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
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.
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