Better Fuel Economy with No Gas Cap - Why?

[kvnschmdt]

Not sure if this is the right place to post this...

My truck has been getting poor mileage for a year or so - city or highway I'd only get 300km (171miles) to a tank (60liters-15.9gallons).

I recently did a trip to the arctic coast from Vancouver BC (Hello, ice highway) and at one of my fuel-ups I misplaced the gas cap. With no fuel cap all of a sudden I was getting 400km to a tank.

On the way back I got a replacement cap, and with it in I was back to 300km to a tank.

Why would this be? Is there something I can fix so I can keep the better fuel economy and put a gas cap back on?

 

[TheOne]

hmmm weird. there may be something with WHERE u were driving....theres literally a BILLION reasons for how u got those numbers...

 

[cammeddrz]

hmmm weird. there may be something with WHERE u were driving....theres literally a BILLION reasons for how u got those numbers...

i live at 7,000 feet elevation. and my fuel econmy sucks. when i go somewhere lower like vegas my power increases and so does my economy........

 

[Ranger Kip]

You adjust the carb or fuel injector to your altitude.

I lived at sea level in Arcadia CA (aka Hell, Cody will vouch for me), and we moved to Wellsboro PA, which our farm is at about 5000 feet above sea level (top of the hill in this area, we are very noticable, haha), and we had to adjust all the vehicles.

Air is thinner as you rise in altitude, so you have to adjust the carb or injector to work with it.

 

[cammeddrz]

You adjust the carb or fuel injector to your
Air is thinner as you rise in altitude,

and here i thought air got thicker as you got higher

so the entire state of utah is to "adjust their injectors" i don't plan on messing with my 1998 explorer. the map sensor coupled with the O2 sensors will do all of that. you can't make something run like its at sea leven by "adjustment" you can't create more oxygen, where i live you see near 25% power losses. i grew up racing dragsters and going from state to state and needing to tune accordingly. in fact my friend that moved here from new jersey with an '03 cobra needed to increase his boost 8psi to get the same hp numbers he got in new jersey. add 8psi to your vehicle that's what its like for us when we go to sea level

 

[TheOne]

he may be thinking the jets on a carbed system.... fyi hence the fuel injector comment.

but yea elevation plays havoc on fuel economy...diesels may have a better time with it then gas tho

 

[Ranger Kip]

Your fuel to air ratio I am talking about =P you adjust it to capture more air.
Wont ever get it exact, but you can get it a little better =)

Just a suggestion =P
You are almost required to do it on snowmobiles >.> we had them at Big Bear in CA, and when we moved, we had to readjust them, the one designed for jumping still needs it. (we have 2).

 

[adsm08]

You adjust the carb or fuel injector to your altitude.

Uh, FI systems adjust themselves for altitude there captain carburetor.

I'm kinda wondering if there isn't a return line obstruction causing high pressure and thus increased fuel into the cylinder.

Removing the pressure building effects of the cap allowed the line to flow better allowing for better mileage.

 

[cody93]

dont forget to put ur fuel cap on when not using it, gas evaporates faster than water and becomes explodable fumes, and yea, kips right, cali is hell, i keep a 12gauge by my pc cause its bad here in cali, there was a crack house down the street from me too

 

[Shran]

That is really weird. I would first see if you can duplicate the results you got by removing the gas cap for the next tank. I'd kind of suspect the check valve on the top of the fuel tank - could be stuck. Could also be anything in the lines connected to that up to the charcoal canister.

I am thinking of the old vapor lock scenario, only it is causing low fuel pressure when the gas cap is on. However, only a fuel pressure test would prove this.

 

[kvnschmdt]

yes its super weird.

the trip i just did was super long - i put over 4000kms on the truck, so I had a lot of tanks of gas to try things out - and after I got the replacement cap the results were the same - cap on 300km to the tank - cap off 400km

so you think a fuel pressure test is the next step?

 

[adsm08]

Well, at the very least it can't hurt anything.

You might also be sucking air into the tank and just getting better atomization or something.

Might also be allowing heat to escape keeping the fuel cooler. Cold fuel is more efficient.

 

[Shran]

yes its super weird.

the trip i just did was super long - i put over 4000kms on the truck, so I had a lot of tanks of gas to try things out - and after I got the replacement cap the results were the same - cap on 300km to the tank - cap off 400km

so you think a fuel pressure test is the next step?

I think you should run a pressure test for curiosity sake more than anything. You'd have to go drive the truck around for a while to build up vacuum in the tank, then do the pressure test before you shut the engine off.

 

[erocka1982]

and here i thought air got thicker as you got higher

so the entire state of utah is to "adjust their injectors" i don't plan on messing with my 1998 explorer. the map sensor coupled with the O2 sensors will do all of that. you can't make something run like its at sea leven by "adjustment" you can't create more oxygen, where i live you see near 25% power losses. i grew up racing dragsters and going from state to state and needing to tune accordingly. in fact my friend that moved here from new jersey with an '03 cobra needed to increase his boost 8psi to get the same hp numbers he got in new jersey. add 8psi to your vehicle that's what its like for us when we go to sea level

it doesnt apply to the 89 ranger but with newer vehicles, thats why there is a barometric sensor. it automatically adjusts for the elevation

 

[fastpakr]

it doesnt apply to the 89 ranger but with newer vehicles, thats why there is a barometric sensor. it automatically adjusts for the elevation

Actually, his '89 probably does adjust to altitude changes. Most MAP vehicles sample air pressure before they start. Some will also be able to continue sampling while they're running (by using two sensors) so they can adjust to altitude changes on the fly, others will just adjust based on that initial reading.