Since "flooding" happens to a significant percentage of RX-8's I wanted to understand what was happening and why. I have looked at a number of threads with bits and pieces and rants and fixes.

Does flooding occur from a leaking of the injectors in a cold condition or poor ECU logic? Or is there a loss of compression/rotor seal due to the cold engine and excess fuel?

It appears to consistantly occur with a cold start and shutdown before the engine warms and air temperature may be a factor. Then the engine will crank but won't start. What actually happened is less clear.

In a piston engine there is too much fuel to support ignition, usually from carburetion problems or pumping the throttle too much, and in extreme cases you could fluid lock the engine. This is hard to do with a modern stock fuel injected piston engine. To restart the engine means cleaning the plugs and allowing the excess fuel to evaporate.

In the rotary engine cleaning the plugs and airing out the combustion chamber only occasionally works. The factory fixes include: haul it to the dealer, reflashing, hotter plugs, faster starter motor, bigger battery, retrain the owner ( I may have missed a few). The DIY fixes include: let the car sit for days/weeks, clean the plugs, replace the plugs, inject water/antifreeze/oil/atf fluid, extended startup startup/shutdown procedure. Some of these fixes are consistant with classic piston engine flooding and some with loss of compression.

My engineering curiosity is tweaked with this one and I appreciate your feedback.

 

As to the exact reason a rotary engine is more suseptable to flooding is unknown to me. But I would lean primarily to an error in ECU logic. This was pretty quickly resolved with a flash. As far as the fixes you listed below most are bogus, these may have been "solutions" made my ignorant dealers, but they were not solutions. The main DIY solution was using washer fluid and I think (not a chemist) the main reason for that is most washer fluids are alchohol based and aide in evaporation of excess gas.

And as far as a flooding happening to a significant percentage I am not sure I agree with that. Using this board is not a good way of measuring the percentage. If you say more often than modern piston engines then, yes it is more common.

 

There was a message in the polling thread area. But not a lot of people responded....

My understanding of what the flooding is, is the Apex seals lose cohesion because extra gas washes away the oil that helps create a seal. Secondarily, the plugs can get wet with too much gas.

 

The following is my understanding of some of the issues. Those of you, more knowledgeable, please chime in with any corrections or additions..

I believe there are several factors that contribute to flooding in a rotary engine:

1. The combustion chamber in the rotary engine is long and flat. That is, it has lots of surface area. When the block is cold it draws thermal energy from the gas/air mixture making it more difficult to ignite. I think this is one of the reasons why two plugs are needed. I would also guess that the shape of the combustion chamber makes it hard to expel the extra fuel when a flood occurs.

2. When you start and initially run a cold engine, the ECU provides very rich mixture as a fixed setting, rather that using engine sensors to provide a mixture optimized based on engine conditions. So you are running a very "fuel damp" environment.

3. For starting, the rotary takes a lot of energy to turn over and to run at the needed RPM. To save weight Mazda apparently undersized the battery. Also, the RPM rate of the original starter is marginal (again a weight trade off).

The rest of this is conjecture on my part. The cold engine is running a very rich mixture and when shut off cold and left, the fuel condenses in the combustion chamber. Because of the weak starter and long cold combustion chamber it is almost impossible to vaporize or expel the liquid fuel on the next start attempt. Unless you hold the pedal to the floor the fuel injection system makes things worse by squirting in even more rich air/fuel mixture into the combustion chamber. Eventually the plugs get wet to the point where they will not spark properly. If enough fuel collects in the combustion chamber it will interfere with the apex seal's ability to seal the chamber resulting in a loss of compression.

When the flood occurs, the combustion chambers must be manually "dried out" and the fouled plugs/oil replaced.

 

Thanks for the feedback. It all makes more sense now. The simple solution may be to install a block heater and plug in every night.

 

Too much fuel to support combustion - same as piston

Fuel washes oil from housing, reduces or eliminates compression seal

In a piston engine, the excess fuel can drain past the piston rings and leave the combustion chamber, thus letting the car sit can relieve the problem. In the rotary there's nowhere for the fuel to go.

Motion of the rotors slings any liquid fuel into the spark plug.