hey whats up i have a 88 7mgte and i couldnt figure out why my car also ran like shit, after tareing the pipes and stuff out to track a leak i found a coupler. check the little pice of pipe that connects the throttle body to the 3000 pipe mine was cracked in half and couldnt tell untill i took it off, look for other cracks to in other pipes also.
tdo you know how to check your car for codes with the jumper wire heres how
http://www.toyota-supra.info/forums/index....ct=ST&f=2&t=333
this link from the forum will help you out.
but i could never figure out why my car was running crappy until................................i took out the plugs and look into the head and i found oil and anti, in the number 1 and 2 cylinders and my car doesnt even smoke on bit of smoke, i have 93000 on mine and you said 173000 on yours might be a seepy head gasket you never kow until you look or do a compression test.
hope this helped you out some good luck