Fuel Delivery and Air Induction: Description and Operation

The fuel system handles two main tasks necessary for diesel engine operation: (1) it provides storage space for the diesel fuel, (2) it delivers the correct quantities of filtered diesel fuel to the cylinders. It is also responsible for admitting an unrestricted supply of filtered air to the engine. The second function-delivering fuel to the cylinders is handled by a high-pressure fuel injection system that is totally unlike the fuel injection systems used on contemporary spark-ignition engines.

Because the diesel engine relies on a high compression ratio (23:1) to generate heat that will ignite vaporized diesel fuel, the maximum possible quantity of air must be admitted to each cylinder on every air intake stroke. So the diesel engine has no throttle to restrict airflow as on spark-ignition engines; the intake ports are always "wide open." In the case of the optional turbo-diesel engine, a turbocharger is used to pump air into the engine under pressure. The diesel engine's power output-hence its rpm-is determined solely by the quantity of fuel injected into the superheated air inside the combustion pre-chambers. At idle, the air-to-fuel ratio may be greater than 100:1; even at top speed, the mixture never approaches the comparatively rich stoichiometric air-to-fuel ratio of approximately 14.5:1 that is common to spark-ignition engines. This lean operation is largely responsible for the diesel engine's excellent fuel economy. (A stoichiometric ratio is one in which all fuel and oxygen are consumed in combustion; a diesel always has oxygen left over.)

The diesel engine's fuel injection pump draws fuel through a water separator and filter from the tank. This fuel is then put under very high pressure. The fuel distributor portion of the pump selects (according to the engine's firing order) the cylinder to which the fuel will go. Finally, the pressurized fuel enters the combustion pre-chamber of the correct cylinder via an injector. Because diesel fuel is not a particularly volatile liquid, thorough vaporization is necessary to obtain efficient burning. Each injector is designed both to restrict the admission of fuel until a pressure of about 125 bar (1786 psi) has been attained and also to spray the fuel into the pre-chamber with a uniform pattern of highly vaporized fuel.

Special equipment is necessary for nearly all kinds of diesel injection system work. So if you lack the skills, the tools, or a clean workshop for diesel fuel injection tests, adjustments, and repairs, we suggest you leave this work to an authorized dealer or other qualified shop. We especially urge you you attempt to consult your authorized dealer before repairs on a vehicle still covered by the new-car warranty.

The diesel engine's fuel system can be divided into two main parts according to their functions. These are fuel storage and supply, and fuel injection.

The molded plastic fuel tank is located beneath the car, ahead of the rear axle. The fuel tank capacity is 53 liters (14.0 US. gallons). When the fuel gauge indicates reserve there is approximately 7 liters (1.8 US. gallons) of fuel left in the tank. The correct fuel to use under most conditions is diesel fuel No. 2.

The tank is equipped with a sending unit for the electrical fuel gauge, incorporating a pickup tube for the transfer of fuel to the engine, and a connection for the return of surplus fuel from the fuel injection pump. To prevent exposing the injector pump to harmful contaminants, the fuel system also contains an efficient water separator and fuel filter.

The injection pump is engine-driven by the same toothed belt that drives the camshaft. It handles virtually all engine fuel functions including pumping fuel from the tank, and controlling the timing and the quantity of fuel injected. A timing advance mechanism changes injection timing in response to engine speed and during cold-start conditions. Beginning in 1989, pumps have additional features that compensate for altitude changes and for different engine loads. The pump meters fuel to the injectors at an operating pressure in excess of 120 bar (1706 psi) and incorporates check valves to prevent backflow of fuel from the injectors. All the internal moving parts are lubricated by the diesel fuel, so routine maintenance is not required.

Repairs to the injection pump require specialized knowledge and equipment, and parts for rebuilding are not generally available. Faulty pumps must be serviced by the pump manufacturer or other qualified diesel fuel injection repair shop. Internal problems usually require replacement of the pump.

The fuel injectors are complex and very precise devices. They open at a specific pressure, in excess of 120 bar (1706 psi), and close quickly to ensure complete fuel vaporization. The injectors can be disassembled for cleaning and repair, and Volkswagen replacement parts are available.

The diesel engine has a rectangular, dry-type pleated paper air filter, held in a plastic housing bolted directly to the intake manifold. Intake air is drawn from inside the passenger-side fender well, through an adapter, in order to reduce the noise caused by intake air resonance.