AFR lean under WOT

[jfigus]

99' Vette with a mostly stock LS1 other than a stage 2 cam upgrade, normally aspirated. I've tuned the VE table but the AFR is still too lean when under power enrichment (WOT). While I can command the AFR lower using the PE multiplier table, the observed AFR is still well above the commanded AFR. The cells in the VE table in play under these conditions are already maxed out at 100. My question is why is the observed AFR still too lean? Looking at a single point in the log, here's what I see:

Observations
----------------------
commanded AFR: 10.6
observed AFR: 13.6
RPM: 4900
Manifold pressure: 95 kPa

Configuration
--------------------------------
My stoich is set at 14.63
PE multiplier at 4900 rpm is 1.375
VE cell at 4800 rpm and 95 kPa is 100

So the math looks good on the commanded AFR. But it appears the ECM still isn't delivering enough fuel. I have verified my fuel pressure while under WOT, it's good at 58 psi. The vehicle has 84K miles on it with original injectors. Could this be an injector problem?

My wideband reads well at idle and cruise speeds, always near 14.7 when the engine is warm. Could the wideband behave different under WOT? I have the wideband mounted about 18" in front of the cats, so it's not at the factory O2 bung on the exhaust manifold.

I do get some heat soak from my IAT sensor. But when I do the math on that, that would only contribute about 5% to the lean condition. My wideband is showing much more than that (from commanded). Given air mass is calculated as follows, I don't see anything else that would cause the airmass to be miscalculated. My MAP readings seem fine in the logs.

Air Mass = (Engine Displacement * RPM * VE * (Barometric Pressure / MAP) * Sqrt(IAT / 273)) / 2

So it leads me back to injectors. Could 84K mile injectors just be partially clogged and not flowing fuel per spec? Is there some other table in the tune that needs adjustment?

[SiriusC1024]

Can we get a log? Be sure to include injector duty cycle.

For your heat soak problem try adjusting the bias more toward IAT at Airflow>General>Cylinder Charge Temperature. It'll be the 0 lb/hr (airflow) cell.

[jfigus]

Here's a log one one pull. Duty cycle isn't going above 70%. Sorry, I should have mentioned that in the post.

[SiriusC1024]

Does changing PE result in a similar change in wideband measured AFR?

[jfigus]

Yes, when I bump up the PE multiplier the wideband reports a lower AFR. So it does respond to that. But that also lowers the commanded AFR. I'm already at a commanded value of 10.6, which seems low for NA.

[SiriusC1024]

A weak pump, clogged filter, or clogged fuel injectors can still show this behavior. You've verified WOT fuel pressure, not just idle?

Wideband might need a free-air cal if it's old.

[jfigus]

Yes, I put a new fuel filter in. I also bought a glowshift fuel pressure gauge and have monitored while WOT. The pressure never waivers below 58. I think the fuel supply is good, this was the first path I went down. That's why I'm questioning injectors now.

84K miles on the engine, 24 years old. I'm sure the injectors have seen some stale gas. Pretty sure the first owner didn't use the car a lot. But I'm a noob at this and wondering if the injectors are just slightly clogged and not flowing at spec. I've had them out before and they look fine. But I have no way to test injector flow rate.

I do find it interesting that GM has the injector flow rate biased up for higher manifold pressure. For example, at 80 kPa it's set to 29 lbs/hr. But at the 40 kPa it's set to 26.4. I would think the vacuum would increase injector flow, not decrease it. It seems backwards to me. I've pulled some stock tunes for other GM models in this area and they all have a similar bias on the injector flow rate vs. kPa.

[Alvin]

Are you sure it's in SD mode? If it doesn't have a 101, 102, or 103 what you are doing in the VE tables really won't change much.

Post the file and we can look over it.

[jfigus]

Are you sure it's in SD mode? If it doesn't have a 101, 102, or 103 what you are doing in the VE tables really won't change much.

Post the file and we can look over it.

I'm logging throttle% and fuel trims. When going full throttle I observe the short/long fuel trims go to zero and the commanded AFR drops. The AFR drops to exactly what the stoich/PE multiplier calculation would command. My open loop F/A is at 1.13, which would command a higher AFR than I'm seeing in the log. So I'm fairly confident it's in PE mode, which my understanding is using the VE table. Is that incorrect?

The one parts I can't get my head wrapped around is the near 100 values on the VE table. For instance, the VE table cell in the stock tune for 4800 RPM and 95 kPa is set at 94. After installing a stage 2 came, I would expect the airflow at 4800 RPM and 95 kPa to be quite a bit more. But if I can only bump this cell to 100 (I'm naturally aspirated), how else can I command more fuel? My new cam wants more. The PE multiplier table is the only way I see, and that works for me. But that's also dropping my commanded AFR a lot. There's something I'm missing about how the computer is commanding fuel while in PE. Or, is this how the pro tuners do it? They don't worry about commanded AFR while in PE???

[jfigus]

c5-27.hpt

Here's my tune

[Alvin]

So I'm fairly confident it's in PE mode, which my understanding is using the VE table. Is that incorrect?

No that is not correct. It is still using the MAF table. It uses the VE table for quick transients and when the MAF is failed / bad. To tune the VE table you need to disable the MAF by unpluggging it or disabling it with diagnostic checks.

You should be setting the OL and PE tables to the values you want.. then tuning the MAF or VE depending on strategy and what I said above to reach those values.

[jfigus]

No that is not correct. It is still using the MAF table. It uses the VE table for quick transients and when the MAF is failed / bad. To tune the VE table you need to disable the MAF by unpluggging it or disabling it with diagnostic checks.

You should be setting the OL and PE tables to the values you want.. then tuning the MAF or VE depending on strategy and what I said above to reach those values.

Correct. I wasn't tuning VE with MAF and DFCO enabled. My VE table is tuned. I re-enabled MAF. Then I collected a log doing WOT. That shows the lean condition even with the relevant cells in the VE table maxed at 100. Please see my questions in the thread above. It's not clear to me how to command more fuel when the VE cell is already at 100. My work around is to use the PE multiplier table, which does lower the observed AFR. But that's also lowering the commanded AFR.

[RDF1]

Adjust MAF calibration to add more fuel at WOT.

[aaronc7]

You can set VE value well above 100. Not uncommon to need more than 100%, there's a lot of other factors baked into it. On other software this table isn't even in VE percent units, but some other more universal units (I don't have the specifics in front of me).

This whole discussion has gone full circle on corvette forum and now here it feels like.

Set PE to desired target AFR/lambda at full load.

Fail MAF to run SD mode and tune VE table so trims are near zero and commanded = actual AFR at WOT/open loop. Edit the secondary VE table to hit these targets... primary is not used in failover VE mode for this OS apparently. Transpose and fill in primary VE table after you're happy with secondary VE table results from SD tuning.

Re-enable MAF/MAF-only and same concept to tune the MAF table. With VE and MAF tables calibrated, you can move onto other stuff and adders and whatnot.

[Alvin]

Correct. I wasn't tuning VE with MAF and DFCO enabled. My VE table is tuned. I re-enabled MAF. Then I collected a log doing WOT. That shows the lean condition even with the relevant cells in the VE table maxed at 100. Please see my questions in the thread above. It's not clear to me how to command more fuel when the VE cell is already at 100. My work around is to use the PE multiplier table, which does lower the observed AFR. But that's also lowering the commanded AFR.

This is all wrong.. You are misunderstanding me.. Someone above re-wrote what I was getting at.

Once you re enable the MAF doing anything to the VE table isn't going to make a meaningful change.