Compression Pressure Check

Required Special Tool: 

MB991958: Scan Tool (MUT-III Sub Assembly)

• MB991824: V.I.C.
• MB991827: USB Cable
• MB991911: Main Harness B

1. Before inspection, set vehicle to the following conditions:
   • Engine coolant temperature: 80-95°C (176-203°F)
   • Lights and all accessories: OFF
   • Transaxle: Neutral
2. Disconnect the spark plug cables.
3. Remove all of the spark plugs.
4. 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.
   Fig 1: Locating Crankshaft Position Sensor Connector

   Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
5. Cover the spark plug holes with a shop towel etc. Crank the engine for a few seconds to clear debris from a round the spark plug holes. After the engine has been cranked, check for foreign material adhering to the shop towel.
6. Install the compression gauge to one of the spark plug holes.
7. Crank the engine with the throttle valve fully open and measure the compression pressure.
   1. Standard value (at engine speed of 250 r/min): 1,128 kPa (163 psi) 
   2. Minimum limit (at engine speed of 250 r/min): 951 kPa (138 psi) 
8. 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 (14 psi) 
9. If there is a cylinder with compression difference 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.
10. Connect the crankshaft position sensor connector.
11. Install the spark plugs and spark plug cables.
    Fig 2: Measuring Compression Pressure Using Compression Gauge

    Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.
12. 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.
    NOTE: If the negative (-) cable has been disconnected from the battery terminal in order to erase the diagnostic trouble code, operate the engine at idle for approximately 10 minutes after restarting.
    Fig 3: Connecting Scan Tool To Erase Diagnostic Trouble Codes

    Courtesy of MITSUBISHI MOTOR SALES OF AMERICA.