Letter Of The Month
Fluid Changes for Better Mileage
Question: For the life of me I do not understand how changing the oil and air filter can improve your gas mileage, particularly the air filter. With vehicles running air mass sensors, the mixture remains controlled by the computer. Air restriction by the air filter would limit horsepower and might actually result in higher mileage, if I am not all wet.
Roger Russ
Ottumwa, IA
Answer: Well, let's look at it in a couple of ways. First off, not all vehicles on the road are fuel-injected. There are quite a few of us who still use carburetors. When the air filter gets restrictive and less air can flow, the air/fuel mixture will become richer. A classic example of an engine running on a rich fuel mixture is when the choke is fully or partly closed.
Something that a lot of people don't understand is that the air/fuel mixture is rated in pounds of fuel and pounds of air used. Gasoline fuel weighs in at about 6.84 pounds per gallon and takes up about 0.13368 cubic feet per gallon. Air weight can change due to density, but you can figure on about 1.2 ounces per cubic foot. You can do the math if you like, but it takes a whole lot of air volume being sucked into an engine to make it run. For instance, my carbureted 383 Chevy engine will suck in something like over 600 cubic feet per minute in the upper rpm range.
The point I am trying to make here is that there is a lot of air flowing through the air cleaner. This applies to a carbureted or fuel-injected engine, and like you say a fuel-injected engine has a MAF (Mass Air Flow Sensor) that sends a signal to the computer that in turn tells the injectors how much fuel to let in. So in theory, the less airflow, the less fuel flow. However, there are other things that come into play, like the percentage of load on the engine and the throttle position. Now with less airflow, the engine will produce less horsepower at a given engine speed. However, the Electronic Control Unit (computer) receives readings from quite a few sensors, such as the coolant temperature sensor, throttle position sensor, oxygen sensor(s), engine speed sensor, and says something like "I am having problems making the amount of horsepower that I need to maintain this given amount of speed. I need to correct this." How does it do this? By increasing the air/fuel mixture to a richer degree, hence more fuel is used.
A dirty air filter can affect the computer's ability to adjust for correct air/fuel ratio. In a mass airflow system, a partially blocked air filter would obviously let less airflow through. The MAF sensor will read less airflow, the computer will adjust the Injector pulse width to gain the optimal A/F ratio. This is called fuel trim. The catch is a computer can only trim to a point. After that the computer is at its limits and performance will go down. The same effects can happen with a vacuum leak after the sensor. A speed density system works about the same way but uses the MAP (manifold absolute pressure) sensor to measure manifold vacuum. A plugged air filter will cause a change in manifold vacuum and again fuel trim can only adjust so far.
It would be nice if I could go into some engineering jargon, and give you a whiz-bang answer, but my shade-tree education just won't produce that, so I gave you the simpler approach that I hope is a sufficient answer.
About changing the oil. I am not sure what you mean this in reference to, but changing the oil can have an effect on fuel mileage. That is one of the reasons that a lot of late-model engines specify 5W-30 oil. It's proven that up to a 5-percent gain in fuel mileage can be gained by using this lighter oil instead of say a 15W-40. It's a fact of physics that a thinner oil will flow easier and faster at a given temperature than a heavier weight oil will. This means that the oil pump will have to do less work and there is less internal friction, or parasitic drag, in an engine.
Oil companies are constantly updating the quality of the oils. However, that does not mean that you can run out, buy some 5W-30, and use it in an '88 Chevy. Late-model engines have much tighter clearances on all moving parts to allow the new style oils. If you tried to use it in a motor that was not so designed, the oil pressure would be quite low and in some cases the engine could be damaged.