The way the o2 sensors won't over come the MAF reading, without remapping the ecu, is if the load calculation is based off the MAF readings and not by a separate MAP sensor. In this case, adjusting the MAF resistance changes the load value at a given rpm, which in turn will modify the target AFR.
Yes I understand that. The change in resistance will give a false reading for the air mass flow and the ECU will set the A/F to a different position (target) on the map. I.E. the ECU thinks the engine is operation at a different load that it really is. But isn't the roll of the O2 sensor to correct for that? It's looking at the exhaust and seeing that the chosen AF isn't correct and telling the ECU to change the fuel delivery.
I mean, the ECU gets reading from the MAF, RPM sensors, Temp sensors, Throttle position sensor and from those it chooses what the fuel delivery is from the engine map. But the roll of the O2 sensor to provide feedback, by looking at the actual composition of the exhaust and telling the ECU, " I know you think you have set the correct conditions, but your wrong. You need to cut/increase (as required) fuel delivery." If not, why would you need LTFT at all? LTFT is basically telling the ECU that for the condition of the sensors and engine the engine map is wrong and the LTFT is the correction to be applied.
I'm really trying to understand where my think is wrong, if it is, or if it is wrong. For example, if everything is to spec and the MAF tells the ECU that X lbs/sec of air are entering the engine then the fuel map would say inject Y lbs/sec of fuel and the AF ratio would be at the target, X/Y and, assuming the engine is in spec, the exhaust gas should be ok and both LT and ST fuel trim should be zero. But if I dick with the MAF so that it tells the ECU that X lbs/sec of air are flowing when the actual air mass flow in (X-z) then it still injects Y lbs/sec of fuel. However the actual AF is now (X-z)/Y and it's rich. So how will the O2 sensor not see that in the exhaust and not tell the ECU it should cut the fuel flow rate, leading to a negative value for LTFT?
The thing is that according to all the diagnostic procedures I have read excessive LTFT, + or -, can be an indication of a bad MAF. And, other than complete failure, an MAF can only go bad by incorrectly reporting the air mass flow to the ECU and the LTFT is what compensates for that. So how any distinction between an actually erratic reading MAF and one that has been intentionally modified can be made by the ECU? It can't know if some potentiometer in the circuit has sifted resistance due to circumstances or intentionally. At this point the only way I guess I could satisfy myself is to mess with my MAFs and we what happens to LTFT, but I'm not about to do that. Well, maybe on my Toyota truck.