Help Diagnosing Throttle/Starting/Electrical Issue
 

Hi everyone, here's today's puzzler:

I've got a lovely 2008 RX-8 40th Anniversary Edition with around 24,000 on the clock (bought it with 18,000) and today it ran into its first serious issue. After a short warm-up, as I was accelerating hard to highway speed (past 8,000 RPM), the engine lost power and revs, threw a CEL and began oscillating near 3,000 RPM, almost as if it was hitting a rev limiter. When I depressed the gas pedal, I got no response, so I was forced to stop on the shoulder where the behavior continued:
I shut off the engine, and the car cranked but wouldn't start. It started on the second attempt with the engine idling high, and I tried driving it but only made it a few yards before it began to lose power. I stopped again and plugged in my OBD bluetooth scanner to pull the CEL codes. I got "P0638 - Powertrain: Throttle Actuator Control Range/Performance (Bank 1)," and "P0116 - Powertrain: Engine Temperature Circuit Range/Performance Problem."

I hoped it was an electrical bug of some sort, and not wanting to be stuck on the side of the highway, I popped the hood and pulled the negative terminal for the battery and reattached it. Car started up fine and I drove the 3 miles home without further incident. Notably, the CEL was gone.

History:
Previously, only once I think, I noticed the car bog down and lose revs, but it only happened once (no CEL) and hasn't repeated itself for months until now. I also had a problem right after I installed my SOHN adapter where turning the key would instantly kill ALL electronics (as if the battery had been disconnected) but opening/shutting the car door a couple of times brought everything back and the car started just fine... I assumed it was a loose battery ground from my recent work so I tightened the negative terminal and the problem did not repeat. Could it be related? The car starts 95/100 times on the first time without issue, hot or cold, and I haven't really had any trouble with it until now.

Major modifications on the car:
I installed a SOHN adapter about 400 miles ago.
AEM cold air intake (replaced previous owner's K&N)
TurboXS catback exhaust

Recent Service:
Oil changed 400 miles ago
Coolant drained/flushed 400 miles ago
Replaced coolant overflow tank for bad sensor 400 miles ago
New plugs, coils & wires 4,000 miles ago
MAF sensor cleaned 5,000 miles ago


So, any ideas where I should start? This is my daily driver and thus I need it to be drivable. Thanks in advance :)

I would.pull the throttle body plug and clean it with contact cleaner and clean up the grounds and maybe the coolant temp sensor connection as well. Often electrical gremlins just need a reboot and they never recur so not likely a huge worry

Ok, I'll give it a shot. The exhaust ground at the rear has rusted away and no longer exists, so that probably isn't helping if the problem is indeed electrical in nature. I'm hoping I can solve this myself and don't have to take it to the stealership...

The exhaust ground isn't very important at all...

Good luck 😊

Happy update:

I had the same issue again yesterday, but pulled a different CEL code as well - one for the pedal minimum range. I limped into a CVS parking lot, did a little searching on the forums and guessed it might be a stuck/dirty throttle body. I pulled my AEM intake off and lo and behold, one of the mesh screens from the AEM intake had come loose and got sucked into the butterfly valve. Luckily it didn't make it any further... I stuck the screen back in place, added some krazy glue for good measure (didn't have any RTV on hand) and obsessively tightened everything back up. Total cost for the repair: $1.99

No problems since :)

Crank Sensor
 

Try the crank sensor I had same problem I changed everything trying to diagnose it Spark plugs, wires, coils, crank sensor, starter, well it finally worked like it should then the radiator went replaced it but I had it on ramps for over two weeks now I'm back where I started not starting does anyone know if there a master reset switch like Ford uses I think there is since Mazda is backed/owned by Ford

You would be better served by starting your own thread.....

The ECU can be reset by disconnecting the battery...and the ESS sensor by doing the brake pedal reset procedure

You sure it isn't flooded?