can't figure this out, no CEL or anything
 

- 2004 Mazda RX-8 Auto 92,000 miles w/upgraded starter

Last June I had some issues with rough idle, the car dying on me at stoplights, and long starts. So I replaced all the spark plugs, bought new battery, and did the sea foam DIY. Still rough idle, car was still dying so I took it to the dealership. $327 later they replaced two of my coils. Car works, however, still a rough idle, dealer said it might need new engine mounts.

Just last month in December, I had my car shut off again at a stoplight and had to take several starts just to get it going. Paid Mazda $90 to run a diagnostic to figure out what the hell is going on, showed them the service bulletin about lack of power, too. Diagnostic came back saying the cat "was out of emissions," need $1600 for a new one. Told them no thanks at the time, I'll look into buying one and putting it on myself. Proceeded to take two weeks of vacation for xmas out of state and didn't drive my car until this week.

Noticed my car was rumbling around 2-3k RPMs and progressively got worse to the point the CEL flashed a couple times and then went away. Felt like my car was going to die like it did in June. Friend drove me to Mazda dealership and I bought two new coils to replace the other two that didn't get replaced in June.

Car worked fine. Still has somewhat of a long start. No issues around the 2-3k RPM. Went to the store, got a haircut, etc. However, on the way back home, I can hear a faint rumbling sound like before around the same RPM range. But once I accelerate past it, it performs fine.

Where do I go from here? Running out of ideas. Replacing the cat is one of my top priorities, with replacing the wires and re-checking the spark plugs a close second that I will probably do soon. Car still idles rough and has long starts.

Appreciate any help as always.

um.. the cat would be my first guess

Cat should be still under warranty...100k miles/8year.
If you are over 100k, just get a straight pipe...

Then go from there...

wrong

Cat is 80K miles 8 year. whichever comes first.

Engine Core warranty on the other hand, is 100K/8 years. whichever comes first.

You can get a Brand new stock cat for around 1/2 the price (more like 900 bux?) but honestly you should just get a high flow cat (if thats what you want) for like 300 bux.

but seriously your problem sound like the engine is on its way to rotary heaven. Don't ask me how I know. The dealership just want you to "pay for this and that" and after they get enough $$$$ out of you they will then tell you oh the Engine is bad ok let's replace it.

My engine did the same thing before it went to heaven but thank god I was covered under Mazda's 8 yr warranty and I had super low K's so I got it covered. I would listen to NYC and get a High Flow cat for $300 and see if that fixes it.

Okay, dumb question here, what is a High Flow cat and how is it different? Why is it so much cheaper? And would it still allow the car to pass required emissions tests?

all CATS are made by some sort of Ceramics or precious metal like palladium and/or platinum.

when you look at the CAT directly, you will see a lot of "tiny" holes. those are called "cells", as gas passes thru those "cells", it will have a chemical reaction to "turn" those bad stuff into some of the more harmless stuff. for example bad stuff like NOx and CM into Nitrogen, CO2 and Water.

High flow cats are cheaper because ---

1. OE CATs usually focus on pollution controls more than flow & cost. so OE/stock cats usually have more "cells" or more "holes" to make sure stuff came out of the tailpipe are as clean as possible.

Stock usually have 600/800/1200+ or more cells/holes in them.

2. More cells = more material = cost more.

So in the other words, high flow cat flows better/more by having "less but bigger" cells/holes.

Also material is another thing to consider, not all aftermarket cats are cheaper, ceramic cats are much cheaper than metal cats. but they don't last as long. some good metal cats for this car will cost you more than 1500+ bux. but they flow a lot better + it will last forever.

High flow cat will let you pass emission test, at least when its still new.

Stock CAT is not that expensive IF you know where to get them. you can get a new one for around 900-1000 bux. Our stock cat is a 3 way CAT, meaning it has "3" layers inside, it left some space in between the catalyst material to create some turbulence to try to improve the efficiency. a lot of stock cats are based on this design.

My FC came with no CAT, it had Full RB Racing Exhaust(header, straight pipe & cat back) which I sold it for pretty good money. then I bought an aftermarket stock replacement for only 80 bux with 5 yr warranty. I also bought a stock CAT with I think 80K miles for 20 bux. comparing the stock CAT vs the aftermarket el-cheapo wooo The stock cat is HUGE. the used stock CAT still looks good but Im not gonna use that until my 5 yr warranty on this aftermarket CAT is over. They should have cheap CATs for RX-8 real soon. RX-8 is already like what 7 yrs old. when more and more rx8 emission warranty expires, there will be more aftermarket CATs.

Wow, great info. Thanks for taking the time to post that.

welcome to the new engine club