Stepson bought a 1985 Marquis recently(2days sgi). It stalls out at lights and runs rough. The engine seems to be good, starts ok, but I’m not familiar with this car at all. Some of the forums have mentioned cleaning air sensors. The exhaust blows white when started and smells of unburnt fuel all the time. Is there an idle adjustment screw? I’ve cleaned the air sensors on the housing for the air filter, made sure all hoses were attached properly too. The former owner said it was “mechanically sound“ and the boy made the deal before I could really look it over. I’m afraid the slimy seller did something to make it run well just long enough to sell it. Any help would be greatly appreciated.