Good morning, gentlemen. I'd like to start with a disclaimer that this will be a long one, apologies.

Yesterday I took my first long trip in my 2005 RX-8, which as around 80k miles on it. As an aside, if I wasn't sold on these cars before, I am now--the trip was a dream and the car was a joy to drive almost the entire 5 hour trip.

After driving for about 3 hours, I stopped at a rest stop to fill up. I had about a quarter tank of gas left and could've kept going, but I'm paranoid due to reading about the horror stories of the fuel pump starving the engine for fuel when below a quarter tank during hard left turns. I did my routine of taking out my funnel, adding some Idemitsu, and then filling the tank up with 93 octane gasoline.

Around 10 minutes after getting back on the highway and steadily going 80 mph with the cruise control on, I got a CEL. The car drove fine the rest of my trip and when I arrived at my destination I pulled out my OBDII scanner and got the following:



I promptly did some research for the remainder of last night and found that many, many people have discussed this. Specifically, my situation mirrors the one described by member Hartsk8s in this thread, https://www.rx8club.com/series-i-tro...96-cel-194571/. My reason for making a separate thread was because of a few unique factors/qualms to my situation.

Some information beforehand: I just cleaned my entire air box, accordion tube, and MAF sensor with MAF cleaner and got a brand new air filter before making this trip. Upon arrival, the intake looked clean and my new air filter was bright white still. I am running a catless midpipe on this car with an O2 "Simulator" from Casper's Electronics. This was something was done to it by its previous owner and has caused the "Slow downstream O2 circuit response" code before. However, that code has since stopped showing up and has been replaced by the "Post Catalyst Fuel Trim System Too Lean" code. If I could I would just go back to having a catalytic converter on the car, as most people who remove it seem doomed to fight CEL's forever, but the previous owner said that it had already been gutted anyway. One last thing: I have a completely new ignition system as of ~800 miles ago. Plugs, wires, coils, battery, starter. Now, for my concerns:

First, could too much premix cause this code? I have simply been eyeballing around 4-6 oz of Idemitsu per tank of gas and have been doing my best to measure out how much I need for partial fill-ups. Despite this, I did add a bit more than I wanted to by accident when I filled up at that rest stop. The gas that was already in the tank was also premixed as well.

A related question I've been asking myself is when to add the premix--before or after adding the gas. Additionally, it has been suggested to me that I add the premix halfway between my fill up while the engine is running. This suggestion inherently doesn't seem right to me.

I see a lot of mixed information here on the forums from the early years of the 8's release that suggest that premixing too heavily can cause your plugs to get fouled up. Would fouled spark plugs alone cause this code?

Second, I am aware that exhaust leaks can cause this code. However, given that I don't actually have an O2 sensor downstream and am running a "simulator" sensor, I'm not sure if this is probable in my case. I have noticed an odd trend in the exhaust coming from this car. When it is first started after sitting for a few hours it gives off a faint smoke that disappears after moving away from the tailpipes. Here is a video I have taken to demonstrate:


Once the car is warmed up and driven, even for just 30 seconds, this smoke disappears and the car idles as demonstrated by this second video:


Given what's going on here, I'm not 100% sure if that's just condensation coming off of the pipes, or if there's something more sinister at play here. A second opinion would be immensely appreciated. For the record, the temperature outside during these videos is roughly 55 F and it was just after a rainstorm.

Third, I sprayed WD-40 in my engine bay whilst it was running to see if I had any apparent vacuum leaks. From what I can tell, there are none, and both service nipples on the left of the car (when facing the vehicle) are firmly affixed. There is no loss of power, no rough idle, and no other signs that are typically associated with this code based on the knowledge base here on the forums. Given these factors, could my fuel pump assembly be on its way out? Mazda actually just inspected my o-ring and fuel pump when I took my car to get recall service and said all was well with it, however I have seen it suggested that this could be the case.

Finally, I'd like to say thanks for reading this book of a post, I really appreciate all the help you guys have given over the past couple of months. I know it's a lot, but I've seen it said more than once here that a "lean rotary is eventually a dead rotary" so I'd rather be overly cautious than to wind up with a beautiful paperweight on wheels.

-Wyvern

 

1) I run Premix and have a Racing beat Dual resonated mid-pipe and haven't ever had that code. I always add pre mix before the fuel but I don't think it matter too much. Fouled plug would more than likely cause a rich condition instead of lean and it would start to missfire or at least run rough.

2) An exhaust leak between the manifold and the mid pipe could cause this if it was a big enough leak. You could hear it or smell it. Could you explain "fake" O2 sensor?

3)vacuum leak would give you a lean code on the primary O2 sensor.

It could just be a bad secondary O2 sensor but only way to check would be replacing it.

As for the smoke, it could be the premix if you ran it a bit heavy. Check your coolant also as it could be caused by a coolant leak into the housing(s).
Might also want to do plugs and coils if you haven't done them lately.

 

Thanks for the suggestions, let me address some of your points.

1.) Okay, right that makes sense. I've had no misfires and an idle that fluctuates between 815-855, usually between 30 and 50.

2.) I don't think I can hear an exhaust leak. Smell wise, what would the leak smell like? Sorry that wasn't very clear now was it. By "fake" I meant I have an O2 simulator installed downstream. I believe it's by Casper's Electronics.

3.) That's what I thought, whereas this code is for the post-catalyst one as I've shown. I can't detect any vacuum leaks from my WD-40 "smoke" test and just by my own inspection.

That's the thing, I have no cat and only this "simulator" that was put on by the previous owner.

My coolant levels have been roughly the same since December. Mazda checked out all of my fluid levels and said that it was all nominal.

Thanks again for your help, I appreciate it.

-Wyvern

 

Ah, I see. Well It could just be the simulator causing the issue then. I'd get rid of it, then reset the ecu and see if the code comes back as a P0420(no cat)

 

Yeah the simulator is super suspicious, however it never gave me any troubles until now. In fact for the first three hours of driving it was fine. The trigger for all of this was my second tank of gas as far as I can tell.

The car is used to getting 91 octane fuel with no ethanol in it. I half wonder if the 93 octane with the ethanol pissed it off. (that was the only premium available at the rest stop).

 

Alright fellas, to anyone who might be having issues similar to what I mentioned I have noticed a trend that I believe is the solution:

As I mentioned in my posts, when I filled up the last tank of gas I ran in this car, I added fuel that was 93 octane, but was mixed with a certain percentage (not over 10%) of ethanol. Almost immediately after filling up with this gas (within 10 minutes of getting back on the interstate) I got a CEL.

I have been driving the ethanol gas out of my car without clearing the CEL, and just filled up this morning with 91 octane with 0% ethanol mixed in. I drove about a sixth of tank of fresh gas out of the car today and the light has gone off. Just to test my theory I took the car out into the country and raced the engine a bit. No CEL.

I will report back here in the coming days to tell you guys if it comes back, but as of now I'm pretty sure that's what it was.

-Wyvern