How can you have a rich or lean condition?

[ssbtech]

Doesn't the ECU compensate for too much or too little air so that once the mixture is in the chamber it's the right AFR?

[bnguyenbb6]

In closed loop the ECU checks the front O2 sensor to determine the afr and compensates. In open loop, it just does what its told: X amount of air seen, provide X amount of fuel. Now if you have a leak, X amount of air may not be the right amount.

 

Typically if your maf is in the stock location:

Pre turbo leak= lean

post turbo, pre throttle body= lean in vacuum, rich in boost

Post throttle body leak= lean

[underground000]

the ecu can only to so much and its also programmed to do so much. if everything is stock and everything works fine. then it is very capable. once you start doing things or adding things 'normal' people wont do then thats when stuff gets weird. making it be able to control everything means paying the programmers and researches more money for scenarios most cars wont see

 

if your also talking about a aftermarket air intake

the spt has shown to cause turbulence around the maf which makes the maf readings off

[iNVAR]

In closed loop the ECU checks the front O2 sensor to determine the afr and compensates. In open loop, it just does what its told: X amount of air seen, provide X amount of fuel. Now if you have a leak, X amount of air may not be the right amount.

 

this is only partially correct. in open loop, trim D is applied. and it's the REAR O2 sensor that it checks.

 

edit: actually, checking the front/rear sensor thing. might actually be the front in which case i take back what i said.

edit 2: yep, sorry, it's the front sensor that primarily affects long term fuel trims but the rear sensor has some minor input in closed loop as well.