Issue with idle after tune
 

I recently put a 6.2 L94 with 4L80 in my 68. It ran great with the stock tune from PSI but I had a check engine light on. So I took it to be tuned a couple days ago to see if they could get the CE light to go off. Now on startup it idles really rough for the first 15-20 seconds before settling down and idling smoothly. It with continuously rev up and down between maybe 500-1000 rpms. I’ve been back to the tuner and they can seem to fix it no matter what they try and I finally told them to just put the stock tune back in it, which they claim they did but it’s still doing it. I suspect they accidentally saved over my stock tune and weren’t able to put it back in because I could see on their computer the stock file had a more recent save date than my first day there. Anyway, does anyone have any ideas on what would cause this? It seems to run fine, other than the rough idle at startup.
I was able to get PSI to email me the stock tune and I plan on flashing it back to the computer on Monday. I just hope that fixes it back to running and idling smoothly.

Re: Issue with idle after tune
 

Makes me wonder if you have a air leak somwhere
Great looking truck btw..

Re: Issue with idle after tune
 

Does the CE light still come on? I so what code is shown?
I know this is a bit out there but on one of my LS swaps the 12 volt constant connection at the fuse block was bad. Mine did a similar thing as yours. On cold start it was fine. Then if I shut it off for a short time it would stumble and not not idle correctly for 20-30 seconds. Finally the connection failed and left me on the side of the road. Found the problem by smelling the melted plastic in the fuse block.

Re: Issue with idle after tune
 

I'm in the process of swapping an LS1 into my 67 but I'm doing a full resto so the body is off. I have the engine and trans mounted and I made a set up so I could start it up.
When I started it the first time it had to hunt for idle speed for 30-40 seconds before it settled in and idled properly.
When I shut it off, I killed all power to the PCM (no constant 12V input) subsequently, at every start up it would again have to hunt for idle.
A few times I left the 12v connected temporarily, when I did that, it started and idled immediately.
As mentioned above, check to see if the "keep alive" 12v constant input connection is good.
Good luck with it