8 Fast 8 Furious: Detroit Drift

P0420 code now.
Anyone ever take the temperature of their catalytic converter on a stock NB Miata?

So after my 30 minute ride home at 70mph i took the temperature and only got a reading of 350F. After doing some reading, i find that this temperature could mean my cat is no longer operational. It COULD explain my weird stuttering problems at low RPM when the engine is warmed up.

I don't want to spend a ton of money to fix this issue, but i may have to.
I'm thinking a test pipe, but i'm not sure if would solve my issue. I don't have emissions or any of that BS to follow, i just kinda want the missfire and now code to go away.

Flaky coil leading to failed (melted) cat and extreme backpressure? This is tragically common on the NB1 with the shitty coils and precat, but its a mode of failure to explore.

Unbolt the 2 bolt flange before the cat and peer in with a flashlight. Damage will be immediately visible if the above is the case.

I'd measure my NB1 midpipe cat (without the precat) for you except that I don't have an infrared thermometer to do it with. :sad2:

Oh, so the precat (the one before the O2 sensor) is also a catalytic converter? I don't know why i thought that was just a resonator. I'll have to take a temperature of both, before and after the O2.
Not that i'm doubting they're both bad.

I felt like it was worse yesterday when it was -8F outside too.

Man, i don't want to buy $200 in coils and $300 in exhaust for a daily beater.

You said 2001 so NB2 and no precat. The main (and only) cat is built into the midpipe on the NB2.

NB2 flow diagram looks like: head -> manifold -> front o2 sensor -> cat -> rear o2 sensor -> resonator -> muffler.

Okay, so the bulge behind the rear O2 is a resonator...
Seriously, it looks more like a cat than the bulge before the rear O2. So the temp i took was actually on the cat. I guess it wouldn't make any sense to put a catalytic converter behind an O2 sensor. haha
Thanks for clearing that up.

Gonna go rip it apart right now and look for apparent damage.

Here is a pic

https://www.miataturbo.net/attachmen...ine=1424209643

Its taken from the rear, the resonator is closest to the camera, cat is the thing further upstream with swiss cheese heatshield on it. Not shown is the rear o2 sensor as its around the top of the pipe, but it would be after the cat, so between the cat and the resonator in the above pic.

Glad I could help! :bigtu:

Welp, no apparent damage.
I did find my slave cylinder is leaking though. Awesome.
I want to complain, but the amount of hell this Miata goes through on a daily basis says i really have no right to.

I guess either suck it up, or replace the midpipe back with something. I really don't want to mod this car though.

So... This morning on my commute to work the CEL is no longer on.
I didn't do anything except unbolt the exhaust then bolt it back up.
Still missfires pretty badly below 2000rpm.

Just because i didn't see damage on the Cat might not mean something is wrong with it right?

Correct. When it does not look like a screen door anymore then you know you have problems. Visual check is just the easy way to do it, infared thermometer is probably the "right way" but again, I don't have the baseline operating temp for you :(

How do your plugs look? I'm running out of ideas :dunno:

Plugs are bran new within the last 3k miles. The ones they replaced only had like 10k on them.

I don't get it. Oh well.

Do you have another set of ignition coils you could try with it? I'm trying to think of what else might be that affected by warming up.

Nope. I may purchase another set just to try it out. RockAuto has an "ultrapower" for $30. Seems like it could be worth a shot to try.

I've got a set of stock ones I can mail you to try out. Provided I get them back of course ;)

Nothing solid to contribute, but it could be many different things. Bad coil/wire, vaccum leak, etc.

Check to see how much vaccum you have at idle. A semi-large leak could cause it to misfire right when load is applied from being lean, but be massaged out due to the stock computer being rich everywhere else.

Do you have the factory repair manual? If not, I can look into it for you if I get a chance. I would probably start by checking some sensors according to the manual. Just because a sensor is messed up, that doesn't mean it will throw a CEL, especially if it's borderline.

I could have a vacuum leak. That should be pretty easy to verify.

I'll just buy some, thanks though.

Also, when I get home from work, I'll run inside and grab my super awesome Amprobe IR-730 thermometer.

I'm going to guess your converter should be much hotter than 350. Should be closer to 1000F. If your IR thermometer has a low spot:distance ratio, you were probably getting an average temperature with other junk under there. Also, the heat shielding will be alot cooler than the converter itself.

Edit: On a side note, I would take E02K up on his coil offer if I were you. A set of used stock coils will most likely be much better than some generic chinese replacement coils. I have literally seen parts store coils, TPS sensor, MAF sensors not work right out of the box. Shoot, I tried 3 different parts store alternators when my stock one went out, all failed in less than a month. I bought a used stock alt, been good for over a year now. Just my :2cents:

Cat no longer has any heat shielding on it.

I have a Fluke VT04 at work i can try it with. My multimeter even has temperature probes i can use to check it.
I'll get a couple different readings from different instruments. All i have at the crib is a cheapo $40 unit.

When I had wrong sized injectors (too small) in my Civic it also gave a particular bad experience with low-RPM and high load. So your problem could also be fuel related and not spark related.

Vacuum leak sounds plausible, this would cause a lean situation in low-RPM situations.
It also would not hurt to check fuel pressure I think.

Do you have a wideband O2 sensor laying around? Sticking it into tailpipe to see if you are running rich/lean in the 'stutter situation' would also be a great help.

If you do not want to fork out a lot of cash to debug the issue, maybe try to find someone with the same car in the area. You could swap out coils / wires / sensors and see if things move along.

Well, I don't think you should be too worried about your cat temps being low. I just checked mine, but mine has heat shielding and I forgot for about 10 minutes, and it was right at 310F. I'm not sure, but I'm guessing it cools down quickly after shutoff.