What causes "bucking"

[zapp168]

Hi, I have a 98 Fbody. Cam is 224/581 W/114 Lobe sep. Long tubes, Patriot stage II heads M6.
I had the car tuned a year ago after the Head / cam and it ran perfect. I have recently added an STS turbo @ 8PSI and had the car retuned. I now am getting a "bucking" at low RPM while driving. It was not there before. I am REALLY new to EFI live and EFI tuning in general. I would like to try to fix this, but I would really like to try to learn something about it if I could. I have read some people adjust the set screw, some open the hole in the butterfly. I would rather try to get the tune to fix it if possible. :help2:
Thank you

[Redline Motorsports]

Welcome to the forum! I don't think your issue has to due with the throttle blade holes or cracking the throttle. It's either timing or fueling issues. Make sure the car is not lean at those engine speeds as a lean condition can cause a "lean miss". I would also suggest bumping up the timing at those areas and see if it cleans up.

As another note; it's not uncommon that the overlap from the camshaft can cause a surge feeling at low speeds. Many times the overlap causes pulses that head back towards the MAF in which the MAF kind of "double reports" loads. A 224 doesn't typically cause this but be aware of it none the less.

I hope you have a wide band so you can at least address the fueling aspect pretty easily!

Howard

[SSpdDmon]

In my experience, I have been able to eliminate a great deal of bucking with the IAC valve. Originally, I thought I needed more than 40* of timing during decel (like my friends' A4s had) to eliminate the problem. However, I believe the cause of my bucking was too much airflow. Think about it....when you're slowing down during off throttle or very light throttle (but not in DFCO), the engine needs a certain amount of air. If it gets more than it needs, the RPMs will remain constant or go up for a second (until the momentum of the car catches up). That initial lurch starts the chain reaction. The car starts jerking...finding itself in different timing cells, which further adds to the problem. Assuming your fueling is where it needs to be and the timing during decel doesn't change dramatically, you need to play with the throttle cracker and follower settings until you find the IAC's sweet spot. That's JMO though...

[TAQuickness]

Bucking can be caused by timing, too much/little airflow, and/or fueling. SSpdDmon makes some good points on tracking it down.

[zapp168]

This happens while I am cruising at low speed, would it still be a problem with decel? The turbo was installed and I drove the car for a week with the old tune with no issue. I know that the shop that tuned the car said that the timing needed to be backed off for the turbo. Yes I have a wideband gauge(AEM), as soon as I figure out how to hook that into the EFIlive I can start checking the fuel aspect. Little busy at work please be patient with me, but thank you all.

[lplott]

I would also check B3313, DFCO, and see what it is set to. If it is 120c +, then DFCO is turned off. This is usually done to prevent lean spikes during decleration that can send erroneous data to the tuner during tuning.

I just found out what this setting does, and have been fighting surging during decleration for a long time. I have reset B3313 back to 40c and deceleration is as smooth as a baby's behind.

As you can see in my sig tag, I turbos on a LS1 so we have a little in common.

Good luck with EFILive, you will really appreciate all the help you can get from the forum's members, they are a willing and knowledgable group.

Lonnie:notacrook:

[Chuck L.]

This happens while I am cruising at low speed, would it still be a problem with decel? The turbo was installed and I drove the car for a week with the old tune with no issue. I know that the shop that tuned the car said that the timing needed to be backed off for the turbo. Yes I have a wideband gauge(AEM), as soon as I figure out how to hook that into the EFIlive I can start checking the fuel aspect. Little busy at work please be patient with me, but thank you all.

At light throttle, should not need to be reduced "due to the turbo". The engine runs as if it's N/A, with the turbo being "along for the ride". [Til the boost hits, anyway:D ]

As has already been posted.. Look at the timing..