Engine Compression

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.
Ensure engine coolant temperature does not exceed 86°F (30°C). Remove oil filler cap. Remove 2 nut covers and nuts securing cylinder head cover(s). Remove cylinder head cover(s). Disconnect ignition coil connectors.
Remove ignition coils. 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 COMPRESSION SPECIFICATIONS table.

COMPRESSION SPECIFICATIONS

Application	Specification
Compression Ratio	10.0:1
Compression Pressure	142-156 psi (10-11 kg/cm² )

DME & FUEL PUMP RELAY LOCATION

Application

Location

325i, 325iC & 325is

⁽¹⁾ Behind Cover, Along Engine Compartment Firewall

525i & 525iT

⁽²⁾ Right Inner Fender Panel Of Engine Compartment

⁽¹⁾	First relay (viewed from left to right) after ground wire is fuel pump relay, followed by Digital Motor Electronics (DME) master relay.

⁽²⁾	First relay (viewed from top to bottom) is Digital Motor Electronics (DME) master relay, followed by fuel pump relay and then oxygen sensor heater relay.