wwjessedo

I'm going to look at a used engine today that's not in a car. Is it possible to do a compression test by cranking it manually? I'd apreciate any help.
Thanks.

hypntyz7

Rotary 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 test(free):

1. remove lower sparkplug and wire from front rotor.
2. remove EGI fuse from underhood fusebox to prevent fuel and spark while testing.
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 of air.
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.


Compression 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 more airflow
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.

wwjessedo

Thanks, that's exactly what I needed to know.
Jesse