* Consistency is Key: The most important factor is *consistency* across all eight cylinders. A significant variation (more than 15-20 PSI) between cylinders points to a problem (worn rings, valves, etc.) even if the individual readings are within the "good" range.
* Engine Temperature: The engine should be at operating temperature for accurate readings. Cold readings will be lower.
* Throttle Position: The throttle should be fully open during the test.
* Worn Engine: Older engines, especially high-mileage ones, might show slightly lower readings, perhaps in the 130-140 PSI range, and still be acceptable if consistent across cylinders.
* Test Procedure: Incorrect testing procedures can yield inaccurate results. Make sure you're following the instructions for your compression tester meticulously.
If you're getting readings significantly below 130 PSI, or if there's considerable variation between cylinders, you'll need to investigate further to determine the cause. This could range from worn piston rings or valves to head gasket leaks.
