So i got an RX8 willing to throw money in it long term (even rebuild the engine if needed)
2004 RX8 with 95k miles. AT

The car was sitting in storage for maybe a few months before I got my hands on it. So here are the issues:

Day 1: The first time starting it out of storage, it was having a hard time. Eventually it got running but it does not have normal start up procedures (normally the engine should rev around 1.5k-2k RPMs to heat up to operating temp?) the RPMs were hunting hard between 500-1000. had a very rough idle, and stalls after a few seconds. the CEL is on with codes P0107, P2259, and P0031.

Day 2: I changed the starter, coils, plugs and wires. Also went ahead and gave it a fresh oil change. I also cleaned the MAF and Throttle Body. The cold start is still a bit rough, but better. It will still stall if I don't feather the gas. The engine will also hesitate at low RPMs (800-1500) but once it reaches around 2k, its able to rev pretty freely (but slowly) up to 5k (hard cap?) at this point I was thinking that the car was in limp mode.

Day 3: Cold started the car and to my surprise it seemed to start perfectly fine, almost felt normal. The engine revved up to 2k RPMs immediately, and as the car began to warm up it slowly dropped to a 800-1k smooth idle. The car seems completely fine but will still not rev passed 5k RPMs. After a few more minutes, the car will begin to idle rough again and eventually stall. I can feather the gas to keep it alive. Revving the engine will hesitate between 800-1.5k rpms, but again be consistent from 1.5k-5k (wont rev passed 5k still)

I turn the car off here, and decide to wait a few seconds and do a hot start. It had a hard time starting up (5-6 seconds?) and immediately started idling roughly again/stalling.

Is the car in limp mode? from searching im thinking the engine may have very low compression since the ignition system is in working order.

The weird thing is though: if i start the car (hot or cold) i can rev the engine up to redline easily if i rev the car within a few seconds of starting it. Then eventually after a few seconds it will drop to the 5k cap even if im flooring it.

The car is also slow as hell, havent gotten it to move passed 30 mph. even in first gear the RPM seems to be limited based on mph or load

I also found a receipt for gas that the previous owner had in the back of the car, the last fill up was with regular gas, i've heard this can cause issues.

So right now im thinking:
Low Engine Compression
Bad Front O2 Sensor? (P0031)
Bad Air Solenoid? (P2259)
Bad BARO Sensor? (P0107)
Bad Cat? The cat looks really rusty/worn.

I was also thinking it could be the fuel pump/fuel injector? I've read that once these get hot it can causes this type of issue as well.

Also would low compression keep me from passing 5k rpms? isnt compression supposed to improve with higher RPM. The engine is not misfiring at all, also sounds fine. I dont think an apex seal is blown since it starts/idles completely fine cold, is this right? Im very sure the seals have worn down, and the compression isnt what it should be, naturally, but would that cause my described issue?

I'm going to get the compression checked, but I wanted to see if there was an obvious issue that would directly cause this problem

 

Before fixing anything you will want a compression test done. This way you’ll know if the engine is still usable.

 

just got the compression checked today

Rotor 1: 7.0, 6.9, 7.0
Rotor 2: 7.2, 7.1, 7.2
@ 250 RPM

these numbers wouldnt cause my issue right? i unplugged the front o2 sensor (others were telling me this was the issue, and ive heard people have been driving without it). and no change

im getting a new lift tomorrow so i can replace the front o2 and clean the ESS. will check out the cat while im under there

 

Right about Mazda minimum spec. Engine is totally ok for many more miles if you take care of it. Dont mess with the front O2 sensor it is very important. You will want to check your cat to make sure it's all good.

 

Had the same problem ^, after looking at the cat it was blocked, so i just replaced it with a catless midpipe and car ran fine after that

edit - thought it might be important to say i did not have the limited revs like you do, so id check the cat make sure its all good like Cayman said.

 

alright so i dropped the cat and exhaust manifold together. i replaced the front o2 sensor and put the exhaust manifold back on. i visually inspected the cat, and the honey comb filter was intact (visually) would there still be issues inside of the resonator of the cat that can cause clogging?

i put the cat back on, reassembled the entire car and reset the fuel trims. o2 code is gone now, only code left is p2259 (air solenoid for smog is bad? im ignoring this for now)

so the car starts much better cold and hot. idle is nearly perfect (sometimes stalls when putting it in park after driving) hot start is about 3 seconds. the car drives much much better but is still suffering some pretty major loss in power. if i turn the car on hot and immediately redline it, ill pop random misfires (p0300) around 7.5k RPMs. the rpms will slowly drop as the engine runs even at full throttle and the power loss is frequent throughout the entire power band

at lower rpms the engine seems to hesitate just a bit. i no longer think this is limp mode, but rather the cat is clogged and creating back pressure to the engine which explains the random misfires, hesitation at low rpms, full power loss, and why the previous front o2 sensor ended up dying

does that sound right? my next step for now is to gut the cat and give it a try then.

 

Don't gut a good cat. They're worth a lot. Get a midpipe if you want to replace it. If you're going to take it out again, pay special attention to the back element of the cat that's not so easy to see. What you can do is lower a flashlight through the pipe past the bend and see if the light shines all the way through to the front.
The 02 code you had was for the 02 heater circuit, I don't think the cat could have killed it.

Stalling in park and the other symptoms don't fit a bad cat anyway. It's not limp mode, that would limit you to 3.5k.
Have you gotten OBD data on airflow, fuel trims and rpm at idle, and when the issue presents? That would be helpful.

 

alright so car isnt stalling at all anymore, it also idles very smoothly around 800 rpms, i also believe the cat is fine after shining a light through it.

the only code im getting is p2259 for the air solenoid. the cold starts are about 1 second, hot starts are 2.5 to 3 seconds. weird thing is.. if i start the car cold or hot, i have full power on the car for about 5 seconds before it starts giving me lower power. i drove the car around for a bit in this "low power mode" to heat up the car, then i turned off the car at the base of a pretty decent straight inclined hill. i turned the car off, waited about 10 seconds and turned it back on. car started in 2.5 seconds, and i floored it up the hill, i felt the full power of the car and it didnt hestitate at all. but after 3-4 seconds it went straight back into this "low power mode" and stays this way until i restart the car again

while in the low power mode lightly pressing the acceleration will hesitate 1-2 times before trying to give me revs, but it revs up very very slowly. it takes forever to gain speed, especially uphill with my foot to the floor

any ideas what this could be? ive cleaned the MAF, ESS, Throttle Body, changed coils, plugs, wires and starter, front o2 (fixed o2 code and rough idle/stalling), Cat is good too. only code im getting is p2259 for that air solenoid

 

well i think i found the issue. after looking into limp mode i checked the OMP sensor. its snapped in half and the connector is just hanging there under the passenger side wheel well. this is what it looks like

where can i get a replacement for this?

 

just bought a used OMP Position Sensor for 35$ off someone parting out an rx8 on facebook martketplace, i've heard they need to be adjusted to fit which can be a PITA. ill keep this thread posted on the results

 

Can you buy the whole OMP ? That way it's already together and properly adjusted

 

i couldnt afford what he was trying to sell it for the whole thing, oddly enough he was down to sell just the sensor for 35.. i dont at all mind trying to adjust it myself. how difficult is it, are there any DIY procedures for it? if im unable to do it then ill suck it up and buy the whole thing

 

just to update this thread for anyone else who has a similar problem. the 5k rev limit was in park/neutral. it wouldnt go passed 3k rpms while driving. car was in limp mode for sure. i got the new OMP sensor for 35$ off someone on FB marketplace, and adjusted it onto my OMP. (abit of a PITA)

for those of you who want to adjust your OMP yourself

Jack the car up on the passenger side, throw a jack stand under it
1: Remove passenger side wheel
2: Disconnect neg battery lead
3: Remove black OMP sensor by removing the two screws holding it in place
4: Open the OMP fully by turning the dial of the all the way counterclock wise using a pair of needle nose pliers until it picks up slack
5: close the OMP slightly about 5-6 degrees clockwise *carefully*
6: put the black OMP sensor back on at an angle with the connector facing downward, the screw holes will not line up because the sensor needs to be adjusted now: to do this, very slowly and lightly turn the OMP sensor clockwise: the key here is not to adjust the actual OMP that its connecting to. do this until the screw holes line up
7: once the screw holes line up, secure them tightly and reconnect the battery
8: check for limp mode, car will idle/run like **** because the fuel trims are reset.

if limp mode is still present, remove the negative battery lead again and lightly unscrew the black OMP sensor and rotate it a bit more clockwise and tighten the screws again.

you may have to do this a few times, and if you adjust the black sensor in every possible way, you need to remove it and adjust the OMP **** again

hope this helps others!