How do I perform a compression check on a 1986 Mazda RX-7 FC?

Doing a compression test on a rotary engine requires a slightly different approach than a standard piston engine, but it’s straightforward once you know how the gauge needs to be set up.

Difficulty: Easy to Moderate

Here is the procedure for checking the compression on your 1986 FC RX-7:

1. Prep the Car

As a matter of general automotive practice, you'll get the most accurate readings if you warm the engine up to operating temperature first. Once warm, shut it off and pull the EGI fuse under the hood to cut fuel and prevent engine damage during cranking ¹. You can also disconnect the Crank Angle Sensor (CAS) to disable the ignition ¹³.

2. Remove the Spark Plugs

Remove the spark plugs. The standard method is to thread your tester into the top (trailing) spark plug holes ³. However, testing from the bottom (leading) plug hole can sometimes give you a slightly higher and more accurate peak reading ⁷.

3. Set Up the Gauge

This is the most critical step for a rotary. A specialized Mazda rotary compression tester is ideal because it automatically records the pressure of all three rotor faces independently ¹.

If you are using a standard piston-engine compression gauge from a parts store, you must remove the Schrader valve from the tip of the tester ¹⁰. If you leave the valve in, the gauge will just hold pressure from the highest-compressing rotor face, masking a potentially blown apex seal on another face ¹. Alternatively, you can leave the valve in but manually hold the pressure release button down on the gauge while testing ⁷.

4. Crank and Read

Thread the modified gauge into the first rotor housing. Hold the throttle wide open, crank the engine, and watch the needle ⁷.

Because you removed the Schrader valve, the needle will bounce with every compression stroke. You are looking for three strong, even bounces per revolution of the rotor ³.

• Healthy Specs: A new or very healthy engine will show around 100+ PSI ¹³. The ideal operating range is 90-110 PSI ¹¹.
• Minimum Specs: The factory service limit is roughly 85 PSI ¹⁰.
• Failures: If you see one or two bounces that are significantly lower than the others, or missing entirely, it points to internal issues like a broken apex seal or a collapsed/stuck seal ¹.

Repeat the process for the second rotor. If you don't have a gauge at all, you can do a quick "listening test" by leaving the plugs out, cranking the engine, and listening/feeling for those three strong, even pulses of air from the plug holes ³. It won't give you a PSI number, but an uneven rhythm will immediately tell you if a seal is gone.