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