STFT Issues

Hey guys,

I'm having a weird STFT issue. When I'm logging just for data, or even just driving, in closed loop MAF only (no VVE) on decel above 45 mph DFCO does not kick on and STFT maxes out to 29.7% equally both banks lambda goes down to like .82~ depending on rpm. It will also max out STFT if I coast in neutral above 45 mph. If I get up 60-70 and make a hard downshift get the decel rpms up to around 3000-3500 it will correct it self and then for the remainder of that drive it will be fine. I've asked this in the HP tuners pages on Facebook no luck, searched google, looked around on here no with no luck. I have attached a short data log and the tune file I have on the car. Its a 2015 Camaro SS, TSP F35 cam, TSP 1 7/8" catless headers 3" Exhaust, FID 1000 CC injectors, 93 pump gas right now. Please help I'm pulling my hair out here and I don't have much hair

Are you running a cold air intake? How about a K&N air filter? I over-oiled my K&N which fogged the little heated MAF wires that then drove my MAF bonkers that then drove my LTFT nutty. Cleaning the MAF wires helped here.

My intake piping also provided too much chaotic air to the MAF. Once I bolted my K&N (with a lot less oil) straight onto the throttle body, my fuel trims dropped to random plus and minus values around zero.

Rick

I am using a Variram intake it does use an oiled filter, I have not added any oil to it from when I got it. I’ll go get some stuff and clean the MAF and see if that helps.

I know how to fix it. I'll post back later. On phone now.

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Originally Posted by RobZL1

I know how to fix it. I'll post back later. On phone now.

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Please do I cleaned the MAF as mentioned above and that didn't work.

As best I understand, it has to do with the removal of the post-cat O2 sensors. The ECM will try to run some sort of test that can't be disabled, regardless of how you set the DTC's. Then it just stays rich while it waits for a response from the sensors. Of course there is never a response from those sensors since they are gone. Thus, it just stays rich until something kicks it out of the test- either a timeout or a driving event. If you were to log fuel trim cell (FTC), you'd notice that it always goes into FTC 15 during this rich period. See my picture #5 in this post- it looks just like your log:

https://forum.hptuners.com/showthrea...l=1#post570561

The fix is to keep it out of FTC 15 (one of the two deceleration trim cells). To do that, you have to force idle trim cells to override the deceleration trim cells:

Engine>Fuel>Oxygen Sensors>LTFT Idle Cell Thresholds>Enable VSS : change to 317 mph (or any other value so high you'll never reach it)
Engine>Fuel>Oxygen Sensors>LTFT Idle Cell Thresholds>Disable VSS : change to 318 mph (or greater than the value you put in the Enable VSS field)

That forces it into FTC 6 or 14 (typically 6) instead of 15, which are the two idle trim cells. You'll never have the problem again. DFCO and everything else will still work as it's supposed to, but you'll also want to change these:

Engine>Spark>Advance>Idle Adaptive Spark Control : change both overspeed and underspeed "Gear" tables by dividing by about 8 (make them run up to a max of +/-3 instead of 24, for example. Do this, otherwise you'll be pulling up to 24 degrees from your idle spark while coasting down at low RPM, running negative advance and popping like the 4th of July.

Unsolicited feedback, but one other thing I saw in your log and tune that might be worth your time:
In your log, upon light acceleration you'll sometimes see spark waiver a bit like a saw tooth as it climbs. Smooth out your high octane advance table- get rid of those shelves as RPM climbs, and remove the large down-up-down direction change as you get higher in the g/cyl direction and it will smooth out the log.

You could also look into tuning your O2 sensors for a little better response with the headers. It will get your rich/lean swings to tighten up around 1.00 lambda

Thank you this fixed the issue!

Is this the same issue that screws up Gen3s when downstream O2s are deleted? Just curious, I see threads about that often, suggesting that disabling the cat test doesn't fully disable some part of the test.

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Originally Posted by Skooba

Thank you this fixed the issue!

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Excellent! Glad to hear it.

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Originally Posted by blindsquirrel

Is this the same issue that screws up Gen3s when downstream O2s are deleted? Just curious, I see threads about that often, suggesting that disabling the cat test doesn't fully disable some part of the test.

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Possibly. I don't have much Gen3 experience. I guess someone could try pumping up the LTFT disable threshold on a Gen3 and see if it solves it (if it's a similar issue). My buddy has a P59 ls6- if I get a chance I'll play with it.

I stopped the downstream issue by just leaving the rear o2 sensors plugged in all the time when my truck had no cats. Disabled the codes and COT and it never ran the test again. This was on a 2006 P59 truck.

Could it be possible, that the "test" some users are decribing here is the catalyst learn function which is part of the GM driving cycle learn procedure? Im refering to the emission DTC?s which have to be cleared for regular checkups in some states. I assume that the rich condidition created under decel is used by the ECU to learn the catalyst capacity by whatching the downstream O2 response time. Can anyobe confirm this assumption?