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

• Why Do I Want To Do This?

  • Cylinder compression is one indicator of overall engine health. Low compression can mean you have worn piston rings or worn or non-functioning valves. High compression can mean that your engine has some kind of buildup in it, typically carbon deposits on the pistons.



• How Often Should I Do This?

  • Whenever you feel like it and you have your plugs out, or when you have [car] problems and suspect that your compression is low or high.



• How Long Will It Take Me To Do This?

  • It takes a few minutes to check each cylinder. The hard part (by far) is taking out the spark plugs.



• Anything I Should Do Prior To Attempting This Procedure?

  • Make sure you have all the parts and tools you'll need.
  • Remove your spark plugs.



• What Mitsubishi Parts Will I Need?

  • [none]



• What Other Parts Will I Need?

  • [none]



• What Tools May/Will I Need

  • (1) Compression Gauge
  • (1) 8mm wrench or socket

• Under the Hood...
  

  
  
  To get your bearings: The crankshaft (and camshaft on 93+ models) position sensor harness(es) are on the right of the engine near the timing belt covers. Also, the cylinders are numbered with #1 being the front right (driver's side) and #2 being in the right rear. To it say another way, the front bank is numbered, driver to passenger side (right to left): 1, 3, 5 and the rear bank is numbered, driver to passenger side (right to left): 2, 4, 6.
  

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.