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The rotary engine is very reliable. With only 3 major moving parts, it is immune to the little problems that may plague piston engines. However, because the rotary relies on such few major parts, this also means that it is more likely that a failure of an internal engine part will be catastrophic. Combine this with the general lack of knowledge on the rotary possessed by the general public (with regards to operation, maintenance and modification) and you will find that the "blown engine" is more likely to happen in a rotary then a piston engine. This is simply the reality of the RX-7, and should not be taken to mean that the rotary is unreliable and prone to destruction. It just means that proper maintenance and operating procedures should be observed.
Many potential buyers look at cars with a supposedly blown engine as a way to save money, or start the buildup of a high horsepower car by installing their own fresh power plant. Others are experiencing weird symptoms that can indicate a blown engine, but are not sure. For example, stuck fuel injectors can produce the exact same symptoms as an engine that has lost compression, as can malfunctioning ignition systems. This document is to assist this person in determining whether or not the engine is blown.
But first, let's decide on what exactly "blown" means. For this document, blown means that the engine has lost compression on one or more rotors. This is normally due to apex seal failure. The seal cracks, pits or just plain gets ejected from the rotor, causing two rotor faces to lose their compression (one apex seal is responsible for sealing two rotor faces). Often, the damaged seal will travel around the housing as the engine rotates, crashing into other apex seals and damaging them as well. The result is a bad housing, and probably a damaged rotor. Small cracks can also form in the apex seal, resulting in much lower compression then normal, but no other damage to the housing or rotor. The point is that it is fairly easy to find a blown engine with a simple compression test.
The Mazda compression tester is the quickest and easiest way to determine the compression of a rotary engine. However, since these units are quite expensive, we can assume you don't have one. If you do, then obviously you wouldn't be reading this document so there's no real point in telling you how to use it. A simple tester can be made by using a standard automotive compression gauge. Remove the one way check valve, and install the tester into the leading (lower) spark plug hole in the front rotor. Make sure your battery is fully charged, remove the EGI fuse, then floor the pedal and crank the engine while an observer looks at the gauge. You are not looking for excellent numbers, just three even bounces above 70 PSI. If the rotor has lost one apex seal, you will get one strong bounce followed by two very low bounces. If all seals are damaged, then you will get three bounces that barely register on the gauge. Repeat for the other rotor.
If you don't have a compression tester, it is actually still very easy to determine the state of an engine. Remove the leading spark plug from the front rotor, make sure the battery is fully charged, and then crank the engine while listening to the air exiting the spark plug holes. It should be an even rhythm of "psssst" sounds, one per every half revolution of the eccentric shaft. Something like "psst...psst...psst...psst...psst...psst". An engine with a bad seal will be very obvious in that you will hear an irregular pattern of "psst" sounds or one "psst" for each revolution. For example, you would crank the engine and hear ".........psst...psst..................psst...psst...." or something like "psst.............psst...........psst.............psst.". To help you recognize the sound, a recording of a blown engine is below. This engine lost a seal in the front rotor, leaving only one rotor face with compression. It is in Microsoft Wave (.WAV) format and is playable on probably every machine out there.
Notice the long pauses between "psst" sounds. A good engine will have a steady beat of "psst" sounds that are strong, and well pronounced. On the off chance that you have a bad side seal, you could hear a pattern of two "psst" sounds, a pause, then two more. This will look like "psst...psst..........psst...psst.......". The engine is still blown, but all the apex seals are intact. An apex seal failure will always result in the loss of compression on two rotor faces, and thus one "psst" with a long pause before another.
What if the engine is out of the car? Well, you can't do a proper compression test, but you can at least check the general condition of the seals, and normally tell if it has low compression.
You will need to arrange the engine so that you can get access to the 19MM bolt at the front pulley. Remove the exhaust manifold, but leave the spark plugs in place. Now, using a 19MM wrench, turn the engine over three times. While you are turning it over, listen for a distinct "chug" every half turn. This is caused by the pressure differential as the rotor sweeps by the exhaust port. If a chug is missing, or sounds significantly weaker then the others, then you have a problem. As with the "psst" test, a chug will normally be missing for two rotor faces.