Signs of Worn Apex Seals on a Rotary Engine

April 10, 2012

A rotary engine with a worn apex seal tends to run underpowered due to poor compression. Learn about the 4 main symptoms of worn apex seals.

Engine Work

In a vehicle that uses a rotary engine, such as the popular Mazda RX-7, apex seals are a constant source of frustration and usually one of the first parts to fail in an otherwise very reliable engine design. In fact many people consider the apex seal to be the weakest link of a Wankel rotary engine.

The "piston" on a rotary engine is usually triangular in shape and made out of cast iron. The apex seal is the seal placed at the tip or "apex" of the device. The apex seal is the only moving part that comes in direct contact with the rotor housing. A damaged or faulty apex seal can cause a host of problems. So, here are some signs that the apex seal in a rotary engine powered vehicle may be bad.

Poor Compression Symptoms Associated to Faulty Apex Seals

Most problems associated with faulty apex seals have to do with compression, or lack of it. Poor compression can cause many different types of problems in any type of internal combustion engine. So, here are some of the most common compression related symptoms that may indicate a cracked or damaged apex seal.

  • Hard cold starts. If you notice that the vehicle is hard to start after the engine has been turned off for a considerable length of time (like after having been parked all night), or the vehicle is otherwise hard to start when the engine is cold, this may be a compression related problem that can be traced to a faulty apex seal. While other factors such as poor maintenance, carbon build up in the engine and other problems may cause hard starts, the most common culprit is a faulty apex seal.
  • Idling or cutting off problems. Another very common problem associated with faulty apex seals is "lumpy" or irregular idling of the vehicle when the vehicle is stopped. Another symptom is when the vehicle simply cuts off and is not able to maintain a proper idling speed. You can use a compression tester to verify this. You will probably find that compression is considerably decreased because of a defective apex seal.
  • Frequent misfiring. If your Wankel rotary engine powered vehicle is starting to frequently misfire, it is a sign of bad compression.which is of course the biggest symptom of a damaged or faulty apex seal. You may want to remove one of the spark plugs to verify that it is a compression problem. Also, listen for an uneven or irregular sound when attempting to start a vehicle with a spark plug removed.
  • Noticeable reduction in power. An almost sure sign that the apex seals in your vehicle are defective is when you're able to rev your car's engine up to considerable rpm levels and the vehicle barely moves. If you're having trouble climbing hills, or getting your vehicle to move at decent driving speeds, you should have your vehicle inspected and the apex seals checked.

What Apex Seals on Rotary Engines Do

A rotary engine works by creating three separated chambers inside of a hollow ovoid housing "cylinder." A spinning triangular rotor inside the case regulates the four-part cycle of intake, compression, combustion and exhaust. This design saves weight, providing a higher power-to-weight ratio than a normal four-cylinder engine. The Mazda RX7 and RX8 sports cars are the notable commercial car models with rotary engines.

In order to keep compression in the chamber, the three tips of the triangle must form airtight seals against the interior casing housing. These seals at the three apexes of the triangle are called the apex seals. Apex seals are usually made of metal, and are pushed against the housing by springs. Since the seals are in contact with the housing case, in order to minimize friction they are usually covered in engine oil. Thus, by design, a rotary engine burns a little bit of oil. Like many engine parts, the quality of apex seals varies, and some are more robust than others.

If apex seals become worn, pitted or cracked, this can lead to decompression and reduced power in two of the three chambers. (Remember that due to the nature of a triangle, each seal is responsible for the integrity of two adjacent chambers.) Though this decompression will likely not destroy the engine, it can result in a complete engine teardown and rebuild. This is akin to blowing a head gasket in a conventional engine. Depending on the car and mileage, some owners will just go ahead and junk the car rather than pay for such extensive engine work.

Related Questions and Answers

Does a Rotary Engine Use a Certain Type of Oil?

Rotary engine vehicles require special care when choosing a rotary engine oil. The first warning is to avoid synthetic motor oil. In fact, while some synthetic oil manufacturers say their oil will work fine in a rotary engine, using a synthetic oil in a Mazda rotary engine actually voids the warranty. Mazda recommends using Mineral Oil 10/30W in their rotary engines. When in doubt, it's best to consult the owner's manual for the specific vehicle. If one can't be found, then check with a mechanic who specializes in the type of vehicle in question.

What's the Biggest Issue You Run into After You Rebuild Your Rotary Engine?

In the case of a rebuild on a rotary engine, the biggest issue is the same, as the most common cause of engine failure in rotary engines is imbalance of the rotary assembly. When the rotary assembly is improperly balanced, it causes excess strain on various engine parts. In the short run, that affects performance. In the long run, it will lead to broken or worn parts and eventually engine failure. Perhaps the best way to ensure that a rotary engine is properly balanced is to take it to a professional. Balancing is a common service offered by rotary engine mechanics.

Are Rotary Engine Parts More Expensive than Parts for other Engine Types?

Rotary engine parts are generally manufactured in smaller quantities than other engine parts are. For that reason, they tend to be more expensive. However, there are a lot fewer parts to a rotary engine than to a piston engine. They are often easier to rebuild. For that reason, a full rotary engine rebuild will typically cost less than a rebuild on a piston engine. Prices will vary from mechanic to mechanic, but overall, comparing a piston engine rebuild to a rotary engine rebuild will find the cost savings falling on the side of the rotary engine.