. Within the past few months I have had two periods where the car has sat for over a week. Following each instance, the economy drops 2-3mpg, but only for that tank. After filling up, the economy returns to what it should be.

The most obvious answer to me is evaporation, but I thought with the gas cap on, the fuel system was supposed to be sealed. It's not a major issue, nothing wrong with my car, and like I said economy rights itself after that tank. It's just more of a curiosity I am trying to figure out. Maybe I need a new gas cap?

Edit: 2000 btw

AnsonJ

not evaporation, but condensation. water is condensing inside your tank as the car sits and the car does not run as well with a little bit of water in the gas, which will combine readily with the ethanol that is in most fuels. try filling the tank up before letting it sit, that should help some.


AJ

DAlastDON

^^I would have never thought of that. Definitely plausible.

ffcbairn

Indeed AJ, thanks for the answer. That definitely seems like a plausible explanation, as both times I was over half way through my tank. I will definitely try that next time around. Only thing that might not apply is I run ethanol free, but I would imagine that if water is condensing in the tank it would still end up in the gas, even without the ethanol present.

DrewSER

The fuel system is supposed to be sealed with air only being allowed IN to the tank through the cap and not OUT. Since the fuel load remains near the ambient air temperature there should be no condensation forming in the tank. Not to mention there's only a finite amount of air in the tank even when empty.

ffcbairn

Then I am at a loss still. No codes, no weird issues with the way it runs, and on the subsequent tank, my economy goes back to what it should be.

AnsonJ

yes, this is all true. And this is also reason that evaporation is unlikely. If it was evaporation, you should have a code similar to the one that you get when your gas cap is not tight enough. For condensation however, the air that comes IN to the tank is from the outside, and has moisture in it. and a drop in temp outside will cause condensation on the inside walls of the tank. the less fuel in the tank, more condensation is able to occur. the air outside the tank will drop in temperature faster than the fuel which can cause condensation on the inside of the tank. IF the tank is full, there is less surface to allow for condensation.

An easy way to rule this out is to fill the tank before letting it sit the next time.

AJ

DrewSER

Do you realize how fast and by how much the temperature would have to change to cause water molecules to be grouped together faster than they're torn apart inside of the tank? Heres something else to think about.. An EMPTY 20 gallon tank on a hot 100% humidity summer day has the ability to generate 0.08 oz of water. That number drops to less than 0.05 oz on a cool fall day and continues to drop with ambient air temp. Of course thats only if a "The Day After Tomorrow" type situation were to create the proper conditions so...

We haven't even touched on the fact that water and gas are immiscible. Water in the tank would cause misfires. I could go on forever.

DrewSER

Side note: I don't know if you guys have read this thread but its quite interesting and can benefit everyone. I know the OP mentioned that they run ethanol free gasoline but most of us aren't fortunate enough to have that option.

http://forums.maxima.org/general-max...troke-oil.html

I'm on my second tank.

ffcbairn

^^great thread, I am also using the two-stroke oil, even with ethanol free, there is still a noticeable improvement to motor smoothness. I have not seen any increase in economy though, but I think at this point the consensus is that only motors on the very low end of economy actually do.

AnsonJ

yeah I am thinking on trying that as well. My motor has a bit of a rough run to it, just barely noticable... I will hafta try it out.

Might do the pilot test on my 97 Ranger first. it is waaay easier to work on the fuel system than the Maxima...

My guess is that it will be similar to marvel mystery oil as mentioned in one of the earlier posts.

AJ