Just got my swap done. No tune, just vats removed. 68 firebird ls1 with 4l60e. The trans upshifts great. But when I slow back down to say 15-25mph and coast to stop or go into a parking lot, the car bucks and jerks, until I give more gas of hit brakes. Whats the deal? Im told its a tuning issue.

 

Sounds like the converter's lockup clutch is not disengaging. Could be electrical, the PCM tune or a bad converter. If hitting the brakes disengages it then the problem is limited to electrical or PCM tune.
What PCM are you using and where did the harness come from?

 

I used the PSI harness and PSI TCC switch. I am using the stock LS1 corvette PCM with only the VATS removed. The engine has long tube headers and the Spectre custom intake. Thanks for replying. I am on a waiting list for Synergy to tune the engine.

 

Well, your right about the trans not unlocking the torque converter. Finally got my tune at Synergy motorsports in livermore. Rick is the man. My vss values were all out of whack. Trans software incorrect. Rick calculated new values so the ecm knew exactly how fast it was going and loaded a camaro tune for the trans. Plus redid all the fuel, maf maps and everything. The car drives like a dream and is fast as hell. Wow what a difference a good tune makes.

 

Glad you got it running well and giving up an update to explain how important a good tune is.