ok First off i have a 04 rx8 auto 120k on it.
Never had any problems till last oil change.
600 miles after change engine started overheating.
Car sputters at 4k rpm and wont go over 25 mph and cant go up hill faster than 10mph.
Oil and Coolant levels are good.

Check engine light has been forever and have had it cleared 2 times and looked at both times. dealership came up with nothing.

No idea what to do. Im going to look at the thermostat tomorrow and if bad replace it. Any idea from the experts?

 

also car randomly shuts off after starting it and shuts off at stop signs when i come to a stop.

 

cat?

 

^ +1

 

So I assume you meant that the check engine light has been on forever?
If so, go have it read and report back.
Also, do you even have a cat on the car?

 

I have had it checked and cleared twice nothing comes back. and it will not go away

 

Wait wait
So you are saying that there is no CEL Code Stored in the ECU.
But the check engine light is still on?

Weird ... that's a good one

 

Next time the cel is on go to autozone/checkers and get the cel read for free there, do not go to the dealership. It sounds like they are just clearing the cel and not telling you what it said..

 

^+1
I bet it's the cat....but that's so strange. What reason would a dealership have for withholding information on an 8 with that many miles? They wouldn't be liable for anything unless....was your engine replaced recently?

 

Suggest you stop driving until you figure this out. Long and short of it, sounds like your catalytic converter is clogged, which is usually caused by weak ignition coils (+plugs and wires), which would also throw the check engine light. Driving in this condition leads to further engine damage, although considering you're also overheating, it may be too late.

Depending on how handy you are, you can just unbolt the cat from the exhaust and see what it looks like. It should have a clean honeycomb inside.. if the honeycomb is clogged or broken, you'll need a new cat, but don't invest into that unless you replace the ignition coils/plugs/wires at the same time, otherwise you'll just kill the new cat.

But before you do any of that, get a compression test to verify that the engine is even worth saving.