Engine Compression Check
Often people chase after problems with their RX-7 for many hours, even
weeks at a time. My first rule of troubleshooting a rotary engine is to
first perform a compression check. IF this reveals no problems with the
short block/internals, then a control system or condition is at fault.
I have looked at, worked on, or purchased many 7’s which had blown
engines, all the while the pervious owner thought there was a fuel pump,
fuel filter, sparkplug, coil, computer, etc. problem causing the car not
to run properly. Here are steps to performing a compression check with
or without a tester.
Poor Man’s Compression
1. remove lower sparkplug and wire from front rotor.
2. remove EGI fuse from underhood fusebox to prevent fuel and spark while
3. have an assistant crank the car over for you, while you listen under
the hood at the compression pulses coming from this plug hole. IT is recommended
to put your hand/finger right in front of the hole to feel the pulses
4. There should be one strong pulse/whoosh of air per full rotation of
the crank pulley. Use the timing marks on it as a reference. There should
be 3 even pulses in succession, without skips or gallops.
5. IF one or more pulses are weaker or non existant, this indicates (usually)
at least one blown apex seal and severe internal damage. A full rebuild
will be required, and no further troubleshooting will help.
6. IF this chamber passes the compression test, replace this plug and
repeat for the rear. The rear rotor blows more often then the front on
88 and prior engines, and the front rotor blows more often on 89 and later
engines, for reasons unknown.
7. With both lower plugs out at one time, you can listen/feel for compression
on both rotors at once. This should be a rhythmic ch-ch-ch-ch-ch-ch sound,
like an old steam engine train, alternating front to back, once per rotor
per rotation. Any skips or galloping indicate loss of compression.
8. note that this procedure can be used to test junkyard engines or engines
out of the car. You will need a 19mm socket and ratchet to turn the front
crank pulley bolt clockwise in quick, long strokes as possible to get
a somewhat accurate reading. You will obviously not be able to turn the
engine much at a time, so try to count each pulse as you go.
test using a piston engine tester:
1. note battery strength. A weak battery will yield low compression results.
2. Remove both lower plugs and wires.
3. remove EGI fuse from engine fusebox.
4. have a friend floor the accelerator pedal, opening the throttle for
5. insert your tester into the leading hole
6. hold the valve on the side of the tester open
7. have your friend crank the car over for 5+ seconds.
8. observe the needle bounces. You should see 3 in succession without
skips, even bounces, in roughly the 30-35psi range.
9. let out on the valve now, and let the tester reach an overall compression
value for all 3 faces(highest of 3 will be displayed). 115+ is like new,
100-115 is healthy, 90-100 is getting weak(1 year or less in most cases)
below 90 could blow at any moment.
10. repeat for opposite rotor. Note difference in overall compression
between rotors, which should be no more than 20psi max.