Compression test using a piston engine tester:
1. note battery strength. A weak battery will yield low compression results.
2. Remove both lower plugs and wires.
3. remove EGI fuse from engine fusebox.
4. have a friend floor the accelerator pedal, opening the throttle for more airflow
5. insert your tester into the leading hole
6. hold the valve on the side of the tester open
7. have your friend crank the car over for 5+ seconds.
8. observe the needle bounces. You should see 3 in succession without skips, even bounces, in roughly the 30-35psi range.
9. let out on the valve now, and let the tester reach an overall compression value for all 3 faces(highest of 3 will be displayed). 115+ is like new, 100-115 is healthy, 90-100 is getting weak(1 year or less in most cases) below 90 could blow at any moment.
10. repeat for opposite rotor. Note difference in overall compression between rotors, which should be no more than 20psi max.