This is true if the amount of fuel delivered to the engine is constant because the air/fuel mixture will become rich, i.e., too much fuel relative to the amount of oxygen.
Your vehicle has a sensor that determines how much air is entering the engine and will adjust the amount of fuel to maintain the correct air/fuel mixture.
It is normal for the inside of the exhaust pipe to become black over time. While the emissions controls do a pretty good job of removing tailpipe emissions, they do not remove all of them. Mixed in with the exhaust gases are hydrocarbons, which are unburnt fuel in the form of particulates, or soot. The particulates eventually coat the inside of the exhaust pipe, giving it a black appearance.
If your engine was running rich, your car would not pass any periodic emissions test. Assuming that your car has passed emissions tests, then the electronic fuel injection system is functioning properly.
As an aside, if you were to add additional intake air through the use of a fan, turbocharger, or supercharger, the engine control module will sense the additional air and command additional fuel, so although your car's performance may improve, your fuel economy may get worse unless you are very careful to apply less throttle.
Not with a correctly functioning EFI system. All consumer fuel injection systems are designed to adjust the fuel air ratio to perfect stoichiometry for gasoline.
Then your engine is performing at normal standards, that is good.
Perhaps a bit, but nothing abundant, and as you mentioned, it passes emissions.
No, it really does. They are designed and tested to do so.
Or, waste a lot of money on some after market rigged system that isn't going to address your problems.
You may want to consider selling it and buying something that is more suited to the power and MPG targets you are looking for, perhaps a car that is supercharged or turbocharged. But, of course, it's your call.
Does that need any special adjustment for the altitude?
Is there a way that I can lean that setting out but adjusting it?
Oh ok.
Like I said, not only passed them but 2-3 times at least, the standards.
I see.
Here's another question;
Back during the oil & gas crisis of the 70's, people were adding vacuum gauges to their cars. It would be mounted behind the steering wheel on the dash.
They had green, yellow and red zones, so you could lighten up your foot and get better MPG.
Is there any point in using something like that nowdays? Do they still sell them for that?
Back when engines had carburetors, you could change the carburetor jets for high altitude applications to maintain the proper air/fuel mixture of 14.7 parts air to 1 part fuel. A modern fuel injection system, which your Corolla has, does this by controlling how long the fuel injector sprays fuel, so at higher altitudes, the injector sprays less fuel than at sea level for a given throttle position.
There is no way for you to lean out the setting because there is no need to. The system is programmed to maintain the 14.7:1 air/fuel ratio, which supports complete combustion of the fuel. If you were to lean the adjustment to, say, 16:1, the fuel would not burn as completely so you would be wasting fuel by sending unburnt fuel out the exhaust, and to make things worse, you would have to depress the throttle more for a given amount of acceleration or cruising, so fuel economy would go down even more.
If your goal is to improve fuel economy, then modifying the air/fuel ratio is not the answer because doing so will make fuel economy worse.
If your goal is to improve fuel economy, then the same things that work at sea level will work at altitude. Try these simple things that many people overlook:
1) Use either Denso or NGK brand spark plugs, ignition wires, distributor rotor, and distributor cap that are recommended for your car and stay away from other brands. Some other brands may work fine, but many do not so rather than experiment, stick to Denso or NGK.
2) Check ignition timing to make sure it is within factory specifications, probably around 10 degrees before top dead center.
3) Make sure the engine air filter is clean.
4) Rather than buying the cheapest gas you can find, stick to a name brand and buy from a gas station that does a good sales volume to reduce the chances of sediment getting into your fuel system.
5) Inflate your tires to 5 PSI over the automaker's recommended tire pressure. This will improve tread life and improve fuel economy by reducing rolling resistance, with a slightly harsher ride.
6) Remove all unnecessary payload from the vehicle because the engine has to work to move that payload around. Even 50 extra pounds in the trunk can make a 1/4 to 1/2 MPG difference.
7) Make sure the engine thermostat opens at the proper temperature. If the thermostat is stuck open, missing, or opens at the incorrect temperature, the engine will take longer to warm up, and if the engine takes longer to warm up, it will run rich for a longer time.
A huge side benefit from a clean running engine is a very efficient engine. Mess with the engine and efficiency will probably be lost.
The principles behind using a vacuum gauge have not changed, so I suppose you could still add a vacuum gauge. I think most people could learn to drive more efficiently just by listening to the engine and changing their driving habits. The drawback to a vacuum gauge is that you are adding a potential source for a vacuum leak, which would hurt fuel economy.
Turn off any unnecessary electrical accessories like TV's, video games, unneeded fog lights, etc. and use the AC sparingly, if at all.
If your car has an automatic transmission, the sooner you back off the throttle, the sooner the transmission will upshift, and the sooner the transmission upshifts, the better the fuel mileage. When accelerating while the car is already cruising, depressing the throttle gradually will reduce downshifts. The adage that you can improve fuel economy by driving as if there is an egg between your foot and the gas pedal is true.
Lots of short trips where the engine has to warm up will hurt fuel economy, as will idling the engine for long periods. If the engine has to idle for more than a minute and you are not at a traffic intersection, shut it off.
The fuel economy that you posted is pretty good for a 14 year old car, especially at altitude. I would try the stuff I listed above and drive like the egg is there, and you may see a 1 or 2 MPG gain.
Ignition timing cannot be manually adjusted on a vehicle with a distributor-less ignition, but if the car has a distributor ( IIRC the 1993 Corolla has a distributor), then timing can be adjusted.
Timing is kind of adjustable on some engines with a distributor-less ignition by adjusting with the crankshaft position sensor.
Vacuum gauges have fallen out of favor with consumers because it is very easy to learn how to drive to maximize fuel economy without one. If you really want a vacuum gauge, you can get a diagnostic one and snake it into the passenger compartment and use it until you get the hang of driving without it, then put the gauge away in your toolbox.
Your car does not have points or a condenser, which is why I did not mention them under #1 above. The function of the points and condenser is handled electronically by the igniter (which does not need routine replacement).
I knew you could still get vacuum gauges, although I don't really see a reason to install one unless the driver is not disciplined enough to drive with a light foot.
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