Where the Volkswagen Bosch diesel injector can leak from (left)

1) Loose or damaged hose or injector

2) Worn out fuel return hoses. Return tube is NAPA 4219 LC Fuel Line PCV 3.2mm Performs to J30R7

3) Bad injector

4) Gases leaking from combustion chamber

Also see page Rebuilding a Bosch VE injection pump

Cutaway view of the Bosch diesel injection pump (left)

Volkswagen diesel injection pump distributor head with high-pressure chamber.

Schematic of the Bosch diesel injector delivery valve (right)

Volkswagen Bosch diesel delivery valve with return-flow restriction

The delivery valve closes off the high-pressure line from the pump. It has the job of relieving the pressure in the line by removing a defined volume of fuel upon completion of the delivery phase. This ensures precise closing of the injection nozzle at the end of the injection process. At the same time, stable pressure conditions between the injection pulses are created in the high-pressure lines, regardless of the quantity being injected at a particular time.

The delivery valve is a plunger-type valve It is opened by the injection pressure and closed by its return spring.

Between the plunger's individual delivery strokes for a given cylinder, the delivery valve in question remains closed. This regulates the high-pressure line and the distributor head's outlet port passage. During delivery, the pressure generated in the high-pressure chamber above the plunger causes the delivery valve to open. Fuel then flows via longitudinal slots, into a ring shaped groove and through the delivery valve holder, the high-pressure line and the nozzle holder to the injection nozzle.

As soon as delivery ceases (transverse cutoff bore opened), the pressure in the high-pressure chamber above the plunger and in the high-pressure lines drops to that of the pump interior, and the delivery valve spring, together with the static pressure in the line force the delivery valve plunger back onto its seat again. (Fig. 11)

Precise pressure relief in the lines is necessary at the end of injection. This though generates pressure waves which are reflected at the delivery valve. These cause the delivery valve to open again, or cause vacuum phases in the high-pressure line. These processes result in post-injection of fuel with attendant increases in exhaust emissions or cavitation and wear in the injection line or at the nozzle. To prevent such harmful reflections, the delivery valve is provided with a restriction bore which is only effective in the direction of return flow. This return-flow restriction comprises a valve plate and a pressure spring so arranged that the restriction is ineffective in the delivery direction, whereas in the return direction damping comes into effect (Fig. 12).

In other words, it keeps the pressure in the steel fuel lines relatively constant.