4.3 Fuel pump

The electric fuel pump is mounted in the fuel reservoir. The pump is cooled by fuel and therefore may be damaged if allowed to run dry. The pump runs whenever the ignition switch is in the start or run position and the engine is turning over or running. The important fuel pump performance measurements are fuel pressure and volume of fuel flow. Another important aspect of fuel system performance is residual pressure-the pressure which is maintained in the system after the engine is off to aid hot-starting.

On cars with CIS, CIS-E, and CIS-E Motronic, residual fuel pressure is maintained by the fuel accumulator, located on the side of the fuel reservoir and shown in Fig. 4-6, and by a fuel pump check valve. On these cars, the fuel pump check valve is replaceable. See Replacing Fuel Pump Check Valve later in this section. To test fuel system pressure and residual pressure, see the headings covering pressure tests under 5. Continuous Injection System (CIS) and 6. CIS-Electronic (CIS-E) and CIS-E Motronic.

Fig. 4-6.

Schematic cross-section of fuel accumulator to damp pressure pulses and maintain residual pressure in CIS, CIS-E and CIS-E Motronic systems.

On cars with Digifant, residual pressure is maintained by a one-way check valve at the fuel pump outlet, and a one-way check valve in the fuel pressure regulator. The fuel pump check valve is not replaceable. To test fuel system pressure and residual pressure, see the headings covering pressure tests under 7. Digifant II Fuel Injection.

To test fuel pump delivery rate:

1.	Before testing the main fuel pump, check that the transfer pump is working correctly as described above in 4.2 Transfer Pump.

2.	Disconnect the fuel return line as shown in Fig. 4-7 or Fig. 4-8 and route the end of the fuel return line to a measurement container of at least 1000 ml (1 qt.).

Fig. 4-7.

Container being used to measure fuel delivery rate at fuel return line (CIS or CIS-E and CIS-E Motronic). Return line connection (arrow) is located near right (passenger side) front suspension strut tower.

3.	Run the fuel pump, as described in 4.1 Fuses and Relays, for exactly 30 seconds.

4.	On models with Digifant, the pump should deliver at least 500 cc (17 oz.) of fuel.

Fig. 4-8.

Container being used to measure fuel delivery rate at return line (Digifant). Return line from fuel pressure regulator (lower right) is disconnected (arrow) near right (passenger side) front suspension strut tower.

5.	On models with CIS, CIS-E or CIS-E Motronic, the correct amount of fuel delivered depends on voltage at the pump while the pump is running. See the chart in Fig. 4-9 for exact specifications.

Fig. 4-9.

Minimum amount of fuel to be delivered by fuel pump in 30 seconds of operation. Chart shows fuel delivery vs. voltage at fuel pump with pump running (approximately 2 volts less than battery voltage).