Better Fuel Economy with No Gas Cap - Why?

[waynesburgjay]

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.

Highest point in PA is Mt. Davis in SW PA @3213 ft. above sea level, I never had to adjust any car to run anywhere in PA

 

[Boggin]

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).

what kinda sleds?

 

[cammeddrz]

there is a barometric sensor. it automatically adjusts for the elevation

correct me if i am wrong but as far as i was aware thats is what a MAP sensor was. Manifold Absolute Pressure and i'm pretty sure that the 1989 BII i had with a 2.9 had a map sensor

 

[fastpakr]

Yes and no. Both measure air pressure. By definition, a MAP sensor is designed to measure it in the intake manifold. However, if the engine isn't running it will measure barometric pressure..

 

[cammeddrz]

a MAP sensor is designed to measure it in the intake manifold

exactly why would an engine care what the pressure is outside the engine? that's why an ecm is called an "engine control module" and not an "outside-whole-world control module" either way barometric makes it into the intake manifold before the cylinders

 

[Psychopete]

exactly why would an engine care what the pressure is outside the engine?

The MAP measures the difference between outside air pressure and intake manifold vacuum. It needs to know both. On my MAF conversion, I had to disconnect the MAP from vacuum, but leave it there electrically, for altitude.

Read this:
http://www.aa1car.com/library/map_sensors.htm

 

[pillen140]

I would take a guess.... Highway gas mileage vs everyday city driving? Would result in more miles per tank. Fuel cap probably had nothing to do with it.

 

[adsm08]

exactly why would an engine care what the pressure is outside the engine? that's why an ecm is called an "engine control module" and not an "outside-whole-world control module" either way barometric makes it into the intake manifold before the cylinders

Non-MAF systems are also called "Speed/Density" systems.

It needs to know the temp of the air and the volume that is coming in to calculate the oxygen volume and content.

It needs to know the atmospheric pressure and the pressure/vacuum in the manifold and the difference between them to calculate that.

 

[kimcrwbr1]

I wonder if you could just disconnect the battery after you get to a different elevation and let the computer set new values for the difference in barometric pressure

 

[Psychopete]

I wonder if you could just disconnect the battery after you get to a different elevation and let the computer set new values for the difference in barometric pressure

It should happen dynamically. Like people going up or down steep grade mountains, the computer should be able to compensate for the air density without them having to worry about the engine running poorly. Never really had a reason to kill power to a PCM, usually once something is resolved, it's good to go (stored codes can be erased in the CM portion). Plus when you kill power and let the settings go, sometimes the computer can learn the wrong values from bad sensors making the problem that much harder to find.

 

[OilPatch197]

I would take a guess.... Highway gas mileage vs everyday city driving? Would result in more miles per tank. Fuel cap probably had nothing to do with it.

Or he could be going downhill more and getting a better mileage, where he goes uphill his mileage drops because of gravity.

So we need to know how many miles he drives flat, and incline, decline acceleration, deceleration, how often he uses the brakes, and how much brake pressure is used, how much time is idling. We also need to know his speed and momentum, the grade of the inclines and declines and some mathematician to come up with a formula to work out actual gas mileage.

 

[kvnschmdt]

As I said in the original post, I discovered this weird thing of better milage with no gas cap while on a very long road trip (around 6000km) all on a highway, travelling about 100km/h average. Driving on the highway, regardless of elevation, or whether I was going up a mountain range or coming down the results were very consistent - 300km to a tank with gas cap on and 400km to the tank with it off.

Of course once I discovered that I got better mileage with the cap off, I left it off. So I only used two tanks of gas with it on and probably about 12 with it off.

This many tanks over various terrain should rule out factors such as braking/elevation/uphill/downhill. There has got to something weird going on with my fuel system venting or pressure. Any more guesses?

 

[Shran]

Well the vent system is not very complex. You have a check valve held in place by a rubber grommet on the top of the fuel tank, and a line connected to that which runs all the way up to the vapor canister under the hood. There is one other line connected to the vapor canister which runs up to the upper intake manifold. So if it is in fact clogged, it will be somewhere in there.

 

[adsm08]

Or he could be going downhill more and getting a better mileage, where he goes uphill his mileage drops because of gravity.

So we need to know how many miles he drives flat, and incline, decline acceleration, deceleration, how often he uses the brakes, and how much brake pressure is used, how much time is idling. We also need to know his speed and momentum, the grade of the inclines and declines and some mathematician to come up with a formula to work out actual gas mileage.

Yeah, there are a lot of variables, but let's not turn this poor kid's difficult question into something simple like 400-level college physics.

 

[Ranger Kip]

what kinda sleds?

1998 Arctic Cat 440, both of em are. Can hit 70mph easily. Fun to jump!