Compression Test

Procedure

1. If plugs do not have anti-seize coated threads, then with engine cold, loosen plugs and coat threads with anti-seize, and tighten snug.
2. If plugs have anti-seize, then with engine cold. crack plugs and tighten snug.
3. Take car for a 5 minute drive to get engine temperature to middle of gauge.
4. Pull parking brake and put car in neutral.
5. Block front wheels.
6. Remove all plugs (read each plug and record date, plug location and plug condition in a journal)
7. Connect compression tool to cylinder 1.
8. Disconnect ignition cable from starter solenoid.
9. Connect remote starter to solenoid first THEN to battery +POS terminal.
10. Double check that car is in neutral.
11. Reset Compressor
12. Hold throttle linkage open.
13. Press and hold remote starter for "5 Mississippi's"
14. Record date, value on compression tester, and associated plug in journal.
15. Repeat for all cylinders.
16. Disconnect remote starter at +POS terminal first then at starter solenoid.
17. Reconnect ignition cable to starter solenoid.
18. Clean and re-gap plugs.
19. Coat plugs with anti-seize.
20. After the engine cools, reinstall plugs to proper torque. (11-14ft-lbs) 

Analyzing Compression Test Data:

• For a '77 280z, the  factory service manual says (164 to 178psi)
• Most people with older worn engines get between 125 and 155psi. This is usually due to worn piston rings.
• Usually all six cylinder results are within 10 PSI of each other.
• The FSM says a result that is less than 80% of the highest is a problem.
• Two adjacent cylinders with lower readings is usually an indication of a bad head gasket

Improving Compression Results

• Do the test correctly:

• cold engine decreases results by 5 to 15psi as rings do not seal
• closed throttle decrease results by ~ 5 psi
• having any plugs in decreases results up to 5psi
• loose compression tester fittings can decrease psi significantly

• Carbon build up on valves prevents them closing properly and decreases psi. De-carbon the engine with Seafoam, diesel or similar. Good for 5 to 10psi improvement
• Mechanical stretch in timing chain results in improper valve closing sequence thus a decreased psi result. Advancing the timing chain on the cam sprocket is good for 5 to 10psi improvement on a worn engine
• Oil additives such as Lucas Oil are good for 5psi or more on an older engine
• Re-torqueing head bolts is good for 5 to 10psi or more on an older engine.

If you feel spanky.... repeating a compression tests on a cylinder but adding a few teaspoons or squirts of motor oil in the plug hole prior to the measurement will tell you if the rings are bad. The oil acts to create a better seal around the rings. If the compression results improve then the rings are worn. If the compression remains low then the valves are not sealing or the head gasket has a problem.