I wasn't responding to you there...mine were directed at the OP.

Ditto, I knew you were joking as well:p....all my responses were directed to OP.

i've found the best solution for gas milage...i borrow my sister's honda for going to the store! ha

The fuel lines are completely full of fuel unless you run out of gas, so no "extra" air can get into the engine through the fuel lines.
If you want extra air you need a turbo or super charger.
a loose or missing gas cap will actually lower your MPG as your gas evaporates out of the filler neck.

I doubt the veracity of the experiment. However, if the vent for the fuel tank was obstructed it could restrict the fuel-pump operation (draw a vacuum in the tank). And if another vent was provided, by loosening the filler cap, the problem is alleviated. If the vehicle is operated under the right conditions it would not test itself and not throw a code. It would have to be a compound problem with a faulty cap as well because the cap should allow air into the tank.

First of all, never you a scan gauge to get accurate MPG, do it the old fashion way and divide your mileages by your gallons used. Scan gauges or any other devices like it do an average that is not close to accurate.

Ok guys it looks like mike is right and some of the other comments also. Here are my findings.

the scangauge is not the way to test this, with the cap half on the the truck will consistently show better mileage but it is because it is throwing the truck into open loop (not sure what that is) and the scangauge is forced into reading the mileage at 9999 mpg at times. Anyway it looks like it will have to be checked the "old fashion way" but my guess is that there is no difference but I am not sure.

Anyone with a scangauge can run these test as I have with just one tank, no need to run through 4 full tanks to find out, the scangauge will recalculate the Average Miles Per Gallon every 2 minutes.

My prognosis is no difference in Gas Mileage. Sorry for the wild goose chase.:(

You're just adding wrong on top of wrong here. Time to give up.

Running without a gas cap does not keep you in open loop.

Just let this thread die, and maybe no one will remember.

Jaydog - thanks for the honesty. You found an answer to a question and everyone benefits. Too often the thread "is allowed to die" w/o closure.

OPEC doesn't know their own ass from a oil well in the ground. 100 years ago they were running around in the desert cutting each others heads off and in another 100 years they'll be back doing it again.