Compression check... WTF?
 

Ok, I been having trouble starting my 88 T2 that I bought last weekend. The fella I bought from said compression was just done and read 95 on 1, and 90 on 2. I did the test today myself and got 70 on 1 and 50 on 2. Did I do this right? Unplug crank angle sensor, EGI fuse pulled, throttle held to the floor, and took readings from trailing spark plug holes one at a time. Please let me know your thoughts. Also, see my "Likes to die when stopped" thread for the ONLY other problem I have with this car and see if they are linked somehow. And just FYI, the car responds beautifully when driven, no lag or hesitation. Strong as a mule. Thanks guys. I gotta know what I got before I go ordering tons of stuff for the cars exterior. Brian

Sounds like you're doing it right. Here's an FAQ that may help out a bit: http://mazdarx7.iougs.com/myfaq.shtml

I'd suggest testing the individual bumps per rotor face, rather than the cumulative pressure. If the totals are actually 90 & 95 psi, then each bump should be about 30 psi. You'll need to either hold open the bleeder valve or remove the schrader valve from the end of the tester, depending on how yours is set up.

If one bump is particularly weak, you likely have a side seal damaged. If two bumps are weak, it's likely an apex seal. Hopefully your second test turns out better numbers, but low compression can cause idle to be very rough. Of course, you should also notice a definite loss of power if you have numbers as low as 70 & 50 psi.

You need to test for compression on each individual compression face. That is, 3 faces per rotor. The "overall" compression number is generally meaningless. Why? Because you can have one failed apex seal and still pull decent compression on the one remaining good compression face. So you crank the engine and the one good compression face will pump your tester up to whatever that single face is generating. The other two--on either side of a failed apex seal--will not contribute to the "overall" compression reading.

So, in your case specifically, when using a conventional tester to record one "overall" number, you COULD have one compression face that is producing 70 psi while the other two are producing zero.

As RR88 suggests, remove the shraeder valve from the hose or hold in the button on the side of the compression tester and watch for 3 even bounces of 60 psi or better, with emphasis on EVEN BOUNCES. At 70 psi on all 3 faces of both rotors, your engine is not in the best of health, but it should at least run. However, you should be saving for a rebuild or replacement.

Better yet, have a proper test done by someone with a rotary-engine compression tester and who know what they are doing.