Starting in the 1991 model year, Volkswagen introduced the ECO Diesel engine in the Jetta. This is a standard diesel engine with a turbo charger and a catalytic converter. The turbo charger provides only about 6 psi boost but about a 40 percent increase in air flowing through the engine. This provides a modest power increase, but the objective is to greatly improve the engine's emissions performance. Since the system is designed for improved emissions rather than power, there is no fuel enrichment device on the injection pump. The Turbo Diesel Injected (TDI) engine was later introduced. It provided far greater performance with slightly less fuel economy than the ECO Diesel.
The speed of a diesel engine is controlled by the amount of fuel that is sprayed into the cylinders. There is no throttle plate, therefore no vacuum in the intake plenum. A diesel engine is stopped by cutting off its fuel supply.
Clean, dry diesel fuel is the key to a well running engine. Any dirt in the fuel system can get caught in the fuel rail and injectors and inhibit the quantity of fuel sprayed into the engine. Any reduction in the flow and or pressure of the fuel system can greatly reduce the performance of a diesel engine.