I have an 08 G8GT. It has cam, heads, headers, FAST102. The transmission is a 6L80 built by Circle-D and the Stall is a 258mm, 2800 C2 also from circle-D. The trans and stall were installed 5-6 months ago.
A few days ago I started to notice what felt like a very slight "miss" when under a load. I'm due for plug change and wrote it off to that. It would come and go and seemed load dependent and would all but disappear when eng and tran were warmed up. (My trans temps are typically 165-175 when fully warmed up). The couple mornings ago, with cold engine and going up a slight incline, Dash Boss showing +/-0 Converter Slip RPM in 6th, I felt what almost felt like slight "trailering" that went away on level ground. It was a slightly different feel than the earlier described "miss". This happened a couple more times going to work that morning.
On the way home, the slight "miss" and "trailering' feelings were a little more pronounced and when I fed throttle to move around some slower traffic there was a deffinate shuddering or bucking. It felt like a serious "miss" with the bucking hitting about twice a second or so. I was doing mid-to-upper 40s in 6th with Converter Slip RPM showing "0". When I lifted, the shudder stopped. When I rolled in again, it returned. I pulled the shifter to Manual 6, the stall unlocked (Converter Slip RPM was not at or near zero) and when I fed throttle there was no shudder, bucking or "miss" feeling. I repeated this twice with the same results.
When I got home, I modified the tune to add more ramp pressure and regulator offset. Test drive produced the same or maybe even slightly worse results. I then changed the tune to prevent any lock-up in any gear, speed or throttle position. Dash boss confirmed Converter Slip RPM was greater than zero (up to a few hundred RPMs). Car drove 100% normally. No shudder, miss, bucking, etc.
I parked the car and have contacted circle-d who are totally great to work with. The guess is that the converter lock up clutch is toast for some reason. C-D will know for sure when they open it up.
Can any of you with expertise with aftermarket stalls and tuning, please look at the tune I've been running to see if the problem could have been caused by the TCC settings.
TIA,
Ed