Fuel Pressure: Testing and Inspection
Required Special Tools:MB991502: Scan Tool (MUT-II)
MB991637: Fuel Pressure Gauge Set
MD998709: Adaptor Hose
MD998742: Hose Adaptor
1. Release residual pressure from the fuel line to prevent fuel spray.
WARNING: To prevent a fire, cover the hose connection with shop towels to prevent splash of fuel that could be caused by some residual pressure in the fuel pipe line.
2. Disconnect the high-pressure fuel hose at the fuel rail side.
3. Attach the fuel pressure measurement tools.
1. Remove the union joint and bolt from special tool MD998709 (adapter hose) and instead attach the special tool MD998742 (hose adapter) to the adapter hose.
2. Place a suitable O-ring or gasket on assembled special tools MD998709 and MD998742 and install the fuel pressure gauge.
3. Install the assembled fuel pressure measurement tools between the fuel rail and high-pressure fuel hose.
1. Remove the union joint and bolt from special tool MD998709 (adapter hose) and instead attach special tool MD998742 (hose adapter) to the adapter hose.
2. Install special tool MB991637 (fuel pressure gauge set) into assembled special tools MD998709 and MD998742 via a gasket.
3. Install the assembled fuel pressure measurement tools between the fuel rail and high-pressure fuel hose.
4. Connect scan tool MB991502 to the data link connector.
5. Use the Actuator test 25 to drive the fuel pump. Check that there is no fuel leaking from any section when the fuel pump is operating.
6. Stop the fuel pump.
7. Start the engine and run at idle.
8. Measure fuel pressure while the engine is running at idle.
Standard value:
Approximately 270 kPa (38 psi) at curb idle
. Disconnect the vacuum hose (blue stripe) from the air intake filling and then plug the nipple.
Measure the fuel pressure in this condition.
Standard value:
330 - 350 kPa (47 - 50 psi) at curb idle
10. Check to see that fuel pressure at idle does not drop even after the engine has been raced several times.
11. Racing the engine repeatedly, hold the fuel return hose lightly with fingers to feel that fuel pressure is present in the return hose.
NOTE: If the fuel flow rate is low, there will be no fuel pressure in the return hose.
12. If any of fuel pressure measured in steps 8 to 11 is out of specification, troubleshoot and repair according to the table below.
13. Stop the engine and observe fuel pressure gauge reading. Normal if the reading does not drop within two minutes. If it does, observe the rate of drop and troubleshoot and repair according to the table below. Start, then stop the engine.
1. Squeeze the fuel return line closed to confirm leak-down occurs from defective fuel pressure regulator.
2. Squeeze the fuel supply line closed to confirm leak-down occurs from detective fuel pump check valve.
3. If pressure continues to drop with both fuel lines squeezed closed, injector(s) are leaking.
14. Release residual pressure from the fuel pipe line.
WARNING: Cover the hose connection with shop towels to prevent splash of fuel that could be caused by some residual pressure in the fuel pipe line.
15. Remove the fuel pressure gauge, and special tools M0998709, MD998742 and MB991637 from the fuel rail.
16. Replace the O-ring at the end of the high-pressure fuel hose with a new one.
17. Fit the high-pressure fuel hose into the fuel rail and tighten the bolts to specified torque.
Tightening torque: 5 N.m (3.6 ft.lb)
18. Check for fuel leaks.
1. Use scan tool MB991502 to operate the fuel pump.
2. Check the fuel line for leaks, repair as needed.
19. Disconnect scan tool MB991502.