Cadillac Repair: BLACK SMOKE, cadillac fleetwood brougham, high pressure hose

Question
Recently I took my 1982 Cadillac Fleetwood Brougham for repairs. My mechanic performed the following:
1. A major tune up- wires, plugs, Distributor cap, and rotor button
2. Installation of a water pump, timing cover and chain
3.High pressure hose
4. Distributor and rotor gear
5. oil change w/ filter-20/50 motor oil
After picking up my car and driving it for a day or so.It began to have problems and it overheated. Since then I have had to change out the spark plugs, and oil, and replaced them with new ones and 10W-30 motor oil. I felt 20/50 was to heavy, my car has about 68,000 miles on it. I have also added a gas treatment. Now, there is black sooty smoke coming out of my tailpipe and the car sluggishly runs  as if it  is getting too much gas or there is water in the tank. Please note that I have never had any problems with the running of my car with the  exception of leaking powering steering fluid, the water pump and it needing a tune up before taking it to the mechanic. My major question  is, why is my car running like this? What could be the cause of the smoke and what can be done to possibly repair the situation? I was told that a possible problem could be with the car's computer, is this true?  

Answer
Hi Judson, I'm sorry to hear you are haveing a bad time with your caddy. This mechanic isn't much up on servicing them if he put in 20w50. Good call in changing it back to 10w30. The computor might be the cause of your problems but not likely. I would look at causes of too much fuel by checking fuel pressure and makeing sure the pressure regulator isn't leaking fuel into the vacuum hose. What also happens alot is the map sensor hose develops a hole in it and then the map sensor gives the engine all it can. Scan for codes and see what you get. Hope that helps. Bill