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 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.
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.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Mechanical Inspection: Compression