Upstream vs Downstream O2 Sensors

[Hi-psi]

Hope you don’t put your issue. I know how annoying the little things are.

If you already have the sensors might as well put them in. Doesn’t the system go into open loop st full throttle runs? Wouldnt that take the sensors out of the equation... ?

I'm certainly no tuning guru... But, my (limited) understanding of it all says it isn't quite that simple.

For late model GM's from what I gather there are three modes essentially...

1. Closed Loop
2. Open Loop
3. PE (Power Enrichment) mode, essentially WOT although not technically.

When in closed loop mode, the O2 sensors are used for correcting the fuel mix, targeting a perfect AFR using a combo of your STFT and LTFT's. When in open loop mode, O2 sensors don't really have a say and the PCM uses some defined tables/conditions. When in PE mode, close to open loop operation, but different tables/conditions are used. As I understand if your LTFT's are positive (i.e the PCM generally thinks your engine is running lean) it can carry over into PE mode and impact the fueling table.

Now, all that said, if I think through this logically (and correctly), if my LTFT's are high that could be from any (or a combination) of the following:

1. Too much timing
2. Too little fuel due to worn out injectors not performing adequately
3. Too little fuel due to low fuel pressure (faulty regulator, weak pump, etc)
4. Old (or defective) O2 sensor(s) reading inaccurately
5. Low quality fuel

Now, you would think if generally speaking the PCM is thinking the engine is lean and throwing more fuel at it that it would not contribute to detonation at WOT or heavy load part throttle as you'd be running richer than needed, but I suppose that really depends on what the root cause for the PCM thinking you were running lean is/was.

O2's are relatively cheap, and easy to replace. Since I purchased my Denali used early last year, I have no idea of the age or condition of the existing O2 sensors. I battled a EVAP issue the first couple of months after buying it which was causing a rich condition, fouling plugs, etc, so that could have also impacted the health or functionality of my sensors. So, it's a good idea to go ahead and replace the sensors, see how it reacts to that. If it doesn't impact the behavior any, then I'll start working through what I feel like may be the other potential culprits.

Had no intentions of typing out a novel when I started, but just kind of kept rambling on. LOL 90% of this is probably completely off base. Hahaha...

 

[Hi-psi]

I’m curious as to what the OP trims look like at idle I can post mine as well.

I'll try to do an idle log today after work and post a vid.

Don't expect much excitement.... LOL Fuel Pressure, both banks STFT, both banks LTFT, and rpm. Started at idle, then after a bit ran it up to 1000rpm for a bit, then 1500, then back to idle.

 

[Hi-psi]

This thread has kind of morphed from the original intent I had when starting it...and that's ok. I also feel a bit of a need to collect all the relative details together in a bit of a summary as it feels a little frayed at the moment, largely at my own fault.

Ok, so, 2008 Yukon Denali, 200+k miles. I purchased it in mid March, '18. It ran ok, but not great, but I got it for a LOW price so I was ok with having to address some things. First thing is it was setting a P0455 EVAP leak code, fouling the plugs, and had a pronounced miss/stumble around 1500-2000rpm. So, in my efforts to address the EVAP issue, I ended up replacing the entirety of the EVAP system. Gas cap, Vent Valve solenoid, EVAP canister, purge valve, and the fuel pump assembly with an OE Bosh unit. EVAP issue solved. Next order of business were fresh wires (AC Delco) and plugs (NGK Iridiums). Around this same time I picked up a Diablo Sport i3 crazy cheap. I know a good tune from a reputable shop is better, not wanting to start a debate on that. Again, I picked it up really cheap and it gave me something to play with and tweak with. I had also by this point run about 2-3 cans of Seafoam and 3-4 bottles of Techron through the fuel system, and done one Seafoam oil treatment. Now, it ran pretty good at this point, but still would detonate audibly at WOT and occasionally under part throttle heavy load. This was on the stock tune. On the Diablo tune it would do it more severely, but outside of WOT and part throttle heavy load ran GREAT. I always run 93 from a name brand (usually top tier rated) station. So, at this point since it would detonate even on the stock tune I began my train of though I referenced in my above response. That it had to be due to lack of fuel, or maybe severe carbon build up. High miles, no clue how the previous owners drove it, nor the quality of fuel used. I figured the carbon build up wasn't likely anymore since I had run a fair amount of Seafoam and Techron through it. So, I had been leaning more towards lack of fuel, maybe either caused by tired, worn out original injectors, or after seeing a few mentions about lazy O2 sensors causing some problems, that. As mentioned earlier, I have two new upstream sensors ready to install once the weather cooperates, which hopefully will be this weekend. Now, in another thread on here I had seen someone mention an engine/fuel system cleaner called Kreen by Kano Labs which many people were swearing by. I began reading up on this stuff across various forums and in the process of that saw frequent mention also of Berryman B-12 Chemtool fuel system cleaner, and that it was considerably better than Seafoam (but not as aggressive as Kreen). Kreen is mail order only, and since the B-12 stuff was locally available, while traveling out of town for Christmas I ran a can of it through the fuel system, honestly not expecting much. However, after running a single full can of it, I have ZERO detonation under any condition on the stock tune now. It's still there in the same places on the Diablo tune, but completely gone on the stock tune. This makes me think that perhaps one of the contributing factors was indeed just severe carbon build up or other fuel system deposits. A friend of a friend actually runs an injector clean/flow/rebuild business so here in the next couple of months I'm probably going to pull the injectors and get him to bench clean and rebuild them. I'll probably run another can of the B-12 through it soon, and maybe order some of the Kreen. After I throw the new O2's in this weekend, since that will now be eliminated from the equation, I'm going to throw the Diablo tune back in it, see if I still have the detonation, and if so start doing a bunch of data logging and tweaking on the tune. With the i3 Diablo Sport you can adjust not only the injector slope but also the PE value.....so if I'm maybe still picking up some detonation from not enough fuel/lean condition, I can play with the injector slope and PE values to throw a little more fuel at it to see if it helps. The stock L92 injectors are 31lb/hr units and supposedly more than adequate, so if it's not getting enough fuel then it has to be just worn out injectors or a fuel pressure issue since I've already replaced the fuel pump with a new Bosch unit and eliminated that possibility.

So, I know that's a lot of random info to digest all at once, but as I said I felt as though at least my part of this thread had started to get a bit scattered and in the spirit of someone finding the thread at some point in the future fighting their own issue, I didn't want there to be any missing points of information that could potentially factor into the big picture.

 

[kbuskill]

This thread has kind of morphed from the original intent I had when starting it...and that's ok. I also feel a bit of a need to collect all the relative details together in a bit of a summary as it feels a little frayed at the moment, largely at my own fault.

Ok, so, 2008 Yukon Denali, 200+k miles. I purchased it in mid March, '18. It ran ok, but not great, but I got it for a LOW price so I was ok with having to address some things. First thing is it was setting a P0455 EVAP leak code, fouling the plugs, and had a pronounced miss/stumble around 1500-2000rpm. So, in my efforts to address the EVAP issue, I ended up replacing the entirety of the EVAP system. Gas cap, Vent Valve solenoid, EVAP canister, purge valve, and the fuel pump assembly with an OE Bosh unit. EVAP issue solved. Next order of business were fresh wires (AC Delco) and plugs (NGK Iridiums). Around this same time I picked up a Diablo Sport i3 crazy cheap. I know a good tune from a reputable shop is better, not wanting to start a debate on that. Again, I picked it up really cheap and it gave me something to play with and tweak with. I had also by this point run about 2-3 cans of Seafoam and 3-4 bottles of Techron through the fuel system, and done one Seafoam oil treatment. Now, it ran pretty good at this point, but still would detonate audibly at WOT and occasionally under part throttle heavy load. This was on the stock tune. On the Diablo tune it would do it more severely, but outside of WOT and part throttle heavy load ran GREAT. I always run 93 from a name brand (usually top tier rated) station. So, at this point since it would detonate even on the stock tune I began my train of though I referenced in my above response. That it had to be due to lack of fuel, or maybe severe carbon build up. High miles, no clue how the previous owners drove it, nor the quality of fuel used. I figured the carbon build up wasn't likely anymore since I had run a fair amount of Seafoam and Techron through it. So, I had been leaning more towards lack of fuel, maybe either caused by tired, worn out original injectors, or after seeing a few mentions about lazy O2 sensors causing some problems, that. As mentioned earlier, I have two new upstream sensors ready to install once the weather cooperates, which hopefully will be this weekend. Now, in another thread on here I had seen someone mention an engine/fuel system cleaner called Kreen by Kano Labs which many people were swearing by. I began reading up on this stuff across various forums and in the process of that saw frequent mention also of Berryman B-12 Chemtool fuel system cleaner, and that it was considerably better than Seafoam (but not as aggressive as Kreen). Kreen is mail order only, and since the B-12 stuff was locally available, while traveling out of town for Christmas I ran a can of it through the fuel system, honestly not expecting much. However, after running a single full can of it, I have ZERO detonation under any condition on the stock tune now. It's still there in the same places on the Diablo tune, but completely gone on the stock tune. This makes me think that perhaps one of the contributing factors was indeed just severe carbon build up or other fuel system deposits. A friend of a friend actually runs an injector clean/flow/rebuild business so here in the next couple of months I'm probably going to pull the injectors and get him to bench clean and rebuild them. I'll probably run another can of the B-12 through it soon, and maybe order some of the Kreen. After I throw the new O2's in this weekend, since that will now be eliminated from the equation, I'm going to throw the Diablo tune back in it, see if I still have the detonation, and if so start doing a bunch of data logging and tweaking on the tune. With the i3 Diablo Sport you can adjust not only the injector slope but also the PE value.....so if I'm maybe still picking up some detonation from not enough fuel/lean condition, I can play with the injector slope and PE values to throw a little more fuel at it to see if it helps. The stock L92 injectors are 31lb/hr units and supposedly more than adequate, so if it's not getting enough fuel then it has to be just worn out injectors or a fuel pressure issue since I've already replaced the fuel pump with a new Bosch unit and eliminated that possibility.

So, I know that's a lot of random info to digest all at once, but as I said I felt as though at least my part of this thread had started to get a bit scattered and in the spirit of someone finding the thread at some point in the future fighting their own issue, I didn't want there to be any missing points of information that could potentially factor into the big picture.

You can also run water through the brake booster hose or PCV hose to break up the carbon build up.

Just crank the truck and get it up to operating temperature and then pull the PCV line from the valve cover and start slowly trickling water into the hose. Do it slowly so as not to kill the engine. Let it stumble and recover before continuing.

If it freaks you out using water you can use Seafoam, B-12, or Kreen instead.

Also when you install the canned time from the Diablo make sure you use a higher grade fuel than the tune you install.

In other words, if you flash an 87 tune fill up with 89+... 91 tune fill up with 93+... if you flash the 93 tune find somewhere that sells 100 octane.

If not, expect detonation at WOT as the canned tunes are pretty aggressive with timing.

I believe they do that in order to get as much power as possible knowing that the ECM will detect detonation via the knock sensors and pull the timing back to protect the engine.

My 2 cents... YMMV

 

[swathdiver]

Robert, make sure all those additives and cleaners are O2 sensor and catalytic converter friendly. You can pick up a little bore light on Amazon for under $20 and stick it in the spark plug hole to look at the pistons for carbon build up.

 

[Hi-psi]

Also when you install the canned time from the Diablo make sure you use a higher grade fuel than the tune you install.

In other words, if you flash an 87 tune fill up with 89+... 91 tune fill up with 93+... if you flash the 93 tune find somewhere that sells 100 octane.

If not, expect detonation at WOT as the canned tunes are pretty aggressive with timing.

I believe they do that in order to get as much power as possible knowing that the ECM will detect detonation via the knock sensors and pull the timing back to protect the engine.

My 2 cents... YMMV

On the Diablo GM tunes, the truck and SUV models, the 91 and 93 tunes are identical. Only on the cars are all three tunes different. The 87 actually makes less power than the stock tune on good fuel.

I've used my tuner to test the tunes out on a couple of other vehicles....a friends '14 Sierra, my uncle's '09 Yukon Denali, and both ran just fine on the 91 tune....so I'm pretty sure it's something unique to my configuration/vehicle. I'm still leaning towards weak injectors, and I should be able to test that theory now that it's solid on the stock tune by tweaking the Diablo tune using the injector slope and PE variables.

Robert, make sure all those additives and cleaners are O2 sensor and catalytic converter friendly. You can pick up a little bore light on Amazon for under $20 and stick it in the spark plug hole to look at the pistons for carbon build up.

I'm fairly certain they all are, but I'll probably run another can of the B-12 through it before the new O2 sensors. I actually have one of the little bore cameras for use with a phone/tablet, just haven't taken the time to pull a plug and look.

I'm not terribly concerned about the cats. As most of us do, I've got a broken exhaust manifold bolt or two. Only the driver's rear was leaking, which I fixed with a Kral auto clamp just after buying it. I've justified to myself that whenever I get all the other little things addressed, while pulling the manifolds to throw a stud kit in place of bolts I'd go ahead and slap a set of longtubes on it and remove the cats all together.

 

[Dmascari1984]

Don't expect much excitement.... LOL Fuel Pressure, both banks STFT, both banks LTFT, and rpm. Started at idle, then after a bit ran it up to 1000rpm for a bit, then 1500, then back to idle.

Those numbers look way better/ more stable than mine. Was that test before or after the b-12? Glad to see you’ve got it back to normal without the tune. Must mean you’re on the right track.


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[Hi-psi]

Those numbers look way better/ more stable than mine. Was that test before or after the b-12? Glad to see you’ve got it back to normal without the tune. Must mean you’re on the right track.


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The above one of just the fuel trims at idle was today, so after the can of B12 had been fully run through.....and on the stock tune.

The log I had posted a screenshot of before while running was prior to the B12, and also was while running the Diablo 91/93 tune.

Here's a short clip of that log, starting at idle at a redlight, then accelerating and cruising.

 

[Hi-psi]

What I find interesting about the drive log above is that generally it looks as though Bank 1 was reading more lean (higher fuel trim command), so the PCM was throwing more fuel at that bank than Bank 2. Not sure what's up with that.

To be honest, I haven't been very consistent with my data collection. A log here, log there, sometimes on the stock tune, sometimes on the Diablo, pre-B12, post-B12. LOL I think now that it seems to be running solid on the stock tune.....and after I get the new O2 sensors in it, I'll get a little more serious about building up some good objective logs where I can start really looking at the data to see if it points at any trends.

 

[Dmascari1984]

What I find interesting about the drive log above is that generally it looks as though Bank 1 was reading more lean (higher fuel trim command), so the PCM was throwing more fuel at that bank than Bank 2. Not sure what's up with that.

To be honest, I haven't been very consistent with my data collection. A log here, log there, sometimes on the stock tune, sometimes on the Diablo, pre-B12, post-B12. LOL I think now that it seems to be running solid on the stock tune.....and after I get the new O2 sensors in it, I'll get a little more serious about building up some good objective logs where I can start really looking at the data to see if it points at any trends.

I keep doing the same thing with the logs. My trims are more on one side as well.


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