Looking for help or advice with E38 surging/bucking at low throttle

The problem:
A pretty violent bucking in all gears at low throttle cruising, feels like misfires but it never counts on the misfire counters and never had a P0300 and I don't see the lean spikes usually associated with misfires on wideband or narrows.

With spark smoothing enabled it makes the car somewhat drivable in parking lots but makes timing curves very choppy and getting steady state data for airflow models impossible.

Its obviously more noticeable in low gears but its still noticeable in 5th and 6th at 65mph.

Idle quality is pretty good considering the cam. Heavier throttle over .30 g/cyl the surging goes away. WOT seems good to me. I'm sure there's more work to be done through out but I think I need to get this sorted out before buttoning up other areas.

Subject:
2008 Corvette base with m6
418 that started life as an ls3, factory heads, BTR .660 springs, factory rockers with CHE trunnion bushings, Lingenfelter rocker stands, 7.400 11/32 pushrods, 10.2:1 comp. ratio, Vortech V3si centrifugal kit from A&A 8~psi
1.875" tri-y headers, o2 sensors are in the collectors.
BR7EF plugs gapped at .028, have tried several different sets and never changed this problem
New set of MSD plug wires bought from MSD's website.
New narrow bands both at the same time, Denso.
A new wide band 4.2 sensor.

Camshaft specs:
Lift .620/.612 int/exh
Duration @.006" 289/305 int/exh 67* overlap
Duration @.050" 236/250 int/exh 14* overlap
LSA 115 advanced +3

Cam events at .006":
IVO 32.5 BTDC
IVC 76.5 ABDC
EVO 90.5 BBDC
EVC 34.5 ATDC

Fuel System:
Injector Dynamics ID850s bought from ID in ~2017 that were flow checked by ID's sublet in 2022 and had a clean bill of health (see attachment). The problem was present before they were cleaned and tested.
dual in-tank pumps
vacuum referenced regulator at 4 bar
E85 capable but this is all on gas and the current tune has no ethanol spark modifiers yet.


Attempts:

• EOIT, everything from stock settings to SOI retarded passed PPV, then advanced to 50* before EVC in 10* increments. The file I'll post is the "best" I was able to get trims and surging.
• Spark table, started with a stock table and advanced 5* in the problem area then retarded from there in 5* increments until it was 10* advanced without much change. I settled on the table in the file posted. around 24* in the problem area.
• Going to open loop doesn't make a difference. I know the o2 controller still needs adjusted but I believe putting it in OL eliminates this as the culprit for my problem.
• Forced it into speed density by failing MAF and zeroing MAF airflow, no change.
• Went "MAF only" with high rpm disable/enable, no change.
• I thought the problem could be lean misfires from the long tube headers so I put it in PE with 0%TPS for the problem RPM range and commanded .90 and .87 lambda with no change.
• My last attempt was today going back to injection timing. I lowered the fuel pressure to 3 bar and changed the flow rate in an attempt to make a longer IPW for a better fuel mix at these low RPMS. I played around a little with EOIT while I was at 43psi and same old story, no noticeable improvements so reverted back to 58psi.

My next thing to try is going to the custom 2 bar OS unless we can come up with a solution here.

Attached:
the current file
as found file (tuned by a shop before I owned it)
a stock read with same OS from the repository
a single log I took, a good visual of the issue is at around 2 minutes in
the layout I use if you feel so inclined
the injector cleaning done

Here are the VVE labels I use:
column
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 6600
row
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160

I really wanted to figure it out on my own but here I am preparing for the roast in the forums! But seriously I'm open to any and all advice you guys have.
Thank you all in advance!

Here is a screenshot from the posted log. This log shows the APP bouncing, which should indicate the severity of the bucking, but setting cruise control doesn't help any.

Attachment 150506

I think if you add Spark (*) to your charts, you will see it jumping from 17-24-18-28-17-26-16-27-etc, without much of any corresponding change in throttle/MAF/MAP/airmass.

Quote:

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Originally Posted by blakesmiley

The problem:
A pretty violent bucking in all gears at low throttle cruising, feels like misfires but it never counts on the misfire counters and never had a P0300 and I don't see the lean spikes usually associated with misfires on wideband or narrows.

With spark smoothing enabled it makes the car somewhat drivable in parking lots but makes timing curves very choppy and getting steady state data for airflow models impossible.

Its obviously more noticeable in low gears but its still noticeable in 5th and 6th at 65mph.

Idle quality is pretty good considering the cam. Heavier throttle over .30 g/cyl the surging goes away. WOT seems good to me. I'm sure there's more work to be done through out but I think I need to get this sorted out before buttoning up other areas.

Subject:
2008 Corvette base with m6
418 that started life as an ls3, factory heads, BTR .660 springs, factory rockers with CHE trunnion bushings, Lingenfelter rocker stands, 7.400 11/32 pushrods, 10.2:1 comp. ratio, Vortech V3si centrifugal kit from A&A 8~psi
1.875" tri-y headers, o2 sensors are in the collectors.
BR7EF plugs gapped at .028, have tried several different sets and never changed this problem
New set of MSD plug wires bought from MSD's website.
New narrow bands both at the same time, Denso.
A new wide band 4.2 sensor.

Camshaft specs:
Lift .620/.612 int/exh
Duration @.006" 289/305 int/exh 67* overlap
Duration @.050" 236/250 int/exh 14* overlap
LSA 115 advanced +3

Cam events at .006":
IVO 32.5 BTDC
IVC 76.5 ABDC
EVO 90.5 BBDC
EVC 34.5 ATDC

Fuel System:
Injector Dynamics ID850s bought from ID in ~2017 that were flow checked by ID's sublet in 2022 and had a clean bill of health (see attachment). The problem was present before they were cleaned and tested.
dual in-tank pumps
vacuum referenced regulator at 4 bar
E85 capable but this is all on gas and the current tune has no ethanol spark modifiers yet.


Attempts:

• EOIT, everything from stock settings to SOI retarded passed PPV, then advanced to 50* before EVC in 10* increments. The file I'll post is the "best" I was able to get trims and surging.
• Spark table, started with a stock table and advanced 5* in the problem area then retarded from there in 5* increments until it was 10* advanced without much change. I settled on the table in the file posted. around 24* in the problem area.
• Going to open loop doesn't make a difference. I know the o2 controller still needs adjusted but I believe putting it in OL eliminates this as the culprit for my problem.
• Forced it into speed density by failing MAF and zeroing MAF airflow, no change.
• Went "MAF only" with high rpm disable/enable, no change.
• I thought the problem could be lean misfires from the long tube headers so I put it in PE with 0%TPS for the problem RPM range and commanded .90 and .87 lambda with no change.
• My last attempt was today going back to injection timing. I lowered the fuel pressure to 3 bar and changed the flow rate in an attempt to make a longer IPW for a better fuel mix at these low RPMS. I played around a little with EOIT while I was at 43psi and same old story, no noticeable improvements so reverted back to 58psi.

My next thing to try is going to the custom 2 bar OS unless we can come up with a solution here.

Attached:
the current file
as found file (tuned by a shop before I owned it)
a stock read with same OS from the repository
a single log I took, a good visual of the issue is at around 2 minutes in
the layout I use if you feel so inclined
the injector cleaning done

Here are the VVE labels I use:
column
400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 6600
row
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120 125 130 135 140 145 150 155 160

I really wanted to figure it out on my own but here I am preparing for the roast in the forums! But seriously I'm open to any and all advice you guys have.
Thank you all in advance!

---

This should help, let me know...but with that cam you really need to ditch the MAF and go to speed density.

Attachment 150518

236 intake duration .050 can run MAF unless you have a botched maf "tube", lol. Put the EOIT back to stock for now. Add some spark timing, check, if worse then pull some timing. EOIT being wrong is going to be a lethargic response to actual engine response. That table is damn near only for emissions.

Carburetors had no injection timing. They also never gave an "F" and yet they ran better than EFI as far as bucking/throttle response.

You can pull some out of your Injection EOIT Adder Vs ECT table to help some for time being, I mean like 30 when i say "some" for that cam. Maybe more, maybe less.

Also address that 0 to -11 fuel trim during the bucking.

VTT can play a part but that seems too violent of swings for solely VTT to affect.

Quote:

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Originally Posted by blindsquirrel

I think if you add Spark (*) to your charts, you will see it jumping from 17-24-18-28-17-26-16-27-etc, without much of any corresponding change in throttle/MAF/MAP/airmass.

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I think you're saying to have the the spark PID displayed in my charts, or the line graph. If so, its in the 4th group from the top, white line, 1st series. A screenshot of my layout is in the first comment. Maybe I'm missing the PID you're talking about though. When spark smooth is enabled timing advance definitely bounces like you're saying, assumingly trying to smooth the bucking going on. The log posted, along with screenshot, has spark smooth disabled and the timing is bouncing by tenths. Let me know if I took your comment the wrong way though.

Quote:

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Originally Posted by Cringer

This should help, let me know...but with that cam you really need to ditch the MAF and go to speed density.

Attachment 150518

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Will try it out as soon as possible. And yep that's the next direction I was headed. Does going to the 2 bar OS allow pure SD, or still have some MAF blending?

Much appreciated

Quote:

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Originally Posted by ns158sl

236 intake duration .050 can run MAF unless you have a botched maf "tube", lol. Put the EOIT back to stock for now. Add some spark timing, check, if worse then pull some timing. EOIT being wrong is going to be a lethargic response to actual engine response. That table is damn near only for emissions.

Carburetors had no injection timing. They also never gave an "F" and yet they ran better than EFI as far as bucking/throttle response.

You can pull some out of your Injection EOIT Adder Vs ECT table to help some for time being, I mean like 30 when i say "some" for that cam. Maybe more, maybe less.

Also address that 0 to -11 fuel trim during the bucking.

VTT can play a part but that seems too violent of swings for solely VTT to affect.

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The tube the MAF is in is 3.5" or 4" DOM coming out of the intercooler going to the throttle body. No dents or weird bends other than roughly a 60* bend in one dimension. The MAF card is probably 6" from the end of the bend and 4" from the throttle body. There is a BOV but its on the other side of the intercooler so I didn't put a lot of thought into it messing with the MAF other than when it opens at 15-20 kpa.

The only real noticeable difference I've gotten out of changing injection timing being slightly retarded from stock is a bit smoother idle, but mainly got rid of about 80% of the raw fuel smell. So I'll definitely give stock a try again.

The fuel trim swings I had tamed down with prop and integrator under the o2 controller in previous versions. It didn't make any difference with this issue. Out of frustration I reverted everything I could back to stock and ended up here. OL also doesn't stop the bucking so I didn't put too much weight on the trims throwing it off.

I haven't touched the torque tables at all, and don't know where to begin. No access to a dyno I imagine would make that difficult. But I'm open to all suggestions!

Certainly appreciate your input though. Will let you know if we come with a resolution.

Disable spark smoothing. You're welcome.

Quote:

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Originally Posted by edcmat-l1

Disable spark smoothing. You're welcome.

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In the log that's posted, and the screenshot, spark smooth is disabled. It's enabled in the current file posted because its the only thing that makes the car somewhat drivable through a parking lot.

Quote:

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Originally Posted by blakesmiley

Will try it out as soon as possible. And yep that's the next direction I was headed. Does going to the 2 bar OS allow pure SD, or still have some MAF blending?

Much appreciated

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The 2 bar OS allows you to run hybrid (with MAF) or pure speed density. The nice thing is that the 2 bar OS fixes the inherent defect with the OEM OS in SD. I have a similar cam as you and I resisted the move SD for a long time. I finally gave in and the car runs much smoother over all. I am not going to say it will fix you low speed issues here. But it did solve a lot of low speed drivability issues caused by reversion and even surging from OEM O2 PI tuning.

Quote:

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Originally Posted by Cringer

The 2 bar OS allows you to run hybrid (with MAF) or pure speed density. The nice thing is that the 2 bar OS fixes the inherent defect with the OEM OS in SD. I have a similar cam as you and I resisted the move SD for a long time. I finally gave in and the car runs much smoother over all. I am not going to say it will fix you low speed issues here. But it did solve a lot of low speed drivability issues caused by reversion and even surging from OEM O2 PI tuning.

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Awesome, I'll give your file a try first then. But maybe going to the custom OS after that!

Quote:

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Originally Posted by Cringer

This should help, let me know...but with that cam you really need to ditch the MAF and go to speed density.

Attachment 150518

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I took two logs.

The first one being exactly the one you posted, and it still had spark smooth enabled. It drove the same or maybe slightly better than previous versions with spark smooth enabled.

After that I turned spark smooth, DFCO, CFCO, and LTFT off in the file you sent and flashed it. It was smoother under light acceleration for sure. Steady state throttle cruising around 1500 was about the same.

With spark smoothing on, the car is drivable. My goal is to have it driving like that with spark smooth disabled. I know the cam is on the larger side for a street car, and maybe I wont be able to get it the way I'd like for low speed. Maybe the spark smoothing left on wont be a bad thing in the end either? Pulling timing constantly at cruise speeds just doesn't seem proper.

I'll attach the file I changed and the two logs I took.

"Spark smooth enable" is the log with file you sent.
"Spark smooth disable" is with the changes I made to the attached .hpt.

u can adjust the spark smoothing to suit when u want the affect, the 3 values and also the high and low by rpm/gear tables

Try this

Attachment 150553


This would turn out better in speed density. You should test to see if the buck is better or worse with closed loop active. If it is better in open loop you can tune the o2 response a bit to get it better but sometimes open loop is the answer. This should do OK in closed loop.

Quote:

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Originally Posted by blakesmiley

I took two logs.

The first one being exactly the one you posted, and it still had spark smooth enabled. It drove the same or maybe slightly better than previous versions with spark smooth enabled.

After that I turned spark smooth, DFCO, CFCO, and LTFT off in the file you sent and flashed it. It was smoother under light acceleration for sure. Steady state throttle cruising around 1500 was about the same.

With spark smoothing on, the car is drivable. My goal is to have it driving like that with spark smooth disabled. I know the cam is on the larger side for a street car, and maybe I wont be able to get it the way I'd like for low speed. Maybe the spark smoothing left on wont be a bad thing in the end either? Pulling timing constantly at cruise speeds just doesn't seem proper.

I'll attach the file I changed and the two logs I took.

"Spark smooth enable" is the log with file you sent.
"Spark smooth disable" is with the changes I made to the attached .hpt.

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I think it is time to go speed density.

Quote:

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Originally Posted by Alvin

This would turn out better in speed density. You should test to see if the buck is better or worse with closed loop active. If it is better in open loop you can tune the o2 response a bit to get it better but sometimes open loop is the answer. This should do OK in closed loop.

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I'll give it a try as soon as possible!

I have forced open loop in previous versions and it didn't make a difference, but I'll do it again with your file.

I've also had it go into PE at the trouble RPM range with 0% tps to see if it cleared up any. There was no change.

Attached is the log with it going into PE if you're interested. Also a screenshot of it.

[Attachment 150567

Going to the custom OS and SD is my next step.

Quote:

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Originally Posted by Cringer

I think it is time to go speed density.

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Will do. Thanks for your time, I know you're a busy guy!

Quote:

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Originally Posted by blakesmiley

Spark table, started with a stock table and advanced 5* in the problem area then retarded from there in 5* increments until it was 10* advanced without much change. I settled on the table in the file posted. around 24* in the problem area.

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I think you should try on the order of 20 degrees additional advance in the problem area. At least.

Quote:

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Originally Posted by smokeshow

I think you should try on the order of 20 degrees additional advance in the problem area. At least.

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I have a mild LS3 (mild cam, Mamo heads, light flywheel, supporting mods) in my Miata. I had bucking that was most noticeable in 2nd gear, around 1600 rpm, off throttle. It was *horrible* when the car was first running, the car was essentially undriveable in that region. I have it 95% gone by *reducing* timing in that area, which on my car is .08, .12, .16 g/cyl area in the spark table below 2000 rpm. I've seen some guys post that even their stock Corvette or whatever does that a bit, so I'm satisfied.

I *did* try increasing spark there, but the car wouldn't slow off throttle. It's interesting to me that it seems to work for many people ... maybe their vehicles weigh more than 2400#, so that's the difference.