1998 Chevy Cavalier PO171 and very rich fuel condition

MY 98 CAVALIER IS FAILING EMISSIONS, STARTED OUT AS PO171 CODE, THEN OTHERS, LONG STORY SHORT, EVERY SINGLE SENSOR HAS BEEN REPLACED, SO HAS THE FUEL PUMP, PRESSURE REGULATOR AND PCM. IVE SPENT OVER $1800 AND THE CHEVY DEALER WAS ITS LAST STOP WHERE THEY SAID IT WAS THE PCM AFTER CHARGING ME $450 TO CHANGE THE MASS AIR FLOW SENSOR AND OXY SENSOR AGAIN! SO A NEW PCM IS INSTALLED TOO AND IM BACK TO THE ORIGINAL PO171 AND STILL FAILING EMISSIONS. THE DEALER CHECKED FOR VACUME LEAKS AS DID I AND NONE HAVE EVER BEEN FOUND. I KNOW THE PROBLEM IS STARING ME IN THE FACE BUT IM LOST, I HAVE A SCAN TOOL AND HAVE GONE THRU THE DIAGNOSTICS AS OUTLINED IN ALLDATA BUT STILL HAVE YET TO FIX THE PROBLEM. ON A LAST NOTE, WHEN EVER ANY COMPONET IS REPLACED, IT RUNS GREAT FOR ABOUT 10 MILES AND THEN BOOM BACK TO THE ORIGINAL PROBLEM. SINCE THE PCM HAS BEEN REPLACED, NOW I AM SEEING VISIABLE BLACK SOOT FROM THE EXAUST AGAIN (THIS PROBLEM STOPPED FOR A WHILE WITH NO REASONS) AND ITS ALSO RUNNING SUPER RICH AGAIN. HELP PLEASE!