Engine light and rough idle?
 

I have a 97 Miata with 140k on it and the check engine light is on and the idle like to fluctuate between 1 and 2 grand and never seems to stay in the same spot. So I ran an ODBII scan on it and it said "exhaust gas circulation malfunction" I was thinking it could be my egr valve? Then I noticed that it said the cat, O2 sensors, and egr valve were all unable to be scanned completely. Does anyone know what could be wrong with my car? One more thing that might help is that when i bought the car they guy told me that he has the timing set for premium fuel only, he said that he advanced or retarded the timing for premium, I don't remember exactly what he said, the car has a jackson racing intake and a hi flow cat. Other than that it is stock, sso I do not know why he changed the time for premium but could that have something to do with my check engine light and also how would i set the timing back so I could use regular fuel? All help is appreciated, thanks for reading.

Timing is set by turning the CAS on the back of the head on the exhaust side. It is driven by the exhaust cam. You will need a timing light to accomplish this. Some people advance it because there is a slight performance bump in a certain RPM range. Results may vary.

The post-cat O2 sensor exists just to check that the cat is doing it's job. As far as the EGR, try disconnecting the appropriate electrical connectors and seeing if the idle conditions change. I don't know the wiring exactly, but maybe you have some electrical connectors already disconnected and that is the root of your problem.

You can find most of the information you need by searching here or on miata.net's how-to guides. The Haynes repair manual would be a good thing to have as well.

Rough idle makes me think your EGR valve is open when it shouldn't be.

You're going to have to be more specific/post a video.

Thanks for all of the information, I will try to catch it on video. It will happen randomly at a drive through or a red light, just go from idling at 1 grand to around 1800 rpm. I will check all the wiring around the egr valve after work today and post what I find. Could the post-o2 sensor be giving me an error code because of the hi flow cat?

Yes.

How would I fix that? Get a new o2 sensor?

The way to fix it is to not run a high flow cat. I dont know how sensitive the Miata ecu is to the post cat o2 signal, it may not set a code.