Compression Pressure Check

Required Special Tool: 

MB991958: Scan Tool (M.U.T.-III Sub Assembly)

• MB991824: V. C. I.
• MB991827: M.U.T.-III USB Cable
• MB991910: M.U.T.-III Main Harness A

1. Before inspection, check that the engine oil, starter and battery are normal. Also, set the vehicle in the following condition:
   • Engine coolant temperature: 80 - 95°C (176 - 203°F)
   • Lights and all accessories: OFF
   • Transaxle: Neutral (P range on vehicles with TC-SST)
     NOTE: On vehicles for Canada, the headlight, taillight, etc. remain lit even when the lighting switch is in "OFF" position but this is no problem for checks.
2. Remove all of the ignition coils and spark plugs.
3. Disconnect the all of the injector connectors.
   WARNING: Keep your distance from the spark plug hole when cranking. Oil, fuel, etc., may spray out from the spark plug hole and may cause serious injury.
   Fig 1: Identifying Injector Connectors

   Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
4. Cover the spark plug hole with a shop towel etc., after the engine has been cranked, check that no foreign material is adhering to the shop towel.
5. Set compression gauge to one of the spark plug holes.
Fig 2: Setting Compression Gauge To Spark Plug Holes
Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
6. Crank the engine with the throttle valve fully open and measure the compression pressure.

   Standard value (at engine speed of 200 r/min): 1,090 kPa (158 psi) 

   Limit (at engine speed of 200 r/min): Minimum 650 kPa (95 psi) 

7. Measure the compression pressure for all the cylinders, and check that the pressure differences of the cylinders are below the limit.

   Limit: Maximum 100 kPa (14 psi) 

8. If there is a cylinder with compression or a compression difference that is outside the limit, pour a small amount of engine oil through the spark plug hole, and repeat the operations in steps from  5 to  7.
   1. If the compression increases after oil is added, the cause of the malfunction is a worn or damaged piston ring and/or cylinder inner surface.
   2. If the compression does not rise after oil is added, the cause is a burnt or defective valve seat, or pressure is leaking from the gasket.
9. Connect the all of the injector connector.
10. Install the spark plugs and ignition coils.
11. Use the scan tool MB991958 to erase the diagnosis codes.
    NOTE: This will erase the diagnosis code resulting from the injector connectors being disconnected.
    Fig 3: Connecting Scan Tool MB991958 To Data Link Connector

    Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.