LNF STFT Spikes

[morecircles]

I have a 2010 Cobalt SS with stock internals / turbo / full bolt-ons tuned by a local. The car was initially tuned on a dyno and making about 300 whp at 22 - 23 PSI.

The STFT is showing very brief spikes to rich and lean conditions, -20 to +20. These are very brief, to the point that they don't show up in HP Tuners when too many PIDs are being logged (because of the reduced logging frequency).

From what I can tell, it spikes lean when I'm coming in and out of DFCO. While it would spike rich when I'm braking in gear.

My car drives wonderfully. There's no KR, other than mild tip in knock that never goes beyond 5 degrees. My LTFTs are between -2 to +2.

The tuner is well respected in Ontario, and I trust that he knows his stuff. This is an oddball problem he's never seen before. We tried smoothing out the cam tables today but that did not help. Can anybody offer thoughts as to what is going on? Thanks.

[califcarm]

I wonder if it has to due with a differential between pedal position and throttle plate position. If your optimum spark is setup aggressively so that the car is quite responsive ("lively" reaction to the pedal inputs), then the throttle plate gets ahead of your actual pedal position. PE for example (I have read and also looks this way in logs I review) is based on pedal position. So it is possible with aggressive OS tables to have the pedal position at say 60% and throttle plate will be at 90-100% (especially during transitions on the pedal movement). This can end up in high boost and WOT without PE actively richening things up.

So with that concept in mind, if DFCO is based on pedal position and not throttle position, maybe you end up with the ECU still commanding AFR from one, while the physically reality of the other is vastly different for a fraction of a second. I have seen a log on here where the throttle plate hangs open for up to 1/2 second after the pedal is completely released. So maybe a map is calling for no fuel (it initiates DFCO), but the reality of the engine is that it still needs fuel for a fraction of a second longer since the throttle plate is open, so it would respond to the O2 sensors by using STFT to add a large amount of fuel.

This might be correspond to your +/- STFT swings, or be polar opposite of them, not sure. But a differential between pedal position and throttle plate could explain exactly what you are seeing both in DFCO or braking in gear (either including rapid pedal position changes).

I have even seen torqueXrpm limiting (which I use to limit boost/torque in lower 4th and 5th gear) react strangely and inconsistently based on what I think is the same concept of pedal position vs. throttle plate position. Sometimes limiting boost to 16psi (when mashing pedal to the floor instantly), and allowing 22-23psi if I goe 60-65% pedal which might momentarily allow throttle to jump ahead to 90-100%.

You could try putting your OS tables back to stock if they have been setup aggressively and see what happens.

Final thought: like your tip in knock that you don't worry about, if the STFT swings are so fast you barely capture them in a log, then they probably don't really matter or affect things at all and you could just ignore them. Just be impressed with modern electronics and that they are fast enough to even capture those moments of rich/lean and fix them!

[eronq9]

I know on the lsjs, cyl airmass idle the main decider for dfco.

When dfco activates the throttle plate will actually open a slight bit more. On my car it's usually around 10%, it is probably just a result of sensor slightly lagging behind and reading the lean burn from dfco as the injectors turn back on and, it does take a fraction of time for the combustion gases to cross the O2.

Even my car stock would go a little rich after dfco for a split second because of STFT

[Area47]

ltft is global on the lnf. unplug the evap noid and log it again.

[cobaltssoverbooster]

listen to Area47

[morecircles]

ltft is global on the lnf. unplug the evap noid and log it again.

Thanks. My tuner did MAF calibration as per the instructions here: http://www.hptuners.com/forum/showth...nd-LTFT-Tuning

With the exception of yanking the purge solenoid. I will connect with him and try this.

Can you elaborate what you mean by ltft being global? I understand the yanking the purge solenoid will cause disable LTFT. Is that the goal?

[cobaltssoverbooster]

there is technically one cell for the ltft.
when it makes a correction it corrects for the entire load range all in one shot

[morecircles]

I spoke with my tuner about this. He turned off LTFT while MAF calibrating, so unplugging the purge should be irrelevant.

The car was boost leak tested downstream of the turbo outlet with 0 leaks. He recommended doing a smoke test at the filter inlet, but the symptoms do not point to a boost leak.

During throttle liftoff, the exhaust pops quite audibly. Especially coming off of a WOT pull. This was not the case with my previous trifecta tune.

Thanks. Any further thoughts welcome.

[cobaltssoverbooster]

I spoke with my tuner about this. He turned off LTFT while MAF calibrating, so unplugging the purge should be irrelevant.

The car was boost leak tested downstream of the turbo outlet with 0 leaks. He recommended doing a smoke test at the filter inlet, but the symptoms do not point to a boost leak.

During throttle liftoff, the exhaust pops quite audibly. Especially coming off of a WOT pull. This was not the case with my previous trifecta tune.

Thanks. Any further thoughts welcome.

If your tuner told you he disabled ltft in the computer with a direct injection ecotec then your tuner doesn't know anything about the computer it runs. I hate to say it but you got hosed.

[Area47]

I spoke with my tuner about this. He turned off LTFT while MAF calibrating,

OH do tell!

[morecircles]

I'm not sure exactly what he meant by turning off LTFT, but perhaps he meant that he didn't factor it in when multiplying the MAF correction table?

Can you go into more detail why leaving the purge solenoid plugged in will lead to the symptoms I am seeing?

Thanks.

[cobaltssoverbooster]

it does not allow the fuel trims to correct for a single cell which means the cell for 6k rpm and 210% load can affect a cell in say 700 rpm and 30% load.

[morecircles]

Its obvious I do not have enough background information to understand this (hence why I paid for a tune). Rather than annoy you further for more details, I will relay this to my tuner and work with him further.

What's the typical attitude towards posting another tuner's tune on here for review? Is that considered disrespectful?

Thanks for your reply.

[cobaltssoverbooster]

It is highly recommended to contact your tuner and try to figure out a solution. if that still does not resolve itself then you could use the forum as a means to building a case for a complaint. i wouldnt start with us though, give him a chance. some of us are ruthless.

[morecircles]

We tried the MAF calibration again today with the purge solenoid unplugged. It didn't fix the STFT spikes.

Again, I raise the fact that these brief STFT spikes do not show up in HP Tuners during logging. These are showing up only in my scanner (Android OBD app + bluetooth dongle). I wonder if these spikes are actually normal but nobody notices them because of the logging frequency in HP tuners.

[GHuggins]

Sounds like you might have an optimum spark or desired air load issue - possibly even a corrupt tune file as I have seen all of these cause the exhaust popping issues your having...

To start with have your tuner open the OE tune file - make all of the corrections back to this - resave and flash this back into your vehicle via write entire - see if it's any different...

The guys above know their stuff in regards to these motors and ecu's - I would listen to what they have to say

[califcarm]

We tried the MAF calibration again today with the purge solenoid unplugged. It didn't fix the STFT spikes.

Again, I raise the fact that these brief STFT spikes do not show up in HP Tuners during logging. These are showing up only in my scanner (Android OBD app + bluetooth dongle). I wonder if these spikes are actually normal but nobody notices them because of the logging frequency in HP tuners.

I think you have two different things that you are describing here.

One is that you have a stft swing that lasts for probably less than 1/10th of a second. Not even picked up in HPtuners when logging. For me, I wouldn't worry at all about that as almost anything short of a massive knock event that only lasts for 1/10th of a second is not of a concern. AFR changes that brief hardly have a chance to affect anything. That could be a sensor catching a wiff of a single misfire leaving some extra oxygen in the exhaust for example.

Your burble and backfire on decel IMHO is not coming from a AFR even that only lasts 1/10th of a second, but has to be from a continuous event for at least a second or two (or longer) where fuel is still making its way through to the exhaust (which sounds like DFCO is not kicking in, or there is a mechanical reason outside the tune that is allowing fuel to go through).

But I don't think what you are logging in that brief of time would cause what you are experiencing in the real world on decel.

Also, I find that HPtuners logging is a little faster than my OBD blue tooth setup (OBD Link MX doggle and software in my case). I would trust HP tuners more than any blue tooth dongle setup. Maybe there is an issue with your doggle/scanner software possibly?

[morecircles]

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I did some logging this morning through Torque and noticed this strange looking event over 20 seconds. For some reason my engine load jumps from 18 to 95 over 1 second. You can see the STFT is way off as well. Can anybody make sense of this data?

[JimmyS]

I have seen similar situations from a bad connection at the MAF sensor where the harness was pulled too tight and MAF/VE was spiking every once in a while.

[The_Ghost]

I just figured out what was wrong with mine. I was seeing major erratic behavior with STFT. At idle I would see -30.47 and stay there after a WOT. Then the next 5 minutes it was normal. My AFR would be 10.7 and then go normal to 14.7. Try cleaning the connector and plug on your HPFP. I have been getting the CEL of p2188 "too rich at idle" for 8 months and been chasing it!!! Cleaned the connections and it is now fixed!!! I just spoke to a GM engineer that specializes in driveability issues and fixes for the LNF line....