Help with 91 Caprice Classic Problem

Desperately need help with 1991 Caprice Classic. Here is what is going on...has about 120,000 miles, about a month ago it started making a loud noise while in idle, sort of a knocking and was emitting some nasty sooty looking stuff. So, went to 1st mechanic, spent $200 to check computer and fuel pressure regulator, still not right. 2nd mechanic $25 to say it was probably the catalytic converter (didn't do exhaust work), Brought back home before going to exhaust place. Then wouldn't start to go to place. Towed to exhaust place, they changed catalytic converter $300, still didn't start. Back to mechanic #2, for $140 changed out map sensor, tps sensor, coolent sensor, distributor coil, distributor modual. Replaced spark plugs and coolent sensor. Checked timing, fuel pressure. So, HELP. Currently it runs but is sending out some nasty exhaust. It stinks bad, drove in back of my husband for about 5 miles and thought I was going to pass out. The loud noise while in idle is not as loud, but the exhaust is horrible. Any ideas? We just donated a car in November and bought this from a friend in January. Can't stand to donate another car? Anyone have a similar problem or any idea about what we haven't tried already? Thanks.

check the head gaskets my 87 had the same problem i waited to late ans had to replace the motor