I have been watching my mileage very closely for about 4 full tanks now and I stumbled on something that I think might lead us to better mileage on our trucks.
I do alot of driving and it is pretty consistent with a 1/3 city, 2/3 hwy.

So first tank I averaged 18.6 MPG (I am using a scangauge II)
Second tank I averaged 19.2 MPG. At about 1/4 tank my CEL came on (code P0455) Gas cap was not on all the way. I am certain that this was the case on the first tank also.
Third tank with cap on all the way 14.6 MPG.
forth tank with cap on all the way 14.8 MPG.

Anyway I do not have an answer for this because I am not sure what happens when the cap is not all the way down but it will set off the CEL and is probably not good for our trucks.

My guess is that it sucks air into the tank. So if we get more air to the engine will it use less Gas?? I have no idea what is happening here but the facts are that it gets better Gas mileage with the cap not on all the way.

I will be driving the next two tanks with the cap only on half way to verify this once again.

Just fyi my gas cap starts to click after about 1 1/2 turns so that is why I thought it was on, I have to push down on the cap to keep it going all the way on.

 

If your gas mileage has improve by about 4 mpg with the gas cap half on than you could probably get another 4 mpg with the gas cap totally removed. It makes sense, right? Seriously you might be on to something but something tells me that there is a trade off somewhere.

 

Do you really think that if taking a gas cap off could do what you claim it wouldn't be known already and used by the car manufacturers who are getting absolutely beat to death on improved MPG standards?

Occams Razor: The simplest answer is usually the correct one. In this case, you measured wrong on the tank where you got 19+ MPG.

 

I don't think it's the gas cap,

But at the same time car manufacturers are NOT getting beat hard enough on the mpg. Every vehicle sold in other countries gets better gas mileage than us models, and they still can't touch a geo metro even with out "hybrids" it's all a load of crap

 

I'm no conspiracy theorist, but I have to believe after all this time of petrol engines, we haven't come far enough in saving gas. Reminds me of Hoover Vacuums (it think it was Hoover) who turned away the patent for bagless vacuums because they made all their money on bags. They should have bought the patent and shelved it.

I think OPEC is probably the biggest holder of gas saving ideas in the world. And they all got "shelved".

The only way to truly save gas is to keep your car maintained and dont drive aggressively...something I have yet to learn. ;-)

 

So No Gas Cap = Ram Air

....Hate to jump on the pessimist bandwagon and burst your bubble, but I'm going with miscalculation or a fluke. If your gas cap is off, fuel vapors are going to try to escape, air wouldn't be sucked in.....it's science

 

I have never driven with the cap completely off but I can tell you that not being tight will through the CEL so all the way off may not be good.

My thought is maybe some way to get more air in to the engine. but again I will keep collecting data and reporting back so that some of the more mechanically inclined might figure something out.

Code said "Major gross evap leak, or stuck open purge valve, check gas cap"

maybe this will help.

 

Yes, that would be the code. It fails the EVAP test, which only runs when you have less than 1/2 of a tank of fuel and several other criteria are met. The fuel tank is pressurized and essentially a leak down test if performed.

This has no effect on performance or fuel mileage (other than the negligible weight of the system components), nor does leaving the cap off or loose. It is an EPA mandated feature to reduce the amount of hydrocarbon vapors dumping out of our vehicles. Just like downstream O2 sensors. And EGR. None of this is Rover specific.

 

....again, THE GAS CAP IS THERE TO KEEP FUEL IN....keeping your gas cap partially on, will only result in fuel vapors escaping, water or other contaminants getting into your fuel, and screwing with the computer and throwing the codes like you are getting because it is considered a "leak"

Air will not work it's way into the tank, dissolve into the gas, and then make it's way to the engine for better combustion. There's a reason why the intake system isn't located in the filler neck of the fuel tank.

 

Guys I was being sarcastic. I don't expect anyone to drive around without a gas cap or expect and improvement in mpg.