Proud owner of a 2008 RX-8, 40th Anniversary Edition:
I can't shift into gear when the engine is running, but can when the engine is not running. I suspect a failed clutch slave cylinder, so of course I replaced the clutch master cylinder first because it's easier to replace. No joy. BTW - fluid level in the clutch fluid reservoir has never dropped, as it would with a leak.
Please direct me to a link describing the process for replacing the clutch slave cylinder.
Thank you!

 

Foxes.ca has all the manual.
before spend time on the slave cylinder, see if your clutch bracket has broken.

 

If you are replaced the clutch slave cylinder, you are doing the right thing by also replacing the master cylinder as well. Replacing them in a pair ensures that a strong component will not overpower a worn component and introduce another possible leak.

I have seen issues like this previously in an RX-7 and the issue ended up being a seized pilot bearing. The bearing had seized around the input shaft on the trans. It didn't matter that the clutch was depressed and working normally as the engine could never "disconnect" from the trans. It wasn't fun tonget the engine and trans separated. Luckily the pilot bearings needle bearings fell apart and relieved the pressure and I was able to separate the trans from the engine and replace the clutch as well as the pilot bearing and seal. Here's hoping you do not have to go down that path.

 

Thanks, mazdaverx7. Unfortunately, replacing the clutch master and slave cylinders has not solved my problem.
I can shift through gears no problem, with engine not running (with or without depressing clutch pedal). BUT I can't shift into gear with engine running, and if I start the engine with tranny in gear AND clutch pedal depressed, the car lurches ahead.
I have neither space nor expertise to drop the tranny anx deal with internal issues. I will take it to a mechanic.

 

It could be the actual clutch pedal itself. Master/slave cylinder problems are often misdiagnosed because the real problem could be a broken weld on the clutch pedal itself. The clutch weld becomes compromised and makes you think you're pushing the clutch pedal in when mechanically, you are not. Watch this video for more information:

https://youtu.be/7ABrHc6KSxU

If its not the clutch pedal, it's likely an issue with the shift fork and if it's not that, the pressure plate or clutch. In which case, you'll need to order from Exedy.

 

It honestly sounds to me as though the pilot bearing has failed. Based on my experience with this issue, I believe its time to remove the transmission and inspect the clutch pressure plate, disc, throwout bearing, and pilot bearing. I feel as though you or a mechanic you take it to fill find that the pilot bearing is seized.

This is a good time to replace the clutch disc, pressure plate, throwout bearing, pilot bearing, and seal. The clutch fork pivot ball must be inspected for wear and replaced if need be. At the minimum, it should be cleaned and lubricated. The contact points on the clutch release fork that connects to the pivot ball should have a small amount of grease on them. The clutch splines should be lightly greased to aid in reassembly. Sachs does make a clutch spline grease that works great. I also like to lightly grease the snout of the input shaft where it inserts into the pilot bearing.

Mazda makes a special service tool to remove the pilot bearing. I bought one 13 or so years ago and its a must have incredible tool. Best $120 I've ever spent on a tool.

 

Thank you for excellent guidance. Unfortunately, what you describe exceeds my space, tools and expertise. I'm having trouble finding a mechanic in Salt Lake City who works on rotary Mazdas. Nearest one is 30 miles away. They "have worked on RX-7/RX-8s" but don't claim expertise. I'm a bit nervous taking it there but have no other choices.
BTW - while I've been replacing the clutch master and slave cylinders, the battery went dead and when I jumped it, the engine fired once but didn't start, and now it won't fire at all. [Yes, I connected the jump-starter wires correctly.] I have spark from the coils to spark plugs, but no ignition, though fuel is being delivered. I replaced badly fouled spark plugs along with new spark plug wires, because I already had them, and the engine still won't fire. I suck at auto electronics, so now I have two issues for a mechanic to fix.
it's been a great, fun, trouble-free car til now...

 

Clutch pedal, bracket, mechanism are not broken.

 

Well for one it sounds like it's flooded. You can deflood it, google for "deflood procedure". Basically you stand on the gas, crank the car for 5-8 seconds. Stop for 30s, repeat 3 times. Then try to start normally, or try to start with your foot still on the gas, but slowly releasing while you crank it over.

For two, flooding and severely flooded spark plugs point to an ignition coil problem, which are the #1 killer of RX8 engines. If in doubt, replace the coils.

For three, it does sounds the trans will need to come off. Your clutch is not disengaging, and if you've eliminated hydraulic failure and bled the new hydraulics properly, then it's time to have a look inside. Any weird noises while it was running? The good-ish news is there is not much special there. Any reputable non-rotary shops should be able to troubleshoot that, you don't need a rotary expert. But given that you flooded it, it's possible they will as well so caution them not to shut it off cold, or replace the ignition coils before you bring it in.