Engine: Compression Test

Craftsman Compression Tester (~$60 at Sears)

Performing a cylinder compression test is a common and useful way to evaluate the current health of your engine. The results of the test can help indicate specific problems related to your engine, such as worn piston rings or worn/non-functioning valves (usually associated with low compression), or if your engine has carbon deposits or other buildups (high compression).

In order to perform a compression test on your engine, you will need a compression tester. This kit consists of a gauge and an extension tube with adapters, which screws into the spark plug hole in order to measure the compression of the cylinder you wish to test.

Compression testers are generally universal for gas powered engines, so you do not need one specific to the Mitsubishi 3000GT or Dodge Stealth. They can easily be purchased from AutoZone or Sears.

Tools Required

• 8mm socket w/ wrench
• Compression tester kit

Perform a Compression Test on a Mitsubishi 3000GT and Dodge Stealth

Compression test being performed on a Mitsubishi 3000GT/GTO or Dodge Stealth

Compression Range (PSI)

	Standard	Minimum
SOHC	171	127
DOHC NA	185	139
DOHC TT	156	115

1. Warm the engine up. To get accurate readings, it is best to perform the test on a warm engine. A cold engine will provide lower than expected results. Warning: use caution while completing the following steps, as some engine parts will be hot!
2. Disable the car's ignition system. This is an important step that prevents the fuel injectors from firing, allowing you to safely turn over the engine to perform the test. You can do this one of two ways:
   • Remove the ECI fuse, or
   • DOHC Engines
     • Disconnect the Crank Angle Sensor (CAS)
   • SOHC Engines
     • Disconnect the coil wire
3. Access the spark plugs.
   • Front three cylinders
     • Remove the intake arm
   • Rear three cylinders
     • Remove the intake manifold
4. Remove the spark plug of the cylinder you wish to test.
5. Screw the compression tester's tube with adapter into the spark plug hole. If your tester kit came with multiple adapters, simply compare the length of the threads of the adapters with those that are on the spark plug and select the closest fit. Ensure that the adapter is fully screwed in and secure, but do not over-tighten it.
6. Crank the starter for about ten seconds, or five compression strokes. For most compression tester gauges, the needle will stay at the highest indicated compression reading.
   • If you are testing the front bank with the plenum and throttle body on, you will need to have someone hold the throttle body in the wide-open position.
7. Compare your PSI readings to the table to the right.
8. Repeat the previous four steps for each cylinder you wish to test.

Tip: If you find that your compression readings are low, you can perform what is known as a wet test by squirting a small amount (1 teaspoon) of engine oil into the spark plug hole of the cylinder you're testing. From there, if you perform another compression test and find the readings to be higher, there is a good chance that your piston rings need to replaced. The reason for the higher compression in a situation like this is because the oil temporarily creates a better seal on the rings.

Note: While performing these tests, it is possible that your check engine light (CEL) will become illuminated. To reset it, use the 8mm socket to disconnect the negative terminal of the battery for at least 10 seconds. This will erase the check engine code stored in the ECU.

Understanding Your Compression Reading

	Possible Common Problems
Low Compression	Worn, non-functioning or bent valves Worn head gasket Worn piston rings Timing is off
High Compression	Carbon deposits or build-ups

Once you perform the compression test, the gauge will read either a higher number (which is usually good), or a low number (which is very bad). It is possible to have good numbers (high) on some cylinders and bad numbers (low) on the others, as the compression of each cylinder is independent of eachother. Ultimately, there should be no more than 14 PSI (10%) difference between the highest and lowest cylinder compression readings.

A low reading is usually a reasonable indicator of an existing problem. When you receive a low reading, your next step should be to perform a leak-down test to check if the pressure leaks downwards (piston/cylinder) or upwards (valve) and then invest in the necessary repair.