Mechanical Inspection: Compression

1998 Subaru Forester L, AutomaticSECTION Compression

Check engine mechanical condition using a compression gauge, vacuum gauge or engine analyzer. See engine analyzer manual for specific instructions.

WARNING: DO NOT use ignition switch during compression tests on fuel injected vehicles. Use a remote starter to crank engine. Fuel injectors on many models are triggered by ignition switch during cranking mode, which can create a fire hazard or contaminate the engine's oiling system.

Check engine compression with engine at normal operating temperature, all spark plugs removed and throttle wide open. See COMPRESSION SPECIFICATIONSΒ  table.

COMPRESSION SPECIFICATIONS

Application Specification
Compression Ratio 9.7:1
Compression Pressure (1)(2) 156-185 psi (11.0-13.0 kg/cm2 )
(1) Engine cranking at 200-300 RPM with throttle open fully.
(2) Maximum variation between cylinders is 28 psi (2.0 kg/cm2 ).
RENDER: 1.0x

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When to See a Mechanic

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

  • β€’ You smell fuel, burning insulation, or see smoke.
  • β€’ Brakes feel soft, pull hard to one side, or make grinding noises.
  • β€’ The engine overheats, stalls repeatedly, or misfires under load.
  • β€’ You are missing required tools, torque specs, or safe lifting equipment.
  • β€’ You are not confident in the next step or safety outcome.