Why your 4.5 litter Cadillac blows black smoke at an idle?

black smoke mean unburned fuel could be a few things check map sensor manifold absolut pressure sensor many times the rubber vacuum hose cracks or falls off/ check for high fuel pressure. i beleive your car has self diagnostics turn key on but dont start hold the off button and the warm button on your climate conrol panel simutaneously until dash reads codes write all the code down email me with codes