Fuel Pressure: Testing and Inspection
Much of the function of the CIS-E fuel injection system is dependent on precise fuel pressure. Fuel pressure influences all engine operating characteristics, such as idle, part-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-E fuel pressure values: (1) SYSTEM PRESSURE-the basic fuel pressure created by the main fuel pump and maintained by the diaphragm pressure regulator, (2) DIFFERENTIAL PRESSURE-the pressure ratio in the fuel distributor, controlled by the differential pressure regulator, which regulates the flow of fuel to the fuel injectors, (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 needed has a range of:
0 to 150 psi (0 to 10 bar)
CIS-E Pressure Gauge:
It must also have a valve in one line as shown. It is connected between the fuel distributor test port and the end of the fuel line which supplies the cold start valve. Disconnect the fuel line from cold start valve and connect it to the gauge line that leads to the gauge valve. Remove the fuel distributor test port plug, (located on the base of the fuel distributor facing the front fender), and connect the gauge line that leads to the gauge to this test port. Bleed any air from the gauge by hanging it upside down while opening and closing the valve several times. Leave the pressure gauge connected this way for all three pressure tests.
WARNING: Fuel will be expelled when the fuel lines are disconnected. Do not disconnect any 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 CIS-E. Even a minute particle of dirt can cause major trouble if it reaches the fuel distributor or fuel injectors. Before disconnecting any of the fuel line connections, throughly clean the unions. Always use clean tools.
OPERATING FUEL PUMP FOR TEST
1. Unless the fuel pump relay receives an rpm signal from the ignition system indicating that the engine is running, the fuel pumps will operate for only a few seconds when the ignition switch is turned ON. To operate the pumps longer, (for example fuel system tests without the engine running) the fuel pump relay must be bypassed to power the pumps directly.
NOTE: Some vehicles are equipped with a knee bolster to prevent knee injury in the event of an accident. The knee bolster runs along the entire length of the lower dash. To properly access the fuse/relay panel for servicing, the knee bolster must be removed from in front of the fuse/relay panel.
Relay Terminals:
2. One method is to remove the relay and connect a jumper wire between two sockets on the fuse/relay panel. The sockets to be used are the sockets of the fuse/relay panel, which correspond to terminals #30 (battery power input) and terminal #87 (output to fuel pumps) of the relay
NOTE: The jumper wire should be at least 16 ga. (1.5 mm) wire and include an in-line fuse holder with a 15 amp fuse. To avoid fuse/relay panel damage from repeated connecting and disconnecting, also include a toggle switch in the jumper wire assembly.
3. With the ignition switch OFF and the fuel pump relay removed, use the jumper wire assembly (toggle switch OFF) to bridge the sockets in the fuse/relay panel. Turn the toggle switch ON to operate the fuel pumps as needed.
SYSTEM PRESSURE
System pressure is the pressure created by the fuel pump and regulated by the diaphragm pressure regulator. Above the predetermined pressure limit of the regulator, the regulator opens and routes excess fuel back to the fuel reservoir. System pressure is not adjustable and a faulty pressure regulator must be replaced.
1. To check system pressure, connect the fuel pressure gauge as described above and disconnect the harness connector from the differential pressure regulator. Pressurize the fuel system by operating the fuel pump, using the jumper wire assembly at the fuse/relay panel. Turn jumper wire assembly toggle switch ON. System pressure should be:
89 to 95 psi (6.1 to 6.6 bar)
2. After reading system pressure on the gauge, turn jumper wire assembly toggle switch OFF.
3. If the system pressure is too high, check for a blocked or damaged fuel return line. If the system pressure is too low, look for fuel leaks, a clogged fuel filter, or a damaged fuel supply line blocking fuel flow. If system pressure is too low also check fuel pump delivery rate as described in fuel pump testing. If no faults are found with the fuel return line or the fuel pump, the diaphragm pressure regulator is faulty and should be replaced.
DIFFERENTIAL PRESSURE
Differential pressure is the pressure difference between the upper and lower chambers of the fuel distributor, controlled by the differential pressure regulator. This controls the pressure differential at the fuel distributor metering ports, which determines fuel flow to the fuel injectors and thus the air/fuel mixture. Check differential pressure with the gauge connected as described above and the gauge valve CLOSED (valve handle at 90° angle to gauge hose). Run the fuel pump using the jumper wire assembly.
1. To test, first disconnect the differential pressure regulator harness connector and read differential pressure with the gauge valve CLOSED. The correct differential pressure is lower than the measured system pressure by:
4.4 to 7.3 psi (0.3 to 0.5 bar)
CIS-E Fuel Pressure Regulator:
2. If the differential pressure is too low, measure the volume of fuel coming from the fuel distributor return line. Disconnect the fuel distributor return line from the diaphragm pressure regulator and place the disconnected line in a measuring container suitable for catching fuel. plug the open port on the diaphragm pressure regulator and run the fuel pump. After sixty (60) seconds, the amount of fuel in the container should be:
4.4 to 5.1 oz. (130 to 150 cc)
3. Reconnect the fuel line to the diaphragm pressure regulator using new sealing washers and tighten union bolt to specification.
4. If the fuel quantity is correct but differential pressure tested below specifications, the differential pressure regulator regulator is faulty and should be replaced.
5. If fuel quantity is incorrect, recheck system pressure. If system pressure is within specifications but differential pressure and the measured fuel quantity are not, then the cause is most likely a faulty fuel distributor.
Differential Pressure Regulator:
Diaphragm Pressure Regulator Testing Connections:
6. Connect an ammeter between the differential pressure regulator harness connector, (now disconnected) and the differential pressure regulator, using VOLKSWAGEN special tool #VW 1315A/1 or equivalent. This will be used to measure differential pressure regulator current during this test.
CAUTION: The small female terminals in the differential pressure regulator harness connector are easily damaged. If fabricating a test harness, use the appropriate male terminals to make connections with the harness connector.
NOTE: If the engine will not start after connecting the test harness and ammeter, the test harness may be incorrectly wired. Try reversing the wires in the test harness connector that plugs into the differential pressure regulator. Do not change any wires of the factory engine wiring harness connectors.
7. Switch the ignition switch ON and run the fuel pump. Differential pressure and differential pressure regulator current should be:
18.9 to 23.2 psi (1.3 to 1.6 bar) below system pressure
100 mA
8. If the differential pressure regulator current is within specifications but the differential pressure is not, the differential pressure regulator is faulty and should be replaced. If both the pressure and the current are out of specifications, look for an electrical problem. Check that the resistance across the terminals of the differential pressure regulator is between:
17.5 and 21.5 ohms
9. Check all wires and connections. Check for a good ground connection at the cold start valve. If no other faults can be found, the fuel injection control unit is probably faulty.
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.
1. Check residual pressure with the pressure gauge connected as described above for measuring system pressure. Pressurize the fuel system by operating the fuel pump using the jumper wire assembly at the fuse/relay panel, as described above, then turn the fuel pump OFF. After ten minutes, the pressure should not have dropped below:
48 psi (3.3 bar)
2. After twenty minutes, the pressure should not have dropped below:
46 psi (3.2 bar)
3. If the pressure drops excessively, check the fuel pump check valve and the fuel accumulator as described below. Also check for leaks in the fuel lines, the fuel distributor O-rings, and the fuel injectors. If no leaks are found, check the air flow sensor plate clearence.
4. To check operation of the fuel pump check valve and the fuel accumulator, test residual fuel pressure in the supply line from the fuel pump. Disconnect the fuel supply line from the fuel distributor. Connect the end of the fuel supply line to the pressure gauge line that leads to the pressure gauge. The pressure gauge valve must be in the CLOSED position and the gauge line to the valve is left unattached.
WARNING: One gauge line (the one with the valve) is left disconnected. Be sure to close the valve.
5. Run the fuel pump using the jumper wire assembly as described above, until fuel pressure indicated by the gauge reaches:
78 psi (5 bar)
6. Then turn the fuel pump OFF. Once again, after ten minutes, the pressure should not have dropped below:
48 psi (3.3 bar)
7. If the pressure drops excessively and no leaks are found between the fuel pump and the gauge, repeat the test. This time shut OFF the fuel pump and immediately pinch closed the fuel hose between the tank and the pump. If residual pressure now stays within specifications, the fuel pump check valve is faulty and should be replaced.
8. If the residual pressure still drops off too quickly, the fuel accumulator may be faulty. For a quick check, remove the plug from the end of the accumulator. If fuel drips out, then the diaphragm is leaking and the accumulator should be replaced.
9. After completing the fuel system pressure tests, reconnect all fuel lines that have been disconnected during testing procedures. Always use new sealing washers and proper tightening torques when reinstalling each fuel line, refer to SPECIFICATIONS. START the engine and check any for leaks, repair leaks immediately.