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Home >> Mercedes Benz >> 1991 >> 350SDL >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Basic Testing >> Fuel Delivery & System Pressures - Gasoline >> System Pressure Test
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System Pressure Test

  1. There are 2 versions of Mercedes-Benz Fuel Pressure Gauge (100 589 00 21 00) available. One version has junction block with 2 control valve knobs which are numbered "1" or "3" in the following procedure. Looking at control valve knob side of junction block, hose exiting from left side of junction block is No. 1 hose and control valve knob nearest to this hose is No. 1 knob.
  2. The second version of Mercedes-Benz Fuel Pressure Gauge (100 589 00 21 00) has a single control valve knob on junction block. Hose leaving junction block 180 degrees from control valve knob is hose "B". Hose leaving junction block 180 degrees from line to gauge is hose "A".
  3. Remove plug from test port in lower chamber of fuel distributor. Connect fuel pressure gauge hose No. 1 or hose "A" to test port in lower chamber of fuel distributor. Use M12X1.5/M8X1 Reducing Bushing (102 589 06 63 00) to facilitate connecting hose to chamber port.
  4. Remove fuel supply line for start valve from top of fuel distributor. Attach pressure gauge hose No. 3 or hose "B" to cold start fitting on fuel distributor upper chamber.
  5. When testing system pressure using first version of fuel pressure gauge, close control valve No. 1 and open control valve No. 3. On second version, open control valve. System pressure is checked with engine off. Engine temperature has no effect upon system pressure test.
  6. 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.
  7. 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 socket connector. Using fused jumper wire, jumper connector pin socket No. 2 with battery voltage source to establish fuel pressure.
  8. System pressure for V8 engine should be 76.9-79.8 psi (5.4-5.6 kg/cm2 ). System pressure for 4 and 6-cylinder engines should be 29-58 psi (2.04-4.08 kg/cm2 ). If pressures are not as specified, check for defective fuel pump, restricted fuel return line, or defective electrohydraulic actuator or differential pressure regulator. Open pressure gauge valve "1". Remove jumper wire from fuel pump relay connector socket.
  9. After testing is completed, disconnect pressure gauge and clean any spilled fuel. Reconnect all fuel lines and tighten to 88-106 INCH lbs. (10-12 N.m). 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. Start and run engine. Visually check all connections and lines for reassembly leaks.
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