Engine after-run, sometimes referred to as “run-on” or “dieseling,” has existed for many years, but seems to be occurring more frequently with late-model cars and low-octane gasolines. A vehicle study was conducted to gain a better understanding of the factors that influence after-run. Engine operating variables, such as high idle speed, lean idle mixture, and retarded basic spark timing, all increased after-run frequency by increasing the throttle opening (intake charge density). Reduced Research-octane number of the gasoline also increased after-run frequency, but Motor-octane number and hydrocarbon composition did not have any effect.After-run exhaust contained about 125 times more aldehydes than engine idle exhaust, and it caused eye irritation and an obnoxious odor. Compression ignition, rather than surface ignition, is most likely the cause of after-run.