Q:  How do I do a compression check on my engine?

A compression check is a quick and easy method to check the condition of the internals of the rotary engine.  The compression check requires a compression tester or a pressure gauge that goes up to at least 150psi.

As per Mazda standards, the compressor tester fitting is screwed into the TRAILING or TOP spark plugs position.  The other trailing spark plug needs to be removed also, so it's easiest to remove both trailing spark plugs before starting the compression test.  The throttle needs to be opened all the way, so you need to prop the throttle body or step on the gas pedal to WOT.  Mazda specs also require the compression test be done when the engine is warmed up.  Surprisingly, compression is higher when the engine is cold.  Engine RPM cranking speeds should be between 200 to 250RPM.

Make sure the engine doesn't fire when doing the compression test!  The easiest thing to do is pull the EGI fuses under the hood to prevent the ECU from starting the engine.  Please ensure you have a strong battery or a battery charger closeby when you go through this procedure.  You will be cranking the engine over several times, and this can drain even a good battery rather quickly.

If you're using a piston compression tester, you can remove the bypass valve or just hold the pin in.  We will need to see three pulses for all three rotor faces for each rotor, so the check valve needs to be bypassed.

Mazda has set minimum compression pressure to be at 70psi, but with the FD3S, the number has been lowered to 60psi.  Good compression are readings anything above 100psi.  Compression testing under 80psi is a sign of an engine requiring a rebuild soon.  Compression should be even across all three rotor faces on both the front and rear rotors.  It is more important to have all even compression numbers across the rotor faces versus one really high number on one face.

Unless you're using an official Mazda compression tester, you might need to "calibrate" your (piston) compression tester.  Some compression testers read a little low, so it's nice to know what is good for your tester.  If you're getting lower readings than expected, test a known good engine for it's compression to make sure the readings are valid.  I had a compression tester that tested my motor at around 80psi, but this was what a normally good engine would read on this particular tester.

Questions?  Comments?  Send email to:  reted@fc3spro.com