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Rotary engine seals and ethanol in fuel

I have a 2005 mazda rx8 with a little over 30K miles. I learned of the engine flooding problem early the hard way, a.k.a, you don’t run the engine for short periods of time when it’s cold. you must let it warm up or you will flood it and have to be towed in. I live in new orleans and on a balmy december day (63 degrees) I backed the car out of my carport down the driveway and turned it off to put air in the tires. I subsequently moved it back into the carport. the next day it wouldn’t start and sounded very odd when I turned it over. it got towed in to the mazda dealer. their immediate response was that I had flooded it and temperature didn’t matter. the next day the dealer told me that I had no compression, had probably blown a seal and that the engine would have to be replaced. I read on the web that seals were a problem with these early rx8 engines and that that mazda had thankfully extended the warranty. I have also heard from a small engine mechanic that ethanol in gas was killing the seals in lawnmowers and the like.

so, my question is: should I avoid gas with ethanol to save my engine?


Everybody Should Avoid Ethanol.
It Belongs In Corn.

This can happen with piston type engines too. I’ve started cold engines and just backed the vehicle out of the shop and shut it off. Then when I went to start the vehicle five minutes later I would find the engine was flooded and wouldn’t start. But then all I would do is hold the accelerator to floor to clear the flooded condition and the vehicle would start.

When it comes to yard equipment a good rule of thumb is, if it has a steel gas tank you want to avoid ethanol gas. But if it has a plastic gas tank then the equipment was designed to run on ethanol gas.

Your Mazda was designed to run on ethanol gas. If it weren’t, people wouldn’t buy it in some states because ethanol-free gas isn’t available.


Regardless of the fuel tank, ethanol can cause problems on small engines especially in the fuel system. I have recently rebuilt the carburetors on weed eaters, chain saws, lawn mowers and leaf blowers ranging in age from 3 to 18 years old. The chain saw had a metal tank but all others were plastic tanks. If a small engine fails to run unless the choke is partly closed the carburetor needs attention and the fuel line will often be severely deteriorated as will the diaphragms and seals in the carburetor.

The seals referred to in small motors that are harmed by ethanol are rubber seals, often rubber o-rings. You have these types of seals in your motor in the fuel system. If the car was made in 2004-05 the rubber in the seals in your car were made for ethanol in the gas, e-10.

The seals in the rotary piston is a whole different animal and is not affected by the ethanol in the gas. Rubber seals in the pistons wouldn’t last in the heat, pressure, and friction that is encountered by the piston seals. I believe the seals in your motor’s rotary pistons is a very tough ceramic material.

The RX oil injection system (same principal as a 2-stroke engine) is unreliable. If you want to extend the life of your rotary engine, I would add a pint of 2-stroke oil to every 12 gallons of gas which will insure the rotor seals are lubricated…

I used to own a couple of rx 7s. The second one i bought for next to nothing because the owner had flooded it and was told the engine eas shot. Rotaries use a small amount of engine oil that helps seal the combustion chamber. When u flood the engine thebextra gas dilutes this oil and washes off the apex seals. When that happens u have no compression and the engine freewheels when the starter turns it over. Thats why it sounds different. I used to unplug the fuel pump to prevent more gas going in and try to start it. If ir lucky u can burn out the excess

Gas and it will try to run. Then plug in the pump and ur good to go. I actually did this in front of the guy i bought the car from and drove it home. I put a switch in to kill the fuel pump to let it starve out anytime i was just running it for a short time. Dealers dont fix rotaries…they replace them

Its probably too late to help the OP, but if anyone else had a 7 or 8, if it’s flooded too badly to get running by unplugging the pump you can put a small amount of trans oil or 2 stroke oil into the spark plug hole by way of a tube. This will provide enuf sealing effect to make it start. Rotaries are very strong and reliable but not enuf mechanics know enuf about them. I’ve read where the reason they stopped selling 7s here was due to the warranty claims. Too many replacement engines instead of proper diagnosis.

I find this interesting @Kennedy … what about overheating/? I have been told in most cases one over heat can warp the rotors and wreck the motor. Is this true??

overheating a rotary is very bad. I don’t think you warp the rotors, I don’t have experience rebuilding one, but from what I’ve read and heard its the large “O rings” that go between the pieces that make up the engine. You have 2 combustion chambers containing the rotors sandwiched between 3 other pieces. I think the warpage occurs in the metal and causes the Orings to blow out so to speak. Its been several years since I owned the 7s and I tend to forget some of the details. Excess heat is what did in the 3rd generation 7s with the turbos. rotaries make very hot exhaust, combined with extra plumbing for the turbos a lot of heat was under the hood.