aNUT

Hey all, I just got back from a camping trip to Colorado and New Mexico, and had some pretty interesting findings as far as fuel economy.

First off, I have a 1992 4X4 pickup with the 3.0 motor and manual transmission. The motor is nearly new (6,000 miles), as I rebuilt it as soon as I got the truck last winter since the previous owner's son ran it out of oil. The rest of the truck however, has about 144,000 miles on it. I have been getting a repeated code 21 CEL (O2 sensor) and I have just been too lazy and unwilling to spend the $140 to fix it. The light stays off for a few weeks, and then comes on and stays on until I clear the computer.

My milage has been about 16.5 city, and 19 hwy. Everything is stock including the tire size. The truck is aligned, and I keep the tires at 32lbs. Sometimes it stumbles at low rpm when the accelerator is lightly depressed. (<2000rpm, first, second, or third gear, cruising around the neigborhood or comming gently off of stoplights.) I give it a little more gas and it gets up and goes.

So to the point of all this; At home in Austin, the elevation is about 500' as soon as I got into some elevation, around Amarillo and getting into New Mexico, the CEL went off, despite being on for several weeks (it had never shut off by it self before.) I also noticed that my fuel economy was better than tank.

As the terrain got more mountainous, and as the gas got crappier (87, to 86, to 85 octane) to account for the higher elevation (5,000 to 10,000'), I got better and better gas milage. From Trinadad to Durango, going over several passes, I got 24 mpg. That's pretty unheard of for a 3.0. The high milage persisted for the remainder of my time at altitude, averaging about 22mpg highway. The stumbling at low rpm also stopped, in fact, the truck ran smoother than it ever had.

As soon as I started comming down from altitude, the CEL came back on, code 21 as usual, and my milage dropped down to normal. The stumbling came back as well.

Additionaly I went back and looked at the previous owner's gas records from the time the truck was new to the point at which she gave it to her son (about 10 years worth of data) It appears that around Kerrville (similar elevation to Austin) where they live, the milage is consistent with what I got. I did find one trip to New Mexico they had taken, and the gas milage did increase considerably, consistent with my trip at elevation.

So my question is why? The truck had less power due to the decreased oxygen density, but it ran better. Has my bad oxygen sensor lost its ablity to tell the computer how much fuel to provide the engine except when the air is thinner? Do I need to adjust my VAFM so it lets less air in and the engine runs a little richer? -> this seem counter intuitive to get better fuel economy, but if cars didn't adjust mixture for altitude, they would run rich at higher altitudes due to the reduced air density. I don't think temperature is a factor since it ran smooth whether the engine was cold or hot in Colorado, and the truck doesn't apper to have better fuel economy when it gets cold in Austin.

I guess I will start by replacing the O2 sensor, since the computer seems to think its bad, and the owner's manual says to replace it at 120k miles. After that I think I will start fiddiling with the VAFM.

Any suggestion, hypothesis, etc. would be grealy appreciated though.

thanks for your patience,
-aNUT

Ironmike4x4

That sounds really odd to me since I would say the opposite is true. I don't know maybe you have discovered something new? I would have thought the lower the altitude then the more dense the air equals better performance and gas mileage.

4Crawler

iTrader: (3)

I too get better MPG up high. Thinner air has less aerodynamic drag, which is a but issue on my lifted 4Runner. But more importantly, the O2 sensor will detect the exhaust getting rich as the air thins. So the ECU cuts back on fuel to restore the 14.7:1 mixture. Less air = less fuel = less power, but better MPG on some cases. Since the high country often has lots of steep mountain grades, that can kill mileage, but get out in the high flat country and I find I can get 10% better MPG, albeit with maybe 10-20% less horsepower.

Try http://www.sparkplugs.com/ for a lower cost O2 sensor, Denso brand is the one to get.

Roadtripr

I live a sea level and also notice better gas milage when I have gone up to the Sierras. I have heard of people from Colorado complaining about their mpg when the went to LA, only to see it return to better mpg upon return to Colorado.

dave89144

I have a 01 Taco, 3.4/auto, with 265/75 tires. I moved to CO from Las Vegas on July 1. During my time in Vegas, I would average about 17mpg in the winter (crappy oxygenated gas) and about 18mpg in the summer (AC always on). Best I ever got was 19.5 on the highway.
I live at 5900ft altitude and drive everyday to work at 9000ft. When using 87 octane, I get around 21mpg... dropping to 85 octane I average 23mpg. On Tuesday I took a 200 mile road trip, 24.5 mpg.

aNUT

Thanks for the altitude info, seems to be consistent with what I got.

Any theories...I'm really not buying the thinner air giving less drag, at least not enough to make a 20% difference with all the other factor working against it. (lower octane gas, climbing mountains, etc) Sure, jet liners run higher altitues partially for thinner air, but thats 30 or 40 times the sea level elevation, not 5.

Also, no one has addressed, or offered anything as far as the smoother idling and take offs.

4Crawler

iTrader: (3)

Air pressure and density (and engine power) fall about 3% for every 1000' of elevation increase. Drive at 6-7000' and you have about 20% less air density and density is a factor in aerodynamic drag. If you have ever done any bicycle riding at high altitude, you can feel the difference in hill descent speeds. I once rode up to Tioga Pass in Yosemite (10,000') and you can just fly down even the slightest downgrade. Takes a while to get used to having to brake more than you would down lower.

And with less air pressure, you get less air in the engine, it is almost as if you were running a smaller engine, so instead of a 2.4L, it is like you had a 2.0L engine. So even at wide open throttle, you can't burn as much fuel as you could down lower. And with less air pressure, cylinder pressures are lower, meaning you can burn lower octane gas w/o pinging. Octane has little to do with power or fuel economy, assuming that the engine is not getting close to pinging such that the knock sensor backs off the ignition timing.
- http://www.straightdope.com/columns/041008.html
I run the 85 octane gas up high, no problems. Have even had some in the tank when I get back near sel level and it runs fine.

One other thing I find is that near sea level, it is quite easy to peg my air/fuel meter, having it go full rich at even 1/2 throttle in some gears and load conditions. Get up to 6-7000' and you can floor it and keep the A/F meter flipping around the 14.7:1 point, meaning the ECU is still running in closed loop mode, where it is probably most fuel efficient.

Not sure on the O2 sensor issues you are having. Depending on what is wrong with it, the ECU may have been ignoring it, running more of an open loop mode based upon other sensors. Maybe that open loop mode was better suited to the higher elevations and when you got down lower it was worse. Or as you mention, maybe the sensor was able to function somewhat better up higher and you were able to run in closed loop mode and when you got lower it acted up and you went into open loop mode. Lots of good reading on how the ECU and sensor work is available below:
- http://autoshop101.com/
Kind of interesting to read that info, then observe it in operation with the A/F meter. You can basically see the ECU at work as you drive.

geneius

Yup, the octane for cheapo-fuel here is 85 because you don't need that much to drive at this altitude. Can't say exactly why, but that's what I'm told.

As for mpg, my Sequoia does about 17-19 here but did 12-14 while I was in L.A. BIG difference, eh?

Ironmike4x4

Leave it to Roger to explain the difficult questions. Thanks for clearing it up for me

4Crawler

iTrader: (3)

As I explained earlier, less air pressure at higher elevations means less air inside the engine (assuming naturally aspritated engine), less air = less pressure in the cylinder and that lowers the octane requirement. With say 15% less atmospheric pressure it is like taking an engine with 10:1 compression (at sea leavel) and changing it to 8.5:1.