Compression Pressure Check

Required Special Tool: 

MB991958: Scan Tool (MUT-III Sub Assembly)

• MB991824: V.C.I.
• MB991827: MUT-III USB Cable
• MB991910: MUT-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 A/T)
     NOTE: 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 and adjustment.
2. Remove all of the ignition coils and spark plugs.
3. Disconnect the crankshaft position sensor connector.
   NOTE: Doing this will prevent the engine control module from carrying out ignition and fuel injection.
   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.
4. Cover the spark plug hole with a shop towel etc., during cranking. After the engine has been cranked, check for foreign material adhering to the shop towel.
   Fig 1: Disconnecting Crankshaft Position Sensor Connector

   Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
5. Set a compression gauge to one of the spark plug holes.
6. Crank the engine with the throttle valve fully open and measure the compression pressure.
   1. Standard value (at engine speed of 200 r/min): 1,560 kPa (461 in.Hg) 
   2. Minimum limit (at engine speed of 200 r/min): 1,130 kPa (334 in.Hg) 
7. Measure the compression pressure for all the cylinders, and check that the pressure differences of the cylinders are below the limit.
   1. Limit: 98 kPa (29 in.Hg) 
   Fig 2: Setting Compression Gauge To Spark Plug Hole

   Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
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 6 to 8.
   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 crankshaft position sensor connector.
10. Install the spark plugs and ignition coils.
11. Use the scan tool MB991958 to erase the diagnostic trouble codes.
    NOTE: This will erase the diagnostic trouble code resulting from the crankshaft position sensor connector being disconnected.
    Fig 3: Connecting Scan Tool MB991910 To Data Link Connector

    Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.