1985 RX7 compression?

09-02-2008, 09:11 PM
I have an 85 Mazda RX7.
Came out one day and it would not start.
So I made sure it was getting fuel.
Then made sure I had good spark to all the plugs.
So decided to do a compression test. On the 2 spark plugs closes to the radiator I have 60psi but on the back 2 closes to the fire wall there is 0 compression.
I have worked on lots of cars but never a rotary motor, but seems to me there should be the same compression at all spark plug holes.
Any help or advice would be appreciated.

09-05-2008, 09:40 AM
You can try pouring a bit of marvel mystery oil or auto tranny fluid into the spark plug hole that's giving you no compression and try to turn it by hand before cranking it.

even though the rear rotor generally has lower compression than the front (for whatever reason), those numbers are really low. You might be looking at a rebuild. Healthy motors are normally around 100-90 psi, and a little lower for older engines

06-04-2009, 04:52 PM
from the factory 95% of 12a's and 13b's which are found in all models including the 3rd gen and rx8 should have 3 even bounces of 80 to 85psi on a normal comp tester with the shrader valve out and your gogo pedal to the floor. with the exception of the 88gtu that i know of came from the fac with 95psi on all 6 seals

12-12-2009, 04:05 PM
Doesn't sound good with 0 on the rear rotor

when compression testing you should have both plugs removed from the housing not being tested, and one in the tailing (top) of the one being tested with the pressure guage in the lower spark plug hole. With the accel pedal to the floor to open the throttle plates, and a cranking rpm of at least 300. You should do 2 tests 1 st the bounce test look for even pulses more than high numbers (I have seen a good engine with a weak battery give very low results) this test is to see a comparison of the 3 individual seals on 1 rotor. Then you want to check build pressure with the valve in the pressure guage here is where you see bigger numbers cranking speed again is relative to pressure I can't find it right now but there is a graph representing rpm/compression and 100 rpm is a significant change in compression

good luck

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