Timing and Fuel questions

[2007chevy5.3l]

I have a E38 LMG with AFM and was wondering if the PCM will sill revert to the Active Fuel managemet timing table even if the DOD feature is disabled via the tune? In my high octane spark table my highest timing value is 33*. I did a log the other day and noticed that it was showing 40-47* at light throttle and low load conditions. I looked at my AFM timing table and there are values that high so im guessing the PCM was using them?

Second, when in closed loop my commanded AFR is 14.68 at the start of my logs. When I finish logging I noticed it had changed from 14.68 to 14.17 to 14.30 to 14.19 to 13.67 to 14.43 to 14.01... Is it supposed to change that much... None of this was WOT, just light to medium throttle.. Where can I find the closed loop commanded AFR in my tune??

[2007chevy5.3l]

Heres a log of what I am talking about


Tahoe fuel trim5.efi

[Taz]

... Where can I find the closed loop commanded AFR in my tune?? ...

Hello Justin,

In closed loop the ECM commands stoichiometry ... based on oxygen sensor input ... the table is B3671 in your tune. The ECM will command Equivalence Ratio (or Lambda) of "1.00" for stoich in closed loop ... this may not always be the AFR value you expect to see, or that is depicted in B3671. This is why tuning using Lambda or EQR is more consistent than trying to tune using AFR.

AFR is fuel dependent - Lambda and EQR are not.

Post your tune if you wish - and myself or someone else, will take a look at your AFM (DOD) settings - if you want these disabled.


Regards,
Taz

[2007chevy5.3l]

Corey 60 shot tahoe frequency adj.tun


Hey Taz,

I will post up a tune that I have been working on but have yet to flash in and try out. I have the DOD feature turned off via the tune settings and hasnt worked in over a year. But you can double check it all.. I have also made a few changes to the stoich values. 0.0% was 14.6 AFR and has now been changed to 14.1. 6.2% has now been changed from 14.3 to 14.1 as well.

From the above log It looks like it might be sensing a change in ethanol mixture somehow and quite possibly falsely. I think that is strange how it is commanding different afr's when in closed loop so thats why im going to try and change the ethanol % AFR to 14.1

[Dieselman]

OK just a uneducated guess here but I think because your flex fuel table (B0178) is set to YES and sensor type (B0184) is set to calculated the ECM is actually changing commanded AFR to suit what it thinks is in the tank. It will use fuel trims to calculate the ethanol content of the fuel and adjust the commanded to suit. I think your B3671 should always be set at factory values. Pure unleaded fuel has a stoich of 14.68 AFR and 100% ethanol is 8.98 AFR.

As Taz said when playing with ethanol it is easier to work in Lambe or EQ

Someone please correct me if I'm wrong



Ben

[Gregs]

Ahh, those stoich values shouldnt be modified since your vehicle has a flex fuel sensor. Put them back to factory settings and I bet that will solve your problem.

[2007chevy5.3l]

Ahh, those stoich values shouldnt be modified since your vehicle has a flex fuel sensor. Put them back to factory settings and I bet that will solve your problem.

The current tune in my truck right now has stock stoich values and thats why im having the commanded AFR values changing while driving making the truck run rough... The tune I posted is a tune that has not been flashed in because im working on making corrections.

[2007chevy5.3l]

OK just a uneducated guess here but I think because your flex fuel table (B0178) is set to YES and sensor type (B0184) is set to calculated the ECM is actually changing commanded AFR to suit what it thinks is in the tank. It will use fuel trims to calculate the ethanol content of the fuel and adjust the commanded to suit. I think your B3671 should always be set at factory values. Pure unleaded fuel has a stoich of 14.68 AFR and 100% ethanol is 8.98 AFR.

As Taz said when playing with ethanol it is easier to work in Lambe or EQ

Someone please correct me if I'm wrong



Ben

After looking at the flex fuel table Im considering either turning the option off B0178 and setting the stoich values to run 14.1 AFR.
Or leaving B0178 on and setting B0185 to 6.2% as the default. I know our gas around here has been tested to contain anywhere from 6-8% ethanol.
Im kinda leaning towards the flex fuel sensor being bad and thats why my calibrations are off..?

[Taz]

... leaning towards the flex fuel sensor being bad and thats why my calibrations are off..?

Hi Justin,

I haven't had a chance to look at your tune yet ... your vehicle does not use a physical flex fuel sensor ... the ECM calculates the ethanol content in the fuel based on your O2 trims to arrive at stoich. If you look at B0184 (Flex Fuel Sensor Type) you will see that it is a "calculated" sensor (calculated value). Only select GM models still use an actual (physical) flex fuel sensor.


Regards,
Taz

[Gregs]

B1515, B1516 should all be 450 mV. I used to read all about people trying to get better gas mileage by "tricking" the computer think it was running richer that it actually was . whats its doing is confusing the computer and causing you all sorts of weird problems. its not a good strategy and it explains why you are having problems at light part throttle.

I'm worried about some of the table's you've changed. Some of them shouldn't really be changed unless there is a really good reason to change them... not trying to bust your balls at all, just trying to help you out, sorry if it sounds that way