Fuel Pressure: Testing and Inspection
Testing fuel pressure and Internal leaktightnessPreceding work:
Testing, adjusting engine. Refer to "Engine Testing, Adjustment". Testing and Inspection
Fuel Pressure In Bar Gauge:
Special Tools:
Commercially Available Tester:
Note
- The leak test should only be performed if there exists a problem regarding starting when engine warm.
- After switching off the engine, the fuel pressure after 30 minutes must still be 3.0 bar.
Visual Inspection
1 Remove air cleaner.
2 Check all fuel connections for leaks.
3 Check ease of operation of control lever (1) together with air flow sensor plate (4) in the air distributor and of the control plunger (2) in the fuel distributor.
This is done by detaching the fuel pump relay module in the right of the component compartment and bridging the two contacts 7 and 8 briefly to build up pressure.
Engine 102.961 In Model 201
The fuel pump relay module is located in front of the electrical center.
Press air flow sensor plate (4) down by hand. The resistance which is felt must be even over the entire travel. No resistance must be felt when the plate is moved rapidly upward as the slow-moving control plunger lifts away from the control lever (1). During a slow upward movement, the control plunger (2) must follow, maintaining a positive contact.
4 Check control plunger in fuel distributor for leaks.
Briefly fully depress air flow sensor plate and hold in this position; a slight leak quantity is permissible (illuminate with endoscope lamp, if necessary).
Connecting pressure measuring device.
1 Remove screw plug (arrow) on the lower chamber of the fuel distributor.
2 Connect hose line from connection "A" on lower chamber.
Use double fitting M8)( 1/M12 x 1.5, special tool 102 589 06 63 00 for this step.
3 Unscrew fuel line for start valve at fuel distributor.
4 Connect hose line from connection "B" at fuel distributor.
5 Position of valve screws:
a) System pressure
Pressure measuring device = open valve screw.
b) Lower chamber pressure
Pressure measuring device = close valve screw.
Testing
The test of the system and of the lower chamber pressure must be performed with the engine running.
Place pressure measuring device on cylinder head cover.
a) System pressure when engine cold or at normal operating temperature
1 Engine at idle.
2 Take reading of system pressure.
Specification: 5.3-5.5 bar pressure
3 If the specified value is not reached or is exceeded:
- Test fuel pump. Testing and Inspection
- Replace diaphragm pressure regulator.
- Check that fuel return line is not blocked.
b) Lower chamber pressure with engine at normal operating temperature
4 Take reading of lower chamber pressure.
Specified pressure at operating temperature approx. 0.4 bar below system pressure.
If the specified values are not reached:
- Test electro-hydraulic actuator. Refer to "Testing Electrical Components of CFI System". Testing and Inspection
c) Deceleration shut-off
Engine at normal operating temperature.
5 Valve screw closed.
6 Run engine at idle speed. Briefly increase engine speed to approx. 3500/mm. As the engine speed drops, the pressure in the lower chamber must rise to system pressure.
Combustion is restored at approx. 1200 - 1600/min.
If the specified value is not reached:
- Test microswitch. Refer to "Testing Electrical Components of CFI System". Testing and Inspection
- Test current at actuator. Refer to "Testing Electrical Components of CFI System". Testing and Inspection
Vehicles up to 09/88 and model year 1989 with cruise control:
- Test signal for cruise control operation.
d) Acceleration enrichment
"Engine cold" must be simulated for testing acceleration enrichment.
7 Valve screw closed.
8 Simulate "engine cold" by feeding in 2.5 kohm with ohm decade, corresponds to approx. + 20°C, between engine coolant temperature sensor connection cable and vehicle ground.
9 Start engine. Specified value at +20°C approx. 0.5 bar below the previously measured system pressure. Increase engine speed sharply to approx. 2500/min. Lower chamber pressure must drop as engine speed increases.
If the specified value is not reached:
- Test volume air flow sensor potentiometer. Refer to "Testing Electrical Components of CFI System". Testing and Inspection
- Test current at actuator. Refer to "Testing Electrical Components of CFI System". Testing and Inspection
Checking fuel system for leaks
1 Switch off engine. The pressure in the system drops below the opening pressure of the injectors (approx. 3.0 or as from 09/88 approx. 3.2 bar gauge pressure).
2 If the pressure drops immediately to 0 bar, replace check valve at the fuel pump.
3 If the pressure drops slowly, unscrew fuel return line (51) at the diaphragm pressure regulator. No fuel must flow out subsequently from the diaphragm pressure regulator (a slight leak quantity is permissible).
Caution! If fuel flows out in the fuel return line (51), seal line with a dummy plug.
4 Pinch leak line at the fuel accumulator with a clamp. If the pressure does not drop, replace fuel accumulator.
5 If no leak is found after performing the leak test, check start valve for leaks by removing start valve. Replace fuel distributor.
6 Disconnect pressure measuring device, collect fuel with a cloth.
7 Connect fuel lines, once again run engine and check all fuel connections for leaks.
8 Install air cleaner.