Few simple Lexus AFM questions

[Suprapowaz!(2)]

I'm in the beginning of tuning a piggyback to appease Toyota's Vf to satisfaction. Am I supposed to tune it to 3.5v, or 2.5v?
Another is I understand that the stock ECU will remain in closed loop in low boost conditions. At what point does it switch over to open loop? When it does will my voltmeter flatline at 5v at that point?

 

[Nick M]

It is probably more accurate to call open loop the default on these older cars. If you are not in a steady cruise, steady non boost acceleration or idle when warm, you will be in open loop.

 

[Suprapowaz!(2)]

Thank you. So is it better to tune for 2.5v, or 3.5v in close loop? I don't want the stock ECU to kick out my tune.

 

[jetjock]

Doesn't really matter. 2.5 is optimal but 3.5 wouldn't be a problem.

 

[CyFi6]

The WB is probably slightly off. Not the best instrument to use for checking stoich.

Either that or the engine really is a bit lean. What is the average voltage of the NB taken at Ox or Vf over a few minutes at a steady 2500? Unless it's 450mv or 2.5 vdc the engine is not at stoich even though cross counting is occurring. This is also a way of checking for small vacuum leaks.

That's interesting, I was under the impression that the ECU would only cross count if it was able to reach stoich within its fuel trim correction. In my case with engine speed increased to 2500 my VF (showing trim without T/E1 jumped) is 2-3v and wideband shows cross counting, but you are saying its still possible I am not exactly at stoich? I will give the voltage a check.

I was going to say the WB and stock sensor don't know what 14.7 even is. That is interpreted data.

This isn't consistent with the documentation. What you are saying is that the narrowband sensor is linear in voltage according to A/F ratio, but that isnt the case. It appears that there is a very dramatic change in o2 sensor voltage exactly at the 14.7 mark.

 

[Nick M]

The oxegen sensor and A/F sensor do not put out anything that says what the A/F ratio is. It puts out a potential, that you need to interpret. That is what I am saying.

 

[grimreaper]

Something usefull, this is why one WB gauge can be used with different fuels. The signal is the same regardless of fuel type. The scaling to the desired display is what changes

 

[CyFi6]

The oxegen sensor and A/F sensor do not put out anything that says what the A/F ratio is. It puts out a potential, that you need to interpret. That is what I am saying.

Ok well that is consistent with any other sensor on the entire vehicle, it's not like the coolant temp sensor tells the computer a temperature or the oil pressure sensor tells it the oil pressure in psi, all sensors feed the computer a signal that has to be compared to a calibration and interpreted, obviously the o2 sensor is no different.
My point is that the narrowband sensor has a distinct extreme change in voltage from high to low right around the gasoline stoich of 14.7:1, so how can the sensor switch around anything other than 14.7:1 (besides a faulty o2 sensor).

 

[Nick M]

My point is that the narrowband sensor has a distinct extreme change in voltage from high to low right around the gasoline stoich of 14.7:1, so how can the sensor switch around anything other than 14.7:1 (besides a faulty o2 sensor).

Let's establish some things you already know, but maybe are not thinking about.

The switching is from closed loop. The switching is to keep the catalyst in the proper operating range. The computer runs rich and lean on purpose for that catalyst. The only way to switch the sensor would be if the sensor where built different so the difference in oxygen atoms in the sensor was differnet.

 

[3p141592654]

The sensor measures oxygen in the exhaust stream and the reference is the ambient side of the sensor. When the mixture is rich, there is oxygen demand on the exhaust side and when lean there is an oxygen excess. At Stoich there is no free oxygen in the stream. The sensor basically read O2 excess or demand, which is then used by the ecu to compute the stoich point. The ecu constantly ramps up and down its computed stoich point at a controlled rate. Typically 4% per iteration in our ecus. Idle is a little different as the catalyst need to be managed differently.

(hmm: init post was a mess, I blame my browser)

 

[Insidious Surmiser]

from what I understand, tampering w/ AFM scaling is dangerous because it skews your timing maps. one of my engines fried running a SAFC w/ a conservative tune on it (12 psi @ 11.2:1 AFR on a ct26)

by fried I mean one of my pistons melted.

 

[CyFi6]

Lol let me simplify my original question. If the O2 sensor must be richer than 14.7 to give a high reading (o2 demand), and leaner than 14.7 to give a low reading (o2 excess), how can the ecu "cross count" without ever having gone leaner than 14.7?

My case is pretty simple, my wideband shows switching or cross counting happening by the needle fluctuating up and down, but it is fluctuating above and below approximately 14.3 (swings from 14.1 to 14.4).

Unless it's 450mv or 2.5 vdc the engine is not at stoich even though cross counting is occurring. This is also a way of checking for small vacuum leaks.

Just help me understand this. How can you have cross counting without going above and below 14.7 given how the o2 sensor output operates?

http://forums.pelicanparts.com/uploads8/O2+sensor+voltage1150420738.gif

 

[877MGTEGUY]

ok so i am new to foreign cars i have a 87 supra and wondering what is a lexus afm what does the afm stand for and what year and motor does it come off of

 

[Insidious Surmiser]

air flow meter off a 1uz

 

[Nick M]

AFM= Air Flow Meter

 

[3p141592654]

Just help me understand this. How can you have cross counting without going above and below 14.7 given how the o2 sensor output operates?

You can't, but JJ was referring to the average mixture, not the instantaneous.

 

[CyFi6]

You can't, but JJ was referring to the average mixture, not the instantaneous.

I considered this as well, but if the ECU was able to get the mixture above and below 14.7 within its trim, why would the average be anything other than 14.7?

 

[Insidious Surmiser]

that is the question. mine always stayed DEAD on 14.7 (normal driving)

 

[3p141592654]

I considered this as well, but if the ECU was able to get the mixture above and below 14.7 within its trim, why would the average be anything other than 14.7?

Because the dwell time on either the lean or rich side can be different. Its only going to be 14.7 if the times are the same, and the two slopes are identical. As noted, an air leak can cause the two slopes to differ, as can other things. Also, at idle, the ECU purposely makes the slopes different.

 

[CyFi6]

Because the dwell time on either the lean or rich side can be different. Its only going to be 14.7 if the times are the same, and the two slopes are identical. As noted, an air leak can cause the two slopes to differ, as can other things. Also, at idle, the ECU purposely makes the slopes different.

The only part I don't get is why the dwell would be higher on one side than the other if the ECU has the ability to make them equal. For instance if you have an air leak, momentarily the O2 sensor would read a low voltage, but as the trims come up to compensate and the ECU is able to get the mixture richer than stoich, why wouldn't the ECU correct the mixture exactly back to stoich (equal amount of time on rich and lean of stoich). All of this I am referring to off idle where desired is equal on each side of stoich, as I am not sure what the ECU/cat likes at idle.