How does the S-AFC work in closed loop?

[marcmsj]

I was thinking about this the other day and i haven't really figured it out.

the afc works by changing this signal from the maf going to the ecu in order to trick it into thinking there is more or less air going to the engine. alright, that part i understand and is pretty simple. what i don't understand is how this would work in closed fuel loop. closed loop being the ecu takes readings from the O2 sensor and tries to maintain a stoichiometric ratio of fuel and air by either adding or reducing fuel. wouldn't that void any changes the afc would make during closed loop?

i hope i explained my question well enough to be understood.

 

[spyderturbo007]

Basically in closed loop the SAFC is just used to compensate for larger injectors. When you install bigger injectors the ECU has no idea that you have upgraded. It opens the injectors for the same amount of time that it would for the stock injectors (IPW). The O2 sensor then "sees" a rich condition and the ECU adjusts by not opening the injectors as long as it did before. This continues until the ECU runs out of adjustment which results in a DTC for the FT's. If the ECU doesn't run out of adjustment the problem becomes that you no longer have 50% adjustment on the SAFC. This is because the FT's might be a -14% which will only give you a 36% adjustment on the SAFC.

 

[marcmsj]

what do you mean by "DTC"?

my point being, the ecu will always compensate one way or another with the O2 sensor. i understand the conecept of having to adjust the fuel for bigger injectors but let's say for example you get 550's... the ecu will use the fuel trim thinking it's the stock 450 injectors but then realize that it's running rich... so wouldn't it automatically use the O2 to lean out the mixture?

i guess my question would be better phrased as... how much can the O2 reading alter the fuel trim? is it 50%?

 

[DSM90AWD]

what do you mean by "DTC"?

my point being, the ecu will always compensate one way or another with the O2 sensor. i understand the conecept of having to adjust the fuel for bigger injectors but let's say for example you get 550's... the ecu will use the fuel trim thinking it's the stock 450 injectors but then realize that it's running rich... so wouldn't it automatically use the O2 to lean out the mixture?

i guess my question would be better phrased as... how much can the O2 reading alter the fuel trim? is it 50%?

Your 1G 91-94 ECU has "Fuel Trims" for low/med/high airflow values. In closed-loop operation (reads O2 Sensor feedback) the ECU can correct rich/lean conditions at each of these three Trim levels +/- 40% given that your car was running at a perfect 100 before adding injectors (highly unlikely). So once your ECU get to this correction limit it cannot further adjust fuel anymore.

Enter the S-AFC which allows you to alter the airfolw amount the ECU reads so can artificially extend the range of fuel compensation (i.e. -% SAFC values tell the ECU less air is being passed at a given RPM/Trim Level so removes -% of fuel).

Bunch of good info in this S-AFC Article

 

[marcmsj]

Your 1G 91-94 ECU has "Fuel Trims" for low/med/high airflow values. In closed-loop operation (reads O2 Sensor feedback) the ECU can correct rich/lean conditions at each of these three Trim levels +/- 40% given that your car was running at a perfect 100 before adding injectors (highly unlikely). So once your ECU get to this correction limit it cannot further adjust fuel anymore.

Enter the S-AFC which allows you to alter the airfolw amount the ECU reads so can artificially extend the range of fuel compensation (i.e. -% SAFC values tell the ECU less air is being passed at a given RPM/Trim Level so removes -% of fuel).

Bunch of good info in this S-AFC Article

ok thanks, that explains things.. i just didn't know if the ecu had a compensation range for the o2 adjustment or if it was limited (40% according to you)

but that does answer my question, thank you.

 

[psychlow]

DTC = Diagnostic Trouble Code (check engine light.)

 

[marcmsj]

DTC = Diagnostic Trouble Code (check engine light.)

ahh ok.. DTC == CEL

 

[psychlow]

ahh ok.. DTC == CEL

Heh - do you program C++ by chance?