I am a little confused on how the map sensor and o2 sensor work together to adjust air/fuel ratio. I know that the honda ecu can make adjustments based on the feedback from the o2 sensor, but how does the map sensor come into play? Thanks for clearing it up for me.

 

the map sensor along with almost every other sensor will give a certain reading to the ecu and cross reference its reading with practically every other sensor on a car to give it its desired map

 

the ecu uses the map sensor to lookup the base value of how much fuel to inject. the map sensor tells the ecu how much load is on the engine. the higher the load, the more fuel it needs.

the o2 sensor is a feedback device. the ecu looks at the o2 sensor to determine if its injecting too much fuel or too litte based on what values its looking up in the fuel map.

the o2 sensor reports whether the engine is too rich or too lean, and the ecu bumps the value of the Fuel Trim to compensate for this. it then multiplies the base value from the fuel map by this fuel trim value, and this allows it to compensate for changes in the car / atmosphere/driving conditions.

when i made my fuel map for 310cc injectors, i got it pretty close, but i was still too rich at idle and part throttle. the o2 sensor indicated too rich and the ecu used this info to change the fuel trim value to a negative number, so the ecu was always injecting a little less fuel than what i put in the base map, and so my engine runs fine even though my maps were a little off.

same thing would happen at higher elevation (thinner air) or if it got cold out etc.

so map sensor - main fuel value from fuel map

o2 sensor - feedback for small fuel adjustments over time.