ArKay99

iTrader: (1)

I have been having a problem ever since I installed my AFR's and 224/228 .581"/.588" cam. I have it idling very nicely. When I am idling and give it some throttle and let go more often than not the rpm's will drop to around 400 and then come up to idle. It also happens when driving and I let off the throttle. The other issue I have is if I am creeping along in first or second gear at any rpm under 1500 the car will start to buck and get into an oscillation that gets pretty severe and will not stop until I get the rpm's above 1500. Also if I am on the highway and cruising below 1500 rpm ocassionaly I will feel what seems like the engine bucking slightly, but being in 6th gear it feels like a clunk. I have logged this and see no KR. I have tried messing with every table except for the throttle follower and throttle cracker. I've added and subtracted timing at the low rpms' and loads and have done the same with air and fuel.

While logging this condition the other day I noticed that my MAP was following the oscillation and was varying from 30kpa to 80kpa, but the MAF was sitting at 8.27g/s. I would think that if the MAP was changing that much that there should be a change in the airflow the MAF is monitoring. Also, the MAF never goes below 8.27g/s in my car. I am wondering if the MAF is dirty or if it is on the way out. Any ideas? Am I interpreting this correctly?

Also under all other driving conditions the car is a beast.

MNR-0

Sounds like your DFCO settings are interfering with your CAMs ability to hold dynamic compression at low revs. Try raising your DFCO MAP values by 10kpa (entering and exit) and see how that goes. What I assume could be happening is your PCM thinks your are decelerating and so cuts fuel only to add it again as your MAP values deteriorate.

422 ragtop

iTrader: (2)

I am having the exact same problem.

Mine was noticable before, but not intolerable.

I changed to LS-2 intake and 90 LS2 TB thinking I had an airflow problem with the 422 and now it is almost undrivable. Cam is 232 236 @114, stage 3 heads etc

The bucking and idle surging got way worse. Took it to a local shop that fixed start-up idle, but made everything else worse. As soon as I tip into throttle it tries to stall. Letting off throttle at RPM, whether coasting or sitting in the garage, it drops down to approx 500 RPM, up to 1500 RPM, sometimes catches itself after 3 or 4 iterations, sometimes not. I have done numerous idle learns to no avail.

I have single copy LS-1 edit for the car and can email the file if someone out there can help.

thanks

Brad

Another_User

No good. I set DFCO enable to 258 degrees. It isn't the DFCO.

Ragtop 99

iTrader: (7)

You may be rich at at 1500 rpm. Try reducing your VE at 1600 rpm by 2% (i.e. multiply by 98%) and see if that helps with the surging at 1500 rpm.

ArKay99

iTrader: (1)

I didn't mean to mislead anyone by my description. From 1500rpm and up the car is fine. Below 1500 and with foot off the accelerator and just creeping along at 5mph or so is when the problem is most severe. I just looked at my VE table and saw something that looked interesting. In the first row - 15kpa the values start at 967 and climb to 1042 at 1600 then drop to 900 at 2000 and then climb slowly to 1235 at 4000 then level off to 1105 all the way to 8000. The sharp rise at very low rpm's and then the sharp dip, then sharp rise before it smooths has me wondering. It does something similar at 20kpa also. All the other values start at a low level and curve smoothly over the rpm range.

WS6snake-eater

You should not see any dips and/or rises in your VE table. It should all be smooth transitions.

Ragtop 99

iTrader: (7)

There's no direct adjustment at 1400 rpm, so i suggested using the 1600 rpm adjustment. 1200 rpm may be adjusted for idle, so a change at 1600 will cause a 1% change at 1400.

ataylors

iTrader: (2)

check that it isnt bouncing in and out of the main spark table..

Cal

iTrader: (1)

I just installed a new cam (TSP235/240) and now I'm getting some pretty severe bucking any time the rpm is below 2000 and at light load. I tried reducing spark timing everywhere below 2000 rpm and that helped a bunch with the bucking, but made the car fall on it's face trying to pull away from a stop. So I had to put about half the spark advance I took out back in.

I've noticed on EFILive that the spark advance bounces up and down a lot when the car is bucking, but I don't know if this is the cause or the effect. Lugging the engine a bit makes the spark advance back off and and bucking nearly stops.

Just looking for some new ideas; ArKay99 did you get this resolved?

P Mack

iTrader: (6)

I had this happen too, but it's only at no load at exactly 1000 rpm. It's completely unrelated to spark advance cause i made my spark table flat and it still did it, and the spark is pretty much constant when it happens. It also didn't seem to matter whether I was in SD or Maf. I did notice the map oscillating with it, but a little out of phase. By messing with the ve table it seemed like i could either make it stable and die out or unstable and buck worse and worse, but i can't get rid of it completely. I'm gonna try messing with the ve table more in that area as soon as I fix my tranny problems and see if that helps.

Cal

iTrader: (1)

Thanks for the reply, P Mack. I think I will try messing with the VE table next as sugested above by Ragtop 99. That helped at 1200 rpm and below with my smaller 224 cam, so it makes sense that I would have to adjust VE at even higher rpms with the much larger cam I just switched to.

P Mack

iTrader: (6)

What you said about the spark advance is interesting. Mine wasn't oscillating, but i didn't try backing off the timing above and below 1000 rpm so i guess i'll have to try that.

Cal

iTrader: (1)

P Mack, I wound up only reducing the timing between 0.08 Gms/cyl and 0.56 Gms/cyl for best results. My reasoning is to reduce light load bucking without making the car bog on dead-stop take-offs. This seems to work great, and I may go back and reduce it some more at the midpoint and below.

I also just got done tuning the VE as sugested by Ragtop 99; results were so good I went back and did it again all the way from 400 to 2000 rpm. That reduced the raw gas smell and reduced bucking at the same time. Although there is still some jerkyness, I'm very happy with the improvement. For your C1 cam I'm sure this would be overkill, but for others with a 230 to 240 duration cam, here's the multipliers I ended up with for the VE adjustment:

2000 rpm 90%
1200 rpm 65%
800 rpm 63%
400 rpm 54%

I think I could even back it off further because I didn't induce any stumbling on take-offs at this point, I just got tired of messing with it. I left the 800 rpm point disproportionately high because it is below my idle at 1000 rpm, yet should help reducing bogging from a dead stop take-off. (I think the rpm drops below the 1000 rpm idle when I let the clutch out.)

Slowhawk

iTrader: (12)

As alot of people stated it is fuel/timing related.I ussually smooth out the timing tables in that area so the changes are not drastic.That ussually provides a crisp throttle with no bucking.

Another_User

I change my mind on this issue whenever I try something new... I have tried fuel and spark and...well everything. So now I went with an ungodly amount of spark at <.24 g/cyl using a 2005 5.3L truck spark table, and I think it helped a lot. I am going to look at my logs when I am done writing this as a matter of fact... I noticed that in the spots where I have bucking my g/cyl drops below what the car idles at. That is not right. My theory is that because of the overlap timing is not optimal...maybe reversion is causing it...I don't know for sure. But I know the cylinder pressure should not drop that low while you are opening the throttle and letting more air in. Adding timing will increase the cylinder pressure, and hopefully ignite the incoming charge before it has a chance to blow out which ever way it is blowing out. Off to the logs...

P Mack

iTrader: (6)

I can't seem to find any logs where my car is bucking, but can someone look and see what the IAC is doing in relation to the MAP and RPM when this happens?

Cal

iTrader: (1)

The IAC doesn't change or changes very little. I've never seen my IAC change very fast for that mater.

Another_User

Trust me...I have logged it all. I have excellent logs of bucking in action. It does not appear to depend on MAP or IAC, or anything like that. Fueling is a tough call because you can't depend on your O2s at idle with this much overlap. I have noticed some low Dynamic Cylinder Air, which is part of why I have decided to try moving my timing to this extreme (look at an 05 5.3 spark table and you will see what I mean). I think it helped, but I need to make another couple runs to be sure. A cam with a lot of overlap should be pretty timing sensitive because you can easily blow your exhaust back into your intake, or allow too much of the new mixture to blow right into your exhaust...

Also, I have done runs with IAC locked, and fueling locked rich and lean, neither showed a difference. I tried it with timing locked, but I was not about to lock in the values I am working with now...50 degrees is a lot!

luv2spd

iTrader: (1)

Another User - Did you say that turning off idle proportional fueling helped? I tried it a few weeks ago and it seemed to make my bucking a lot worse. Do you still have yours disabled?