Debunking fuel-economy myths

[jimmy101_again]

Originally Posted by Malmensa

Pretty sure this would depend upon the type of fuel injection system, how metering is implemented, type of transmission etc.

A partially blocked air filter restricts incoming air flow, a similar effect to limiting throttle opening. A vehicle with a very dirty filter at full throttle might have a similar air flow to one with a clean filter and 3/4 throttle. Thus it wouod just preform poorly. HOWEVER, most engines measure throttle position, and make some use of that information to determine how much fuel to inject, or what gear to be in. Most modern cars are closed loop and will determine from O2 sensor info how much fuel to meter, but some older cars have no O2 sensor. In a worse case scenario fuel requirements would be determined just via throttle position, so with a blocked filter, way too much fuel would be delivered. This, along with being in a sub-optimal gear could mean a noticeable increase in fuel consumption.

A partially clogged air filter will act as an obstruction in the air flow requiring more energy from the engine to suck air through the clogged filter. This would be independent of whatever the engine control system does to keep the fuel and air properly balanced and will (probably very slightly) reduce engine power per unit of fuel. (A car engine is basically an air pump that pumps air from the front of the car to the back. )