so i am takin some logs and i see that when i crussin about 50-55 at 2000-2200 my mVs are from 480-740 and it goes up and down, there is like a pattern to it........why is this?

 

Its just the ECM correcting the fuel mixture. It can do it many times a second. The oscillating voltage is just the oxygen sensor swinging over the lean/rich threshold at 14.7:1. The ECM is just averaging 14.7:1. It may vary from 14.5-14.9:1 or more. The cat has to have the swing or it won't work correctly.

 

so this is normal then? im just tryin to figure out if im ok to drive on the freeway and around town as long as i dont go WOT, i did a 2nt gear wot a while back and talked to a few people and i was runnin vary lean, need to start tunnin soon i think im goin to do the tunnercat rt and ostritch.

 

A better check would be to log the Block Learn Mode data coming out of the serial data on the ECM. 128 is dead on. 108 is lean, 148 is rich, the closer to 128 the better at part throttle.
Edit- what I meant to say is that at 108 the ECM is pulling fuel and at 148 it is adding fuel.

 

It is entirely normal, expected, and proper.

The o2 sensor is ... an oxygen sensor.

it has high voltage (roughly greater than 550mV or so) when it does NOT sense oxygen in the exhaust.

It has low voltage (roughly less than 500mV or so) when it DOES sense oxygen in the exhaust.

For combustion efficiency, smog purposes, etc. on a stock smog engine, the goal is to run the engine such that you put in just enough fuel to burn all the available oxygen - no more, no less.

So far as the o2 sensor is concerned, there is roughly two states of operation:

low voltage, meaning oxygen in the exhaust - this tells the computer that it can add more fuel because their is excess oxygen.

high voltage, meaning no oxygen in the exhaust - this tells the computer that it needs to cut a bit of fuel out, because there is no excess oxygen.

If you quickly go back and forth between these two states, you get an "average" of just enough fuel for the amount of air you have. this middle point (where the amount of fuel and the amount of air perfectly balance and use each other up with no excess of either component) is called the stoichiometric ratio, which is a big fancy chemistry word meaning you have a 'perfect' mixture (not too much of either component). For gasoline and air, this is roughly 14.7 parts air to 1 part gas.

The computer can't maintain this perfect mixture, it has no way to sense perfection. However, it CAN sense imperfection - richer than 14.7:1 and leaner than 14.7:1. The oxygen sensor is pretty darn good at that.

So, the computer constantly toggles back and forth between "a tiny bit rich" and "a tiny bit lean" based on the input of the O2 sensor.

Just because you see an instaneous momentary value of 200mV does not mean you are "bad lean" nor does an instaneous momentary value of 900mV mean you are "flooding rich".

This is a simplified answer but it should serve well to explain the basic principal.

Next, check the stickies out and learn how BLM works. BLM works in conjunction with the O2 sensor's up and down cycling to see how far off your tune is.

 

Fast, 108 is rich and 145 is lean.....you got it backwards

 

you beat me too it.

as a point of interest when i am closed loop the A/F is erratic swings 14.0 or so to 15.5 or so. and the IAC seems to chase its tail opening-closing. subsequent rpms move around. when i run OL the idle A/F is stable at 13.8/1 and IAC never moves off 0. idle rpm somewhat stable 1000 rpms(just upped it from 800 and over did it).

 

Reduce the proportional gains.

RBob.

 

Say what you mean, mean what you say, I got it. Yesterday was a long day. I meant to say at 108 the ECM is leaning things out and at 140is the ECM is adding fuel, 108 is actually rich and 145 lean.

 

By altering the upper and lower limits of INT and BLM, can the ECM maintain closed loop at something other than 14.7?

 

Probably, but it'll fight you all the way. Best to change the O2 windows terms.

RBob.

 

If this is done, then the BLM and INT wouldn't need to be changed?

 

now my tunercat rt does have a value at stoich in constants. i have it set to 14.3 BUT running OL at about 13.8. TC confirmed i can dictate what A/F i wish to run but within limitations. so i will assume TC program does change the proportional gains otherwise known as 02 constants??