bucking/surging as low eng speeds

[ramjudd]

When creeping through a parking lot at 10mph or less, car bucks like crazy. I just put this big stroke 418 LS2 in a 2001 ZO6, HUGE cam = 258/266 .647 113lsa. I've been out of the tuning loop for a few months with delay's on this project + I've lost my tuning notebook. Shit. So could someone spare me the search engine search and point me in the right direction? I am tuning in SD mode. Runs like a fuggin scalded cat otherwise, and idles nicely, only this bucking issue at very low speed/rpm. Unbelievable motor.

Robert Judd / Gen 3 Motorsports

[EC_Tune]

Sounds like a typical big cam. Nasty near idle & a rocketship everywhere else.

Take a look at your spark while "parking lottting it". If it's jumping around a bunch you may be able to smooth it. Watch the cylinder air when it's jumping. then put the same timing throuout that range. That should help settle it.
After you settle the spark end of things, watch the AFR and make sure it's not jumping a bunch as well.
You may want to put a bit more throttle cracker in around idle RPMs & MPH to help it as well.

Just a couple thoughts.

EC

[ramjudd]

EC

Thanks for the 411!! the advice has affected some change but let me ask you, where should I have the Enable and Disable speeds at? Leave them default? Anyway, 0-32mph/400-1000rpm portion of the table is 0 default, and I have been slowly adding to these cells up to currently .750 (except for the cells in 0 mph). Really has helped a lot, but not completely gone. I have another residual issue that might be contributing to this symptom, see my other post.

Thanks again EC, and let me know what you think about the Enable/Disable speeds.

Robert

[WS6FirebirdTA00]

i assume you are in open loop? what is the AFR when this happens? you may want to disable the throttle cracker below these speeds if you havent already as that cna contribute

[EC_Tune]

I usually leave throttle cracker enable/disable stock. Usually moving it up doesn't help.

EC

[WS6FirebirdTA00]

I usually leave throttle cracker enable/disable stock. Usually moving it up doesn't help.

EC

See and I have seen the total opposite, my throttle cracker is actually disabled under 55 mph and makes a HUGE difference in drivability.

[EC_Tune]

You have an IAC though. His Z06 doesn't.

EC

[WS6FirebirdTA00]

on the vette i am working on the throttle cracker values were causing some issues as well.

[EC_Tune]

Vette, Hmmm... What kinds of issues?

Here's one I had recently:
FYI: Don't taper the cracker values too much near idle RPM's. I did and the idle speed started surging badly at cruise with the car in neutral. 400 & 1000 RPM values should be relatively close. I had a bunch more air in at 400 RPM to prevent any notion of stalling. <-- Don't do that...

EC

[WS6FirebirdTA00]

it was those issues i posted up a while back you were helping me out with. ETC is weird lol

[5_Liter_Eater]

I'm going to around here as my Procharged vette has the same problem. I've done nothing about it yet though aside from some things I found in an A&A base tune BIN file which were to:

Change throttle cracker enable from 2 to 100 MPH
Change throttle cracker disable from 1 to 20 MPH
Changing Idle Spark Advance from -7 to 10 below 1200 RPM's.

I need to log the bucking and see if the spark advance is jumping around. If so which table do I edit, the idle spark or the main spark?

[WS6FirebirdTA00]

the main spark tables, unless for some odd reason you are in idle tables

[EC_Tune]

Make sure the Idle tables are close +/- 2 deg to the Main tables as well. This can cause bucking & kicking at very low TPS.

EC

[5_Liter_Eater]

Make sure the Idle tables are close +/- 2 deg to the Main tables as well. This can cause bucking & kicking at very low TPS.

EC

Very low TPS is where I am seeing it. Like the transition between zero and 1% (although my TPS is never less than 4% I'm guessing because of the ETC). My idle spark tables are not very close to the main spark tables. They were further off with the stock values. I'm afraid to alter the main tables to much because I cruise @ ~1500 RPMs where the bucking happens in any gear so I don't want to alter mileage/drivability.

Attached are my tune and a log I just took. At the end of he log was some pretty good bucking I recreated in the parking lot.

[EC_Tune]

You should move the idle spark tables to match the Main tables then.

[WS6FirebirdTA00]

if the car is lean it can also cause the bucking, I will try to check that stuff out tonihgt

[EC_Tune]

Very good point. Although at very light throttle it is slightly less of an issue unless you are seeing 16+ AFR.

EC

[5_Liter_Eater]

You should move the idle spark tables to match the Main tables then.

Done and done. I will flash this tune and see how it acts on the way home from work.

Something I noticed from the histogram display is that the 0.08 airflow row advance values are very low from ~800-1800 RPM, they are ~11-13 degrees. Both the idle and main spark tables are much higher. So something else is making the timing in these cells be lower than commanded via the idle and main spark tables. DFCO?

[EC_Tune]

Something I noticed from the histogram display is that the 0.08 airflow row advance values are very low from ~800-1800 RPM, they are ~11-13 degrees. Both the idle and main spark tables are much higher. So something else is making the timing in these cells be lower than commanded via the idle and main spark tables. DFCO?

Definitely DFCO. You will probably notice the injector PW go to a minimum value (probably .3 mSec or something) as well. The deeper you go into DFCO the less spark you have as well.

EC

[5_Liter_Eater]

Well I'm going to try it with just matching my idle spark tables with my main spark tables and see what that does. After that I will try disabling DFCO although in looking at how to disable it it would seem that it already is.

Enable RPM VS Gear is 7000 in 1st and it happens in 1st
Speed VS Baro is 30+ MPH and it happens below that
Enable TPS values are <=1 and my TPS value is never less than 4