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5.5 Fuel Pressure Tests and Specifications

As described earlier in this section, much of the function of CIS is dependent on precise fuel pressure. Fuel pressure influences all engine operating characteristics, such as idle, partial-throttle and full-throttle response, starting and warm-up, engine power, and emission levels. Any significant troubleshooting or repair of the system requires the use of a pressure gauge to measure fuel pressure in different parts of the system under different operating conditions.

There are three significant CIS fuel pressure values: 1) System pressure-the basic fuel pressure created by the main fuel pump and maintained by the pressure relief valve in the fuel distributor, 2) Control pressure-determined by the control pressure regulator and used to counter system pressure and regulate the movement of the control plunger, and 3) Residual pressure-the amount of pressure which remains in the closed system after the engine (and fuel pump) are shut off. Testing of each of these pressures is described below.

The pressure gauge shown in Fig. 5-19 (Volkswagen special tool no. VW 1318) has a range of 0 to 10 bar (150 psi) and a valve in one line. It is connected between the fuel distributor and the control pressure regulator. At the fuel distributor, disconnect the line that runs from the center of the fuel distributor to the control pressure regulator. Connect the gauge line without the valve to the fuel distributor. Connect the gauge line with the valve to the end of the control pressure regulator line. Bleed any air from the gauge by hanging it upside down while opening and closing the valve several times. Leave the gauge connected this way for all three pressure tests.


Fuel will be expelled when fuel lines are disconnected. Do not disconnect wires that could cause sparks. Do not smoke or work near heaters or other fire hazards. Keep a fire extinguisher handy.


Absolute cleanliness is essential when working with fuel circuit components of the CIS. Even a minute particle of dirt can cause trouble if it reaches an injector. Before disconnecting any of the fuel line connections, thoroughly clean the unions. Use clean tools.

Fig. 5-19. Pressure gauge used to test system pressure, control pressure, and residual pressure.

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