Ok i have a 98 catera. i just had the valve gaskets replaced and it was running better. The guy that i go to says that my remaining problems may be to some sort of fuel issue. So i put some injector cleaner in and take some more advice from the mec. on what else to clean. he says to use some brake cleaner on the sensors? just after the airfilter. and also on the valves that lead into the manifold. to clean up any oil and dirt for better fuel usage. well when i go to start my car afterwards the ideling is so low and rough that is stalls out. also i have black smoke coming out of my tail pipe for most of the time. well i called around and they said to run it forr 30 -45 mins to get any fumes and chemicals out of the engine. so i do. well it is running a little better so i go and take it to the local jiffylube to clear out the codes and for a quick look over. after they do that and can't tell me anything new, i try to take it home. well it is still ideling low and stalling. it is no better then it was when i first started it up. can someone please help me with this may be. and what i can do about it. thank you for any help.