Sorry in advance for this being so long, but I want to be as thorough as possible!

First here's some info about the car, it's a 2004 GT 6sp with just under 120k miles on an original engine. It's spent the last few years being my daily driver off and on and I have checked the compression within the last year or so and it checked out as being passable by Mazdas standards, although somewhat on the low end. I've never had the car flood on me during my ownership, and I've stayed pretty up to date with maintenance and such. The car does not have a cat or an air pump, or the VFAD in it anymore, all solenoids have been properly capped off, it has a Walbro 255 pump in it with the bypass mod done, and the rear O2 sensor is out of the car with a plug screwed into where it should be. The only CEL codes that it has stored has been for those things being disabled and the front O2 was replaced within the last year. The car has lately been only used in autocross and I have had zero issues with that(besides some minimal cooling issues on the really hot days).

Let me preface the rest of this with the fact that I have a datalogger in the car and all numbers, diagnosis's, and figures come from that and I promise I'm not just guessing or assuming, I like to be very analytical about things.

The issue that I'm currently having with the car seems to have started about 2.5 weeks ago when it all of the sudden started running rich, even once up to temp it was idling in the mid 11's as well as at cruise, but would come back up closer to where it should be during acceleration. It got worse throughout that day until it got to the point that the car would die when I came off throttle and was backfiring at inappropriate times. Once home I pulled a spark plug and saw that it was quite fouled and it was about time to replace the ignition system anyways so I went ahead and replaced coils, plugs, and wires then reset the fuel trims and all. This completely resolved the issue and it was back to where it has always been as far as power and AFRs go.

A week or so after that I had to drive down to Tampa to pick up another car and left the RX8 at my friends house where he was going to drive it back to me once I returned. When I returned and after the RX8 had been sitting for 6 days it would not start for him no matter what. I went over and checked things over and pulled a plug again to see what seemed to be the issue. The engine seemed to not even be trying, it didn't sound like it was firing and watching the temp at the front O2 sensor it would seem that no combustion at all was occurring, it remained at ambient temp even after 10 seconds of cranking. The engine was still getting fuel pressure, the plug I pulled had black soot but was dry, the plug was still firing, and nothing seemed out of place. After poking around a bit more I decided to try the deflooding procedure and after a few tries got it to try and start a little bit, but still far from actually cranking up. To avoid killing the battery any further I decided to tow start it behind the truck and on our first attempt it sputtered a bit but finally fired right up with a large cloud of light colored smoke! I thought I was good to go at this point and ran it around the block a few times but every time I got up in the RPMs it would blow more smoke.

Once I parked it back in the driveway and it idled for a minute it began to make a chirping noise, kind of like a mixture between a sharp electrical noise and a metal-on-metal vibration. We could not positively locate the noise and it seemed to be coming from the 'engine'. While trying to find this noise we also noticed that the front crank pulley was slightly un-true and seemed to almost be wobbling, although I have had this off multiple times I've always taken care to not mess up the front stack with the thrust washers or torrington bearing and it has been over a year since any of that was disassembled. The next symptom was that while it was sitting there idling, the coolant in the reservoir began to boil quite aggressively, I checked the temp and it was sitting right at 216 F, which is quite normal for the Florida summer from what I've seen. I shut the car off and you could visible see steam rising from the engine, but I could not place it, it seemed to be coming off the whole block up around the intake manifold.

To me this seemed like it was either a leak somewhere and the system not properly pressurizing leading to it boiling at that temperature, or combustion gasses getting into the coolant through a water jacket o-ring failure. I used a test kit to see if combustion gases were present in the coolant and that test came up negative. there was nothing wet or any obvious leak anywhere that I could find either.

I'm pretty stumped on this one. The exhaust smelled sweet when it first fired up, although not like coolant normally does. Almost like a mix between that and old gas. My friend described it as smelling like fingernail polish.

So, what do the people of 8club have to say about it? I'm open to any suggestion or test that can be reasonably carried out.