Upstream vs Downstream O2 Sensors

[Hi-psi]

Ok, so, my understanding is that the downstreams are for the most part there to measure the effectiveness of the emissions control components and that the upstreams are primarily the values used for drive-ability, air/fuel management.

Where I'm going with this is as I gradually work through little job after little job on the '08 Denali I picked up back in March, coming up soon I was thinking of replacing the upstream oxygen sensors.

For the most part, it runs/drives fine, no codes. It does have an occasional stumble at idle. I've read a bunch of people saying that's just a tendency of the 6.2, others saying that a working, but lazy oxygen sensor can cause it. I also will sometimes get a little bit of part throttle under heavy load detonation. It isn't a fuel quality issue as I only run 93, and only from top tier brand/stations. Fresh plugs, wires, air filter, new fuel pump, etc, so, to me, it's got to be a fuel delivery issue.

Is it better to just replace all four sensors or would I likely see whatever benefit (if any) that I would get by just replacing the upstreams?

Just interested to hear anyone else's thoughts or similar experiences.

 

[swathdiver]

Robert, if you want to keep good tabs on the condition of the cats, make sure those downstream O2 sensors are working good. You can use a good scan tool to graph them in action and determine if they're doing what they're supposed to do within spec. I was all set to replace mine but they are all performing perfectly.

I'd chase that stumble down though if I could.

 

[Big Mama]

My experience on vehicles the family brings me to fix is chasing the code. I replace The upstream then another code and so on. Follow James’ advice

 

[the_tool_man]

You are correct. Upstreams are used in closed loop to control air/fuel ratio. Downstreams monitor catalyst performance by comparison with upstream output. I just had my downstreams turned off in a tune. It didn't hurt a thing.

My upstreams are due for replacement. Indeed, my tuner monitored them while driving, and found they weren't quite performing to spec. I also have the occasional tremor at idle. So I'm hoping new O2s help. Do keep us posted on what you find with yours.

 

[swathdiver]

I just had my downstreams turned off in a tune. It didn't hurt a thing.

But no way of determining the health of your cats. I suspect that their time remaining aboard your vehicle is short lived though!

 

[SnowDrifter]

What sort of a stumble at idle? Like a 1 or 2 cylinder misfire every few seconds? I have the same on my 5.3. Graph your O2 sensor data. I've found it trends with then the car's ECU bounces to lean. I suspect it's because the factory injectors have a fairly wide tolerance as far as what's deemed acceptable as I've read that a set of balanced injectors tends to smooth the idle.

 

[Hi-psi]

What sort of a stumble at idle? Like a 1 or 2 cylinder misfire every few seconds? I have the same on my 5.3. Graph your O2 sensor data. I've found it trends with then the car's ECU bounces to lean. I suspect it's because the factory injectors have a fairly wide tolerance as far as what's deemed acceptable as I've read that a set of balanced injectors tends to smooth the idle.

Precisely. No codes, no misfires. I've watched the individual cylinder misfire counts live and there's nothing. It's just a very random, very light stumble/miss.

This coupled with the random part throttle under load detonation I get from time to time had me also thinking injectors.

Detonation is typically due to too much timing, low quality fuel, or not enough fuel. It will exhibit the random detonation on the stock tune, the Diablo 91, and the Diablo 93, but WOT is fine. I only run 93, and from a top tier brand/station, so what that leaves is not enough fuel. As far as I know it's the original stock injectors, so with 200k+ miles I'm thinking maybe one or two of them has a funky spray pattern or just isn't optimal. On the list eventually is to replace the injectors. Every 3-4 tanks I run a bottle of Techron through it, so any improvement from cleaning them in place is done.

For now, after seeing some mentions here and there about O2 sensors with a lot of age/miles on them not throwing codes, but just being a little "lazy" in their readings, and replacing them resulted in smoother and better running, I though perhaps my next little job would be that.

 

[iamdub]

I have a random light stutter (what may be referred to as a "miss") at idle sometimes.

*In for results of fresh O2 sensors.

 

[kbuskill]

I have a random light stutter (what may be referred to as a "miss") at idle sometimes.

*In for results of fresh O2 sensors.

Same here... keep us posted.

 

[hagar]

Start by cleaning your Maf with some electronics cleaner, then scan your upstreem 02 sensors and look for a difference between the two in positive or negative fuel trims. If you get an exhaust leak pre 02 sensor for example, one side with be showing more positive fuel trims than the other side. Same if you have a vac leak that is only on one side of the engine.
It doesnt take long for someone qualified watching the data to figure out whats wrong.