Idle AFR Issue

[FastBlackTA]

I'm fairly familiar with ls1 tuning, but the LS2 seems a little different than the ls1.

Currently, I am running in open loop because I was going to try a little leaded C16 fuel for some high boost stock internal applications. I have my open loop pretty much dialed in and want to switch back to closed loop.

Right now in open loop, my AFR will vary from 13.6-15.1 varying on outside temps. I assume that this is pretty normal in an open loop operation.

Once I switch back to closed, how are you guys manipulating your closed loop commanded afr at idle? I assume that the variance I am experiencing in open loop will decrease once the PCM regains control of the air fuel ratio???

[NoMoreFWD]

I'm fairly familiar with ls1 tuning, but the LS2 seems a little different than the ls1.

Currently, I am running in open loop because I was going to try a little leaded C16 fuel for some high boost stock internal applications. I have my open loop pretty much dialed in and want to switch back to closed loop.

Right now in open loop, my AFR will vary from 13.6-15.1 varying on outside temps. I assume that this is pretty normal in an open loop operation.

Once I switch back to closed, how are you guys manipulating your closed loop commanded afr at idle? I assume that the variance I am experiencing in open loop will decrease once the PCM regains control of the air fuel ratio???

I'm having the same problems in open loop. I think for now were just going to have to get it as close as possible and be happy. I haven't tried closed loop yet, so I can't comment. I think the problem is in the bias tables. RedHardSupra and others are working on cracking the tables. Hopefully we will have some info within about a month.

[Michael@L&M Engines]

I'm fairly familiar with ls1 tuning, but the LS2 seems a little different than the ls1.

Currently, I am running in open loop because I was going to try a little leaded C16 fuel for some high boost stock internal applications. I have my open loop pretty much dialed in and want to switch back to closed loop.

Right now in open loop, my AFR will vary from 13.6-15.1 varying on outside temps. I assume that this is pretty normal in an open loop operation.

Once I switch back to closed, how are you guys manipulating your closed loop commanded afr at idle? I assume that the variance I am experiencing in open loop will decrease once the PCM regains control of the air fuel ratio???

I did not understand. One statement is, OL fuel is close, then OL varies from 13.6-15.1, which is 7.5%-3%?

One should develop the air models, VE & MAF, to 1-3% in OL before setting CL and adaptive on. Air temps should not change A/F in OL substantially if all parameters are set correctly.

I assume one is refering to O2 bias? That parameter should not influence the above condition in a large degree as stated.

If I assume the question was in regard to OL fueling?????

Michael

[MNR-0]

O2 sensors will not meter AFR correctly at idle for large CAMS. This is due to many factors some of which include:

overlap
reversion
low IPW injector control
injector timing
poor airspeed

All this leads to erratic combustion where not all the fuel is burnt and is passed out the exhaust. O2s cannot meter unburnt fuel, only O2 in hydrocarbons. End result is even though O2s register lean, it may infact be rich.

The goal for idle AFR tuning is obtain richest O2 mixture with best vacuum and steady RPM. Unfortnuately there is no magic number here, only trial and error.

[Michael@L&M Engines]

O2 sensors operate on the partial pressure of O2 in the exhaust relative to the outside, therefore the sensor body must be in ambient air.

The sensor operates on free O2 in the exhaust stream, not molecular O2 in combustion by-products.

Disable one cylinder and witness O2 output show lean due to the increase of free O2 delivered by the non-firing cylinder.

Generally when combustion is lean O2 increases, rich O2 decreases. Any misfire, erratic combustion (ie. overlap, egr misfire, etc), increases O2 concentrations in the exhaust stream.

Narrow band (Most OEM) sensors will show lean somewhere around 14.8 and rich around 14.6, great for an oscillating fuel control strategy.

Wide band (O2 pumps) O2 sensors as we know are resonably linear.

Michael

[MNR-0]

O2 sensors operate on the partial pressure of O2 in the exhaust relative to the outside, therefore the sensor body must be in ambient air.

The sensor operates on free O2 in the exhaust stream, not molecular O2 in combustion by-products.

Disable one cylinder and witness O2 output show lean due to the increase of free O2 delivered by the non-firing cylinder.

Generally when combustion is lean O2 increases, rich O2 decreases. Any misfire, erratic combustion (ie. overlap, egr misfire, etc), increases O2 concentrations in the exhaust stream.

Narrow band (Most OEM) sensors will show lean somewhere around 14.8 and rich around 14.6, great for an oscillating fuel control strategy.

Wide band (O2 pumps) O2 sensors as we know are resonably linear.

Michael

Thanks for the correction.

FYI...

This was my source ==> http://efi101.com/forum/viewtopic.ph...ean+combustion

In both interpretations, raw fuel still isnt metered. (Though I defer to Michaels expertise here on how O2s meter O2).

Cheers

[Vettepartz]

The original poster to this thread asked this question"

Once I switch back to closed, how are you guys manipulating your closed loop commanded afr at idle? I assume that the variance I am experiencing in open loop will decrease once the PCM regains control of the air fuel ratio???

I see lots of theories, but no answer to his original question. How come?

[MNR-0]

It's difficult to answer without first providing supporting background material, and short of saying "just do it, it will be right" will not educate people as to the why and what fors. Sometimes leading someone to their own conclusions will yield more in the long run than just stating the obvious.

13.6-15.1 is a big swing. It shouldn't swing like that at similar load points just with 10-15 degrees temp. change. Now from cold start to hot running that might be considered OK.

In this case, switching to closed loop will likely yield big trim swings, as they way the car behaves in open loop will work against the way the car behaves in closed loop.

Closed loop, being what it is, and driven by narrow band O2 sensors, purely strives for stoich. Anything that works against that will give the tuner a hard time. Sure the O2s will remove some of the peaks and troughs through learned trimming, but you will have, in this case, excessively lean and rich spots on your STFT histogram. Then the question might be raised how can I fix that? And round we go...

There's not much that can be done in closed loop, really - it works or it doesn't. You have such a short AFR range that O2s trim for. You can stuff with the mv switchpoints but its relatively meaningless in the grand scheme of things, without first bringing fuel delivery under control. You go back to the basics of open loop fuelling like adjusting the VE map or MAF curve to control the swings.

There is no way of manipulating AFR in closed loop. It's stoich or bust. I always trim a car in open loop first, the goal being that when closed loop is applied it should all fall into line.

Oh, and leaded C16 will kill O2 sensors in less than 10 hours. Short answer - dont bother with closed loop and remove your O2s when running leaded fuel.

[NoMoreFWD]

You can VE tune an LS2 and later that day it will be off. It's been discussed. The best right up is probably over at RedHardSupra's blog.

[Michael@L&M Engines]

As I stated in the previous post, that information was in error in regards to a wideband and my apologies to MNR-0 and the forum members.

I know of no valid reason as to why I was not thinking fully and clearly at that time and as such I have verified my rectified post.

In regard to a wideband O2 sensor, the sensor works by reference to the partial pressure of atmospheric and sample gas oxygen with the transfer of oxygen ions across the coated plates in the sensor.

In a chemically correct / theoretical proportion of fuel and air there is just enough oxygen for the conversion of all the fuel to create completely oxidized products.

Such as a simple fuel such as isooctane C8H18, and Air = O2 + 3.773N2, the stoichiometric formula would be in moles, which is the number preceding the molecule.

C8H18 + 12.5(O2 + 3.773N2) = 8CO2 + 9H2O + 47.16N2 which would make the A/F = 15.14/1

Now as stated by others, the O2 sensor reacts to the O2, H2, CO in the gas and in this example the sensor output would be lambda 1.

Now if we add excess air (lean) of 25% the formula would look like this—

C8H18 + 1.25*12.5(O2 + 3.773N2) = 8CO2 + 9H2O + 3.125O2 + 58.95N2 which would have O2 in the exhaust and sensor output of lean.

If we have less than stoichiometric air, we would not have enough O2 to oxidize the carbon and hydrogen to CO2 and H2O and we would form varying amounts of CO2, CO, H2O, HC, H2 and as such the CO, HC, H2 would require the O2 pump to add O2 concentration to form CO2 and H2O and the sensor would show rich.

Now if we were at stoichiometric and had a misfire we would not have combustion and the exhaust gas would have excess O2 and the sensor would read lean when in fact we had no change in actual fueling.

What all of this mumbo-jumbo means is a compilation of what has been said by others is that the O2 sensor reads both O2 levels in free O2 and O2 required to oxidize CO, H2, HC by the O2 pump in the wideband.

I hope I have not made more confusion, but clarified my mistake and a clearer picture of what a wideband O2 sensor really is reacting to.

Thank you in allowing me the time to repair the misinformation I had forwarded.

Michael

[Michael@L&M Engines]

The original poster to this thread asked this question"

Once I switch back to closed, how are you guys manipulating your closed loop commanded afr at idle? I assume that the variance I am experiencing in open loop will decrease once the PCM regains control of the air fuel ratio???

I see lots of theories, but no answer to his original question. How come?

As others stated, there is no magic wand for an answer.

With incomplete data one will have to analyze each circumstance on its individual pieces of evidence.

Additionally stated, at least a dozen items as previosly stated can cause this gentleman's observations on wandering O2 readings.

A properly tuned engine will be able to compensate for atmospheric changes and hold consistent O2 readings.

The basic engine has to be sound as well, or a unsound engine may have consistent O2 readings in OL if on the rich side and when put into CL and leaned slightly to stoichiometric start to have intermittant misfire.

As stated a multitude of issues can cause his symptoms.

Best to you, Michael