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Home >> Mercedes Benz >> 1992 >> 300CE >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Testing >> Fuel Delivery & System Pressures - Gasoline >> Lower Chamber Pressure Test
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Lower Chamber Pressure Test

  1. Connect Mercedes-Benz Fuel Pressure Gauge (100 589 00 21 00) as per steps 1)-4) of previous system pressure test. See SYSTEM PRESSURE TEST under FUEL DELIVERY & SYSTEM PRESSURES - GASOLINE. When testing lower chamber pressure using first version of fuel pressure gauge, open control valve No. 1 and close control valve No. 3. On second version, close control valve. System pressure is checked with engine off. Engine must be at operating temperature.
  2. For V8 engines, remove fuel pump relay (under hood near fuse panel). Bridge fuel pump relay connector pin sockets No. 7 and No. 8 with jumper wire to establish fuel pressure. See Figure .
  3. For 4 and 6-cylinder engines, remove MAS unit from underhood component compartment area, near CIS-E control unit. To remove MAS unit, turn top knob from position No. 1 to position No. 0 and lift/pull MAS unit out of electrical connector socket. Using fused jumper wire, jumper pin socket No. 2 with battery voltage source to establish fuel pressure.
  4. Turn ignition on. Engine must be at operating temperature. Disconnect electrohydraulic actuator connector. Read pressure gauge. Lower chamber pressure should be about 5.8 psi (.4 kg/cm2 ) system pressure reading determined earlier. See SYSTEM PRESSURE TEST under FUEL DELIVERY & SYSTEM PRESSURES - GASOLINE. Reconnect electrohydraulic actuator connector and note pressure gauge. There should be no change in pressure.
    NOTE: During engine warm-up and when engine is at operating temperature, current to electrohydraulic actuator is determined by coolant temperature sensor, and CIS-E control unit.
  5. If test results do not conform to specifications, check coolant temperature sensor, electrohydraulic actuator, and CIS-E control unit for malfunction. See appropriate I - SYS/COMP TESTS article in the ENGINE PERFORMANCE section. Coolant temperature sensor should also be checked if warm-up problems exist. Also verify correct operation of lambda control. If lower chamber pressure is excessive, verify that orifice in fuel distributor is not restricted.
  6. When test is completed, turn ignition off and install fuel pump relay or MAS unit. To install MAS unit, push MAS unit into electrical socket connector and turn top knob from position No. 0 to position No. 1.
NOTE: Perform Internal Leak Testing only if problem exists with hot engine restarts.
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