Bogging At Shift Points

[madscientist91]

Hey, not really completely sure this belongs here because it could be the engine tune but im pretty sure its the tranny. I have a 91 GSX Auto and i got one of those GM MAFT's because i hacked my old stock onecar idled rough. Well for awhile i was knocking and never really gave the car more than a feather of throttle until i got it somewhat tuned. Ive never tuned a car before and all i have is a data logger with o2 volt/Knock sensor so it was torturesome and frustrating. Well couple days ago i figured my tune out and got it all the way to 3/5ths (Mid throttle levels) throttle without any knock. But for some reson my car keeps bucking forward. I think its the transmission trying to shift because it does this only at the shift points, 3000-3300 rpms and 4000-4500 rpms. It also only does this if im not giving it enough gas.. i think its trying to shift but when it does theres to much power and im still trying to accelerate that it gets confused and bounces gears back and forth. I have alot of mods compared to what the stock trannys supposed to take.. turbo back, intercooler pipes, bov, maft, 13 psi. I can avoid it from bucking by giving it more gas when accelerating hard (never full throttle, trying to still tune out WOT) and skipping over the shift point but if im trying to just pass someone without getting a ticket it bucks sometimes. Any thoughts or experances would be greatly appreciated.

 

[spoolamatic]

To me it sounds more like a tune problem .As for the trans it has vairable shift points as they are controled mostly by the throtle position in the shift maps and for a given throtle position it looks up what shaft rpm to shift at not just the few you listed .
It sound like the tcu is working but a bad tps sensor will play havoc on a auto trany car .This could cause it to shift upand down like you are saying but never seen it happen but is posible . Better logging software will let you check the sensor for proper operation .
The stock trany will take quite a bit of modding and then it wont make it do those things ,it will still shift using the shift maps in the tcu ,but to much power will realy break parts or burn the cluthes out and that would be a lot of hp on a good trans.

You realy need more logging software as O2 and knock is not realy very much to go on ,there are a few free logging software ot there to get .
My opinion on the bucking is that it is running way to rich as that will cause this .I would check for boost leaks first thing .And follow the instruction for tuning the translator as they are pretty good and that why you need better logging stuff so you can see what your three trims are .
There are a few threeds about tunning the maft in the other sections .
Hope this helps you some .

 

[madscientist91]

Thanks, that was very helpful. I think your right, its gotta be the tune. Ive been doing some more research/question asking inbetween tune runs and between the two im starting to get the car tuned better and its not bucking so much. Although i do hear some weird rubbing sounds coming from the transmission while im driving it, and only while giving it some gas, but thats another issue ill figure out. Im thinking its probably the clutch packs slipping. Im gonna try to get the wideband i bought alittle while ago installed in a couple weaks hopefully.. my pocket really takes a beating on the holidays, sorta speak. Well anyways, thanks again.

 

[spoolamatic]

Not sure about the rubbing sound ,but a slipping clutch pack will not make any noise like that, the engine will just rev up when you give it the gas .
And if they where realy bad it would kind of slide into the next gear not very firm at all and burt fluid smell also .
If it make the sound only when under some power I would look to see if the drive shaft was hitting on something and all of the cv shaft also ,dont know if a wheel bearing will do this or not but you could jack up the car and see if all is tight .Might look at the motor/trans mount also .

Make sure all of the normal tune up stuff is all ok (wires,plugs,gap ect).