FAQ: Air Fuel Ratios explained

[philipa_240sx]

Gasoline burns ideally at a stoichometric ratio of 14.7 parts air to 1 part fuel. This is sometimes referred to as the air/fuel ratio. This ideal ratio provides enough air for all the fuel to burn completely. In a modern fuel injected engine, the air fuel ratio is tightly controlled for emissions and engine performance. This accomplished in 2 ways:

Closed Loop Mode:

This mode active when you are cruising or the engine is under light load. Initially the ECU injects the amount of fuel needed according to a calculated value based in the mass airflow sensor, engine rpm ,etc. The ECU then reads the O2 sensor located in the exhaust and fine tunes the amount of fuel injected to keep it at the ideal 14.7:1 ratio.

Open Loop Mode:

The mode is active under acceleration and is a calculated value based in the mass airflow sensor, engine rpm ,etc. to provide optimum performance. The O2 sensor is ignored. Typically the engine has to be run richer (more fuel) during acceleration.

Long Term Fuel Trim:

The ECU continuously monitors the O2 sensor and makes small adjustments to the calculations above. This is done to compensate for climate, engine wear, operating conditions over long periods.

Effects on fuel economy:

The Long Term Fuel Trim can vary the amount of fuel injected by as much as 15%. If you happen to get a poor tank of fuel or have an engine problem, it may result in an increase in fuel consumption. Normally it will work itself out by switching to a better quality fuel (you don't need to switch octane rating, just use better name brand fuel) or correcting the engine problem. It may take a few tankfuls of fuel before the adjustment is made. To speed the process up, some suggest clearing the ECU's memory. This can only be done with a Scan Tool or by a dealer. Simply disconnecting the battery will not work.

PLEASE NOTE:

Clearing the ECU memory may not magically improve your fuel economy. In certain cases it may make a difference. In some cases it may make fuel economy worse since the ECU has to 're-learn' the long term fuel trim values.