Checking Engine Cylinder Compression on a 3000GT (6G72)

These instructions apply to a 1995 Mitsubishi 3000GT, but are easily transferable to any model/year of 3000GT or Dodge Stealth.

The Procedure


Disconnect the Crankshaft Position Sensor Wiring Harness
  Disconnecting this harness will cause the ECU to disable the ignition system and to disable the fuel injectors. Then you can safely turn the engine over with the starter to check compression.


Cover Spark Plug Holes and Crank Engine
  Cover the spark plug holes with shop towels (don't stick them INTO the holes as they will be shot out of you engine bay like a cannon. Ask me how I know :-) Crank the engine over a few times (5 seconds or so), and examine the towels for any debris or strange material. Be aware that if you have stuff (oil, coolant, fuel, etc.) in your engine due to some engine malfunction, this stuff WILL be shot out of your spark plug holes. At the very least that will make a mess; it could also injure you or someone else. Be careful!


Check Cylinder Compression
  Place the long-reach adapter onto your compression gauge. If you have multiple adapters, examine them and use the one that is closest to the length of the threads on your spark plugs. Thread the compression gauge into the spark plug hole of the cylinder you wish to test. Make sure the connection is snug. If you were doing this with the plenum and throttle body attached (front bank only), then you would hold the accelerator pedal on the floor to open the throttle body. Since the plenum and throttle body are off the car at this point, you don't have to mess with the accelerator pedal. Then turn the engine over (by turning the ignition key to "Start" with the clutch pedal depressed) until the needle on the gauge stops moving (it may take 10-15 compression strokes). Record this value and repeat for each of the other 5 cylinders. Stock compression for a NA engine (10:1) is 185psi with a lower limit of 139psi. Stock compression for a turbo engine (8:1) is 156psi with a lower limit of 115psi. Further, in either engine, there should be no more than 14psi between the HIGHEST and LOWEST cylinder compression readings. Note that compression readings should be taken with a WARM engine, which means you'll have to do the above steps with rather hot parts - mechanics' gloves help. For reference, my engine reads about 10-15psi LOW when cold when compared to a warm (operating temperature) engine.Note: if you have a cylinder that has LOW compression, you can try squirting a small amount (a mL or two) of engine oil into that cylinder's spark plug hole and rechecking compression. If the compression is better the second time, that could indicate worn rings on that cylinder (oil helps provide a better seal).


Reset Your ECU
  This step is necessary to erase the engine check code stored in the ECU since you turned the car on with the CPS disconnected. Using your 8mm wrench (or whatever size for your non-stock terminals), disconnect the negative terminal of the battery for 10 seconds or more. Then reconnect the negative terminal.

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Last Modified Wed May 26 2004 19:49:43 PDT