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Mechanical Inspection: Compression

  1. Deactivate fuel injection system by removing Digital Motor Electronics (DME) master relay and fuel pump relay before performing compression test. See DME & FUEL PUMP RELAY LOCATION  table. Ensure battery is fully charged.
  2. Ensure engine coolant temperature does not exceed 95°F (35°C). Turn 2 fasteners on electric lead cover 90 degrees, and remove electric lead cover. Repeat for other cover on V8 engines. Disconnect spark plug connectors.
  3. Using 16-mm Socket (12-1-171) and Torque Limiter (12-1-172), remove spark plugs. Use Pressure Gauge (11-0-162), Test Adapter (11-0-166) and Tube Extension (11-0-167) to check compression pressure. See ENGINE COMPRESSION SPECIFICATIONS table.
ENGINE COMPRESSION SPECIFICATIONS

Application psi (kg/cm2 )
Normal Compression Pressure (1) 142-156 (10-11)
(1) Maximum variation between cylinders and minimum compression pressure specifications are not available from manufacturer.
DME & FUEL PUMP RELAY LOCATION

Application Location
3-Series (1) Behind Cover, Along Engine Compartment Firewall
525i & 525iT (1) Right Inner Fender Panel
(1) First relay (viewed from left to right) after ground wire is fuel pump relay followed by Digital Motor Electronics (DME) master relay.
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