smoking and high idle

i have a 88' iroc-z with tpi 305 112,000miles. it smokes alot at start up and continues while driving but not as much. it does have a have a small exsults leak.it produces a real bad exsults smell.any ideas?also i go to start up and sometimes it is fine but alot of the time it idles very high i can get it to go down by pressing the gas hard but when i come to a light it is high again i have spent alot of money to get this problem fixed and it still does it.it also hesitates upon acceleration. did i just buy a lemon or does this happen often.my mechanic told me that it does but i dont know if i trust him too much right now,having spent all the money and it still doesnt run right