What is open/closed loop mode? Why do I need the Air Fuel Meter?

 In a closed loop mode the ECU monitors the oxygen sensor output to adjust the amount of fuel required to maintain a perfect mixture, which will lead to the the best fuel economy and minimal smog pollution. The perfect mixture for gasoline engines is 14.7:1 and the equivalent oxygen sensor output is 0.5 V. The transfer function between rich and lean mixture is really steep. This is why the air/fuel meter readings will fluctuate from 0.3V to 0.7V in a closed loop operation.

At a high load (acceleration), the ECU ignores oxygen sensor output and will switch to the open loop mode using default maps to determine the amount of fuel needed for a certain amount of air. The engine will run rich at a high load and the oxygen sensor output will be in a range of 0.85V to 0.96V. The higher the voltage, the richer the mixture. The oxygen senor output monitored on the Air Fuel Meter  will indicate if the engine is running lean under the load, which is very important to keep  the pistons and the rod bearings happy. The open loop maps are usually poorly optimized for performance driving since the car manufacturer must take into account all manufacturing tolerances. Adding any performance modification to your car will affect the mixture so it needs to be monitored. The Air Fuel Meter is a must for any tuner!

Use the following table for O2 sensor voltage versus AFR.

Happy turbocharged engine likes to be above 0.92 under the load. Naturally aspirated cars can get away with a little leaner mixture.

Stock narrow band O2 sensor is temperature dependent. If you are running closer to the edge and looking for fast responding and temperature compensated air/fuel ratio measurements you may want to look into the Wide Band Air Fuel Meter. Click here for Wide Band details.

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