95 suburban 350 tbi timing advancing way too much

hello all, long time watcher, first time asker lol. i have a 95 Chevy suburban with the 350 tbi motor and am having some issues with the timing. i have set the timing at 0 degrees (bypass wire unplugged) and when i plug the bypass back in the computer advances the timing to like 20-30 degrees at idle. i have replaced plugs, wires, cap, ignition module, checked IAC valve, TPS sensor, fuel pump, fuel filter and have just ran out of ideas. the high advance is making the truck fall on its face with hesitation when you try to take off in gear. idles fine, and revs fine when not in gear. also, it does not trigger the CEL and have checked for codes and there are none registered. please help me shine some light on this issue. thanks in advance and have a great day!