oi812

My car lopes at idle, I think it's a partial misfire every once in a while, it's almost like the combustion process is not 100% constant at idle. I'm not talking about a huge misfire, just enough to "slow" the crankshaft rotation for a split second. I feel there is some additional small amplitude vibration while at idle due to the engine mount tuning, but the "lope" causes an inconstant low frequency "shake" or higher amplitude side to side shift lever movement.

While I agree with RX8_Buckeye on the engine mounts and the NVH issue, if it was purely engine shaking forces and the vibration tuning of the mount system, the shift lever vibration would be consistent and cyclical all the time for a given RPM. This would be true if the vibration was torsional (as RX8_Buckeye describes) or transnational (shaking as in a piston changing direction). This is assuming that the combustion process was consistent and uniform each time the rotor fires.

From my experience with piston engines, if the EGR (Exhaust Gas Recreation) leaks at idle, you always end up with a loping inconsistent idle. The normal EGR operation on piston engine is to have the system closed off most of the time and once fully warm and at a low engine load then the system allows a small amount of burnt exhaust back into the intake manifold via a valve. This reduce fuel consumption on the highway by "diluting" some of the intake charge. Less "good" air in the engine reduces the amount of fuel needed. Less fuel = less emissions. Works well at moderate RPM, not so good for smooth idling. It's kind of like the "race" cam beachdog talked about where the intake and exhaust timing is causing some "mixing" of the two cycles due to valve timing overlap.

According to Mazda's marketing literature, the Renesis has built a built in "EGR system" by virtue of the side exhaust ports. Some small amount of left over combustion gas is carried from the tip of the rotor into the intake charge of the next cycle. The thing with this EGR layout is that it is "on" all the time even at idle, even when cold.

So in my opinion, the shift lever vibration the you can feel and see is a combination of some small amount of engine vibration / engine mount tuning and some inconsistent firing due to the side port design.

r0tor

iTrader: (1)

part of the problem is idling at 900 rpms means the rotors are only going at 300rpms. This means low energy storage when combined with a low inertia engine, and add the lean conditions at idle and it makes the engine have to work harder to keep spinning and overcome compression.

also the 300 rpm rotor speed is putting out frequencies very close to the low lying resonance frequencies that all engines have. This is why every car on the market has some shaking involved during start ups and shutdowns of the engine because you have to go through the resonance frequencies - only in those examples you get through the critical frequencies quickly instead of sitting right at them.

Nubo

I like the low-inertial-mass theory. Makes sense that that would make any momentary bumps more noticeable. Mine feels like a missed or partial combustion that happens every couple of seconds or so. Not big misses - fairly subtle. purrrrrrrrrrrrrrrrrrrrr(bump)purrrrrrrrrrrrrrrrrrr rrrr(bump)purrrrrrrrrr(bump)purrrrrrrrrrrrrrrrrrrr rrrrrrr

RX8_Buckeye

The engine by itself does not have natural frequencies anywhere close to 300 rpm (5 Hz). The "low lying resonance frequencies" that you refer to are the natural frequencies of the spring-mass system that is created when the engine is mounted to the vehicle. Engineers carefully design the mounts so that these natural frequencies are not aligned with the excitation frequencies of the engine at idle or other natural frequencies of the vehicle--such as suspension modes and body modes. As you stated, there is no way to avoid exciting these resonances during engine start up or shut down. However, it is highly unlikely that the Mazda engineers would design a mounting system that has a natural frequency aligned with rotor speed at idle if it could be avoided. Maybe, however, it is difficult to manufacture a mount with such a low stiffness that meets durability requirements.

Omicron

So I guess we've decided there isn't anything that can be done about this?

mysql101

if it's not broke..

md_guy

My AT lopes at idle. As somebody else mentioned, it almost feels like an occasional misfire.

MazdaManiac

iTrader: (3)

Yeah, there really isn't a "solution" because there really isn't a problem.
Its just a feature!:p

greese

I found it odd myself when i got my 8. My first gen (83 GSL Limited) was so smooth and quiet at idle I would often think i had stalled it when sitting at a light. I expected the renesis to be as smooth but it is not. It's as smooth everywhere else. Just not idle

Nothing like being on a date and cranking the starter on an already running car.

rex

iTrader: (2)

You guys spend far too much time idling and not driving

Mine behaves just like Nubo's (though I think his RX-8 is bee powered!). I put it down to an intermittent misfire. It's worse in protracted stop go traffic and was definitely worse prior to PCM rev "L" and "M" when it would foul the plugs so badly it was prone to stall.

I'd be very interested if anyone has an RX-8 that doesn't have the Nubo behavior. I'd also like to know how your cars with lightened flywheels behave. My guess is that the effect would be exaggerated.

RX-8 friend

Listen to the injectors. They cycle. In a perfect world, an idling engine's injectors would be operating in a constant rythm. As Mazdamaniac stated, the poor idle is a result of the design. The injectors can handle a lot more fuel than we are using, even at full load full throttle we see only around 30-40% duty cycle. What this means is one injector pulse will be too much fuel for idle, so the PCM shuts them off, then uses them, then shuts them off... etc. We saw the same thing with the 3rd gen RX-7 when owners replaced the injectors with much larger units (to handle higher boost and larger turbos). Their solution was to bump the idle up.

We've also seen the different flashes affect the idle. The current one (M) sometimes idles rough, other times smooth. We haven't been able to correlate it to anything yet.

thew

MY old Rx7 did it and the new 8 does as well.. its just the way it is.. like algabra...

greese

I don't spend time idling by choice. Bumper to bumper traffic makes for a lot of idle.

I'm not complaining either. As long as it doesn't idle itself into a stall, I don't care how rough it runs.

SpearRX8

I have the same simptoms but I have noticed that reving to 1000 Rpms gets rid of 90% of the bad idle issue. I would love to see Mazda addressing this issue on their next PCM upgrade. I do not know if this will ever happen due to emmissions but it is a wish.

Gomez

Hi guys, I notice that no-one from Oz has responded to this thread and it's probably 'cos we have not experienced this problem down under. All our cars are hi power, and we have a different flash on our PCM's.....My 8 came from the factory with the "F" flash in Jan04, the "G" flash has just been released. The guy's who have had the reflash are reporting a slightly smoother idle and better economy, these guy's have reflashed their 2003 models which had "E" from the factory.

Just another theory to throw into the mix.....

Regards Gomez.