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The short-term effects of an oxidatively unstable diesel fuel were investigated in a 1982 General Motors 4.3-L V-6 diesel vehicle in city-suburban service. Effects on injector coking, Federal Test Procedure (FTP) emissions, fuel economy, cold-idle passenger compartment sound pressure, and driveability were monitored. The unstable test fuel caused frequent severe vehicle fuel filter plugging, despite the fuel having been pre-filtered with a filter having a pore size half that of the vehicle fuel filter. Filter plugging caused by fuel gum resulted in excessive smoke emission and power loss in highway operation and a tendency for the engine to stall easily when cold. It was concluded that vehicle fuel filter plugging can cause some of the complaints (for example, poor driveability and excessive smoke) attributed to injector coking. Injector coking was also observed, but the extent was insufficient to cause adverse effects on the measured parameters.