CIS

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.

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

The pressure gauge shown in Fig. 5-19 (Volkswagen special tool no. VW 1318) has a range of 0 to 10 bar (0 to 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.

WARNING: 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.

CAUTION: 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.

System Pressure

Fig. 5-20. Pressure gauge valve lever in closed position (arrow). Flow to control pressure regulator is blocked.:

To measure system pressure, install a pressure gauge as described above and close the valve in the line to the control pressure regulator, as shown in Fig. 5-20. Start the engine and let it idle. System pressure should be 4.7 to 5.4 bar (68 to 78 psi). System pressure is preset during fuel distributor manufacture and should not require routine adjustment.

Fig. 5-21. Exploded view of pressure relief valve showing location of adjusting shims at (1). Always replace the sealing washer and the 0-rings (arrows).:

If system pressure is too low, look for fuel leaks, a clogged fuel filter, or a damaged fuel line blocking fuel flow. Check fuel pump delivery rate. If no other cause for low system pressure can be found, the pressure can be adjusted by adding shims to the pressure relief valve. See Fig. 5-21. An additional 0.50 mm (0.020 in.) shim will increase system pressure by about 0.30 bar (4 psi). An additional 1.00 mm (0.040 in.) shim will increase it by about 0.60 bar (8 psi).


If system pressure is too high, check for a blocked or damaged fuel return line. If the fuel return line is in good condition, the pressure can be adjusted by reducing the thickness of the shims on the pressure relief valve. See Fig. 5-21. A change of 0.50 mm (0.020 in.) total shim thickness will decrease system pressure by about 0.30 bar (4 psi). A change of 1.00 mm (0.040 in.) total shim thickness will decrease it by about 0.60 bar (8 psi).

If the fuel pressure cannot be accurately adjusted, the fuel distributor is faulty and should be replaced. See Air Flow Measurement and Fuel Metering.

Control Pressure

Control pressure is that which is determined by the control pressure regulator. It helps regulate the response of the control plunger and, therefore, influences the fuel mixture. Measurement of control pressure is the primary way of evaluating the control pressure regulator. Control pressure is not adjustable, and if the pressure is not within specifications the control pressure regulator should be replaced.

System pressure influences control pressure, and should be checked, as described above, prior to checking control pressure. Check control pressure with the pressure gauge connected as described above at the beginning of Fuel Pressure Tests, with the gauge valve open. A major function of the control pressure regulator is to adapt the fuel mixture for cold starting and cold running, so control pressure should first be checked with the engine completely cold.

To check cold control pressure, disconnect the harness connectors from control pressure regulator and the auxiliary air regulator. Open the pressure gauge valve in the line to the control pressure regulator, then start the engine. Cold control pressure varies with temperature. The measured control pressure should correspond to the values listed in Tabled.

If cold control pressure is too high, check for a blocked or kinked fuel line. Also, remove the fuel union at the control pressure regulator and check for a clogged filter screen. If no such faults are found, the control pressure regulator is faulty and should be replaced.

WARNING: Fuel will be expelled as the unions are disconnected. Do not smoke or work near heaters or other fire hazards. Keep a fire extinguisher handy.

To check warm control pressure, reconnect the harness connectors to the control pressure regulator and the auxiliary air regulator. Run the engine until the control pressure is no longer increasing (about 2 minutes). The measured control pressure should correspond to the value listed in Tabled.




Table d. Control Pressure Specifications

If the warm control pressure is too high, check for a blocked or kinked fuel line. Also, remove the fuel union at the control pressure regulator and check for a clogged filter screen. If no such faults are found, the control pressure regulator is faulty and should be replaced.

If warm control pressure is too low, or takes more than about two minutes to reach its peak, test the resistance of the heating element and test for voltage reaching the harness connector, as described in Cold Running Enrichment.

Residual Pressure

The closed fuel system is designed to maintain pressure after the engine is shut off to help prevent the fuel in the injector lines from overheating, causing vapor lock and hard starting. The fuel pump's one-way check valve and the fuel accumulator help maintain this residual pressure.
Check residual pressure with the pressure gauge connected as described above for measuring control pressure (valve in open position). When the engine is fully warm (control pressure between 3.4 and 3.8 bar (49 and 55 psi), shut off the engine. Leave the gauge connected.

After ten minutes, the pressure should not have dropped below 2.6 bar (38 psi) and after twenty minutes, the pressure should not have dropped below 2.4 bar (35 psi). If the pressure drops excessively, check for leaks in the fuel lines, the fuel distributor including the pressure relief valve O-rings, the injectors, and the cold-start valve. If the pressure drops excessively and no leaks are found, check that the air flow sensor plate is correctly adjusted as described in Air Flow Measurement and Fuel Metering.

If the residual pressure still drops off too quickly, the fuel accumulator may be faulty. For a quick check, remove the screw from the end of the accumulator. If fuel drips out, then the diaphragm is leaking and the accumulator should be replaced.