I have a 1999 Silverado ( the 99-02 style) I recently replaced the original 4.8 and 4l60 with a cammed 6.0 lq4 and 4l80e transmission. This is a half ton single cab short wheel base 2 wd truck. 2004 model lq4 engine, new oil pump, beehive springs, BTR stage 2 n\a truck cam Stock 3\4 ton converter 2005 model 4l80e fresh rebuild Cold air intake, shorty headers, having issues with what I believe is the tune. Engine runs fine and strong transmission works mostly as it should. I'm not the tuner but I feel like I'm about to be. Issue is as follows. 70 mph flat highway cruising in manual 3rd gear Scan tool pids Engine rpm - 2800 Input shaft speed- 2800 rpm Turbine speed - 2800 rpm Tcc slip - 0 rpm TC locked Same highway cruising in "D" Trans in 4 the gear Pids Engine rpm - 2100 Input shaft speed- 1950 rpm Turbine speed -1950 rpm Tcc slip - 150 TC locked If I give it throttle and cause it to downshift into 3rd tcc slip goes to 0 once it goes into third If I use the scan tool to command the tcc to lock or unlock, it does and rpms jump and go back down as they should which means it's working. By my logic achieving lockup in 3rd means that the TC, tcc valve and bore, tc solenoid, fluid level and pressure, end electronics are indeed good. So with those being good and working correctly in 3rd they should work in 4th as well. I'm having trouble deciphering if the slip rpm I'm seeing is a glitch or other issue in the PCM logic or if the tune is simply not right like the setup or segment swap either being bad, corrupted or mismatched os, or even a lack of segment swap. I don't have a tune file to upload sadly but I'm at my wit's end with this. I can handle all electrical and mechanical repairs and testing and have done as much as I can find to do but the computer part might as well be an alien language to me.