Idle work, no VE changes but now runs rich

[maudyZ28]

Hi Guys,

I'm working on a tune for a friend who has had quite a lot of tunes in past. Its running COS3 and have around a 230/230 cam with 241 ported heads and LS6 intake. Basically I'm trying to sort idle fueling doing an auto VE but there have been so many changes to his tune I am finding it hard to figure out where to start.

Tune 1 -> has low ish desired airflow, but upto for in the airflow adders table (usually zeroed ? as this is new COS3 table) WB is a dual FAST but is VERY jumpy (+/- 2) in AFR ie 11-17 averages around 15 at idle. The STFT (semi closed loop in COS3) are -15% to -20% so still pulling fuel, even though WB is averaging 15. Like I say it is jumpy and doesnt hold flat, is there an issue?? .
The IAC area has been shifted also so there are *less* zero values and rest steps set to 310 and not 160 as stock tune.
The car hunts but i wanted to sort fuelling then check spark with DTC but I cant understand why if WB is 15:1 that STFT are still negative.


Tune 2 -> To figure out what was happening I tryed to get back to 'stock ish' values. I zeroed the adder idle table (B4338 - engine compensator) and increase the desired airflow to compensate. (set similar to my car with 228/228 cam and 253 heads). Also reset the IAC area to stock and rest steps to 160. The car starts and runs fine but now is PIG rich, WB is 12.5 (still very jumpy) and the NB are 850 mv constant. I HAVE NOT CHANGED THE VE. How would these changes cause the fuelling to be effected so much??

Assuming rest IAC step doesn't make a difference to fuelling or the IAC area, again assuming it is just a reference to deesired airflow telling the IAC how much to open. Then my theory is that the desired airflow is what is used with the VE to calculate fueling. As I have increase only this to compensate the removal of the airflow adders table does anyone know if this why the car has gone so rich with the same VE? I will post tunes if the guy okays it, he has paid a lot of money for them and we've had to edit a lot to make the car run better. Plus the idle is still crap as I've not changed anything as I need to figure out what is going on.

Once I get a handle on what tables do what then I can sort fueling and then spark. Do people just tune the desired airflow table for idle or do you use the extra adders tables? (B4338). Any should the WB hold steady at a given fueling (my innovate MTX serial does) I'm wondering if there is a WB earth issue with the FAST??

[joecar]

Hi Maudy,

Please post some logs showing the FAST WB waveforms, including idling and cruising.

[maudyZ28]

Hi Maudy,

Please post some logs showing the FAST WB waveforms, including idling and cruising.

Mainly the logs are at idle, only a few revs in these tunes I think. I am doing it at a distance for a friend as it is an off-road race car, any times the VSS is above 0 it is on some un-loaded rollers.

I have also edited above post the table with the extra air in is B4338, (engine speed compensator). In the initial tune (corresponding log1) it ramps up from 0 to 7 from 0 to 400 rpms. I zeroed this out in my updated tune as when I updated my car to COS3 it was zeroed. To compensate I added 2 g/s to the P/N and 5-6 g/s to the INGEAR desired airflow. I also changed the IAC area back to stock (99 camaro) and the IAC park position in my tune (log 2)

logs are attached and hopefully screen shots show how same fuel is commanded and it runs much richer? It is hunting as spark is way out for fueling in second log. But i'm still trying to sort the brief hunting out in the first log too, but can't be sure on fueling with the FAST WB not holding steady. I can also confirm from another log the NB O2 are working and oscillate fully.

Log100212log1.efiLog100212log2.efi

[Lextech]

To me, it seems the narrow band O2s are more effective for idle tuning. I noticed in your logs that the timing is also hunting. Here is a procedure that I learned on this forum for idle spark tuning. This will not cure your issue but it should eliminate one more variable to the surging.
Example: Set these Parameters>

Base Spark Closed Throttle B5916 - 1.50%
Base Spark Vehicle Speed B5917 - 30 MPH
This puts you in the base spark map (B5932 and B5933) anytime you're under 1.50% throttle.

Max Speed for Idle B0107 - 4 MPH
Max Throttle for Idle B0108 - 1.50%
set in line with B5916 & B4312

Throttle Cracker Activate B4311 - 5 MPH
Throttle Cracker Deactivate B4312 - 4 MPH

This keeps Timing simple (you know where to make changes if need be) and it keeps
the active idle controls from interfering with the Throttle Cracker settings.

Jeff

[joecar]

Is there the possibility that the wideband AFR is reacting to each individual exhaust pulse...?

[ find out how much time between successive squiggles, invert to convert to frequency, divide by 4 (4 ignition events per revolution), multiply by 60 to convert to equivalent RPM, see if this matches engine RPM ]

[swingtan]

With bigger cams, the overlap will mess with the WB readings. The overlap will cause unburnt intake mixture to be present in the exhaust and result in the WB reading leaner than what is really in the combustion chamber. With a 230's cam, I'd have expected the WB to read around as up to about 15.3:1 when in CL idle. It's no big issue, you just have to know it'll do that. The NB O2's work on switching between rich and lean, not an absolute value so they will control idle mixtures better than is seen on the WB.

To help with idle tuning, I don't usually worry about the actual WB reading, but go by IBPW and idle air flow. You need to know the individual motor, but IBPW of between 2mS to 2.5mS are about where I'd be aiming for. If you want to use the Injector flow rates, fuel pressure and airflow parameters, you can calculate the "applied AFR" at the intake ( assuming the VE / MAF are accurate ).

To help get the idle right, I'd do the following...

1. Set the idle speed high to begin with, maybe 1000 RPM.
2. Set the minimum idle air flow high, maybe around 9.5 gm/S
3. Leave the rest of the tables fairly stock and run the idle in OL mode.
4. Ensure the VE is correct and blend any idle cells down to the lowest RPM range, so that the VE has no big steps and it looks "sensible".
5. Start working on the Idle spark corrections first, these should do 95% of the work to stop hunting.
6. Once the idle settles and becomes more stable, drop the idle RPM to, say, 900 and tweak the spark settings to keep the idle steady. Continue this till you get the idle speed you want.
7. Note the average idle spark in the logs, if it's below the base commanded spark for idle, reduce the min idle air in steps of 0.25 gm/S till the spark comes up. If the logged spark is high, att min idle air.

This normally results in a good idle with a minimum of stress. The idle airflow tables can finally be visited if there are signs of excessive air flow corrections involved. Oh, and don't forget the reset the PCM to clear any learnt idle settings......

Simon.

[maudyZ28]

thanks for the info guys, will take this on board. I have spoke to some of you before and I do employ similar methods to idle tuning.

Ie sort VE out in OL then work on spark with DTC to find a sweet spot (range) Thanks for the additional info on changing the idle air depending on spark , good tip.

The strange thing was the fuel issue, if the car was commanding 14.63 and the WB was 15.2 (which is what is expected at stochiometric on idle) then WHY do the STFT still try to pull 15% fuel??

Secondly, as the tune has been messed with some much the IAC area and IAC steps have been edited from stock, also B4338 have values in this (when i did cos3 on my car these are zero - ie only desired air used to control idle??)

Im wondering what the interplay is between IAC steps, desired air, reference air for IAC at rest and the VE table. My car with a 228/228 cam runs much better and doesnt hunt at temp (only warming up so will add timing with DTC) but I have loads more idle airflow commanded than my buddy, like 2g/s more in all cells. Does the IAC calculate the air input from the IAC area table and the reference step. If so then me chaning the IAC back to stock means the car thinks it getting more air than it is, and so the VE is now out? Hence it is rich? I have sent my friend 4 tunes to test this theory and see which changes did what.

thanks again guys

[Sid447]

........ The overlap will cause unburnt intake mixture to be present in the exhaust and result in the WB reading leaner than what is really in the combustion chamber. .....................
Simon.

Can you explain this one Simon,

I'm missing something here.

Unburnt fuel in the exhaust should result in rich mixture readings, No?

[swingtan]

This is the tricky part. Because the fuel is unburnt, the O2 sensor sees more free oxygen. Remember that normally the O2's are reading the oxygen in the spent charge, not the unburnt charge.

Remember, you have nearly 15 parts of air to one part of fuel in the intake charge. When it burns, the oxygen and fuel are consumed to give a much closer to 1:1 ratio in the exhaust. It's this mixture that the O2's read to work out what the original mixture was. Dump unburnt fuel in the exhaust and you have way more oxygen than expected. At least that's how I think of it.

[joecar]

That is correct, the wideband sees the residual oxygen content as a ratio of the total exhaust gases (Lambda), any liquids do not participate in the measurement of gases;

a no burn looks like a very lean burn.