Compression loss. Most common is apex seal failure, which generally destroys everything in the same common chamber (front or rear) and you wind up with zero compression on one rotor, making it effectively dead weight, while the other rotor remains healthy and functional. You can also have compression loss from a single apex seal breaking or cracking, which decreases compression on 2 of the 3 faces in that chamber. This is a bit harder to diagnose with a piston compression checker or an audible poorman ’s test. Finally you can have a side or cornerseal fail resulting in a smaller loss of compression which will seem more at random than an apex seal failure. You usually only see side and cornerseals fail as a result of improper rebuild/assembly or port work. Compression loss can result in hard starting, a rhythmic cranking sound, sound of an old lawnmower when started, total loss of power, lack of ability to hold a good idle or any idle, severe vibration below 2000rpm, excessive backfiring (due to unburned fuel in the exhaust), erratic or rhythmic vacuum readings, and lack of ability to build much if any boost. Compression loss requires full teardown and rebuild, and very possibly replacement rotorhousings and rotors.

Coolant system failure, otherwise known as coolant seal failure. Coolant seals in a rotary function much the same as a head gasket in a piston engine, but this failure in a rotary requires full teardown. This can allow coolant to enter the combustion chamber, and compression pressure to enter the coolant system and cause excess pressure and overheating. Coolant seal engines are often fully rebuildable unless they’ve been severely overheated, which can cause warpage and cracks in the iron housings. However, if a coolant seal engine is left to sit for a few weeks or months with water inside, rust will form quickly and cause most of the internals to become junk quickly and not rebuildable, increasing the cost of the rebuild later. To avoid this, pump plenty of oil, atf, or wd-40 into the chambers while turning the engine to lube all metal surfaces and displace water to prevent rust, also turning the engine every week or 2 will be helpful in preventing rust and stuck seals until the rebuild.

Oil seal failure. Excess smoke, especially at startup and when at higher rpm’s, is usually a result of worn oil seals. These are rubber orings on the sides of the rotors that flatten and soften due to heat and time. A full teardown and rebuild is required, however oil seal engines are usually fully rebuildable. Any engine that runs low or dry of oil and/or locks up, on the other hand, is likely to be total junk inside, and most parts will not be reuseable in a rebuild if this is the case.