chronic converter failure.

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chronic converter failure.

Postby JeremyJ069 » Wed Oct 17, 2012 2:39 pm

hello out there. i took delivery of a 2011 mazda mx5 back in june of 2011. and in febuary of 2012 my car displayed a check engine light. i took the car to the dealer and [sadly for me] they replaced the manifold primary cat. all under warranty.

fast-forward to nowish. ive been enjoying race track driving. one event at streets of willow, one at buttonwillow and one at big willow. on the trip back to los angeles after buttonwillow my car started loosing power, making sounds of exhaust leaks and getting super hot. took it to the dealer, which replaces BOTH cats this time. under warranty.

then i went and raced big willow. right now i am massively paranoid of the converters being toasted again. pretty much the entire time i have had my car ive been getting piss poor gas mileage. averaging about 16.5-17. granted it takes me 1 hour to go 16miles, so i figured that was the root of it and being a sporty car i do enjoy the full rpm range. but now im thinking its because i have only ever known my car with clogged melted cats.

right after big willow i had all my fluids replaced, i asked my mechanic to check out the cats to see if they had in fact survived. he said it did and handed me a smog check forum... so i dont think there was any physical / visual inspection of it. what i do know is that the exhaust tone has changed since i got the car back from mazda after the dual cat replace. and this morning i swore i heard a chunky clunky sound from the secondary cat area.

so i guess my question to this forum is how often are people melting OEM cats when tracking the car? what can i look for to find the cause of this? one mechanic i talked to said he was tracking failures and most people that he talks to are running chevron gas. i was 100% chevron before and figured it would not hurt to switch to another brand just for the hell of it...

discuss.
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Re: chronic converter failure.

Postby Brian » Wed Oct 17, 2012 7:57 pm

This dovetails with post I made at Miata.net forum yesterday:

Originally Posted by Chuck H View Post
Brian, you can probably settle this question very easily for us. There is almost no difference in price between your Roadstersport Max Performance header ($419) and the Roadstersport Q muffler ($399). If you could only put one of them on an otherwise stock NC2, which one would give you more power gains across the entire power band?

Difficult choice because while there is no debate about the fact that the factory header is the biggest problem in the system.....you don't get all the gains of a header change if you have the factory muffler still stuck at the end of the system. So, I would do BOTH.

Okay, fine...let me imagine that you make me pick one or the other. If, for example, you had a race series with rules that allowed just one change to the system...then I might indeed do the header change first. But I am a guy with full shop with lifts, all the tools, etc (and part of my decision would be reliability, I would want to reduce concern that factory first converter would melt down on me in the middle of a race after lots of constant high rpm use).

As a practical matter, most folks start with the muffler because the muffler is 10 minutes to change....and the header install is 2 hours (longer if your first time doing it). So, most folks move from the back of the car forward with good reasons to do it that way.


Anyway, the point of putting that again here is that we do see this failure of the factory converters in constant high RPM use, which is why my answer to the question above was to change the first one for reliability. What some track guys do is pull their stock unit from the car and save it for when they need to smog the car. Since you are under warranty, you might as well keep driving the snot out of it and let Mazda pay for the converters....at least as long as they are willing to do so.

You can change to any brand of gas and it should not impact converter life. The Chevron Techron is good stuff, but that does not make the other brands bad for the converter.
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Re: chronic converter failure.

Postby mann » Thu Oct 18, 2012 7:01 am

This problem is pretty much universal with many factory headers; not just the mx-5. The stock construction is not very resilient for being so close to the exhaust. For example, in 2002 when nissan first implemented these header cats, they would suck up the chunks even on factory cars--and chew up the motors as a result! Luckily with the MX-5 they either melt or turn to powder and get ejected out the rear. For street use and what the EPA dictates it works, but put them on track or really romp on them with forced induction and their life expectancy shortens significantly. You don't see track cars running cats for this reason, and the race cats with the more durable substrate, are still wear items as Brian says above and their lifespan will depend on use. Poor tuning, NA or FI, will kill them as well and why we have always offered the datalogging/revision free of charge.

With that said, feds mandate that auto manufacturers cover emissions 8 years/80k regardless of warranty period, though I'm not sure if they would honor that knowing the car was used on the track or if they saw a supercharger bolted on 8)

If you go to auto-x events around here you'll find many subaru and evo drivers with masking tape on their plates for this reason!

Tuning for the cat itself is a whole other issue. It is a delicate balance because if too rich, the unburned fuel will decrease the overall life of the cat. Though cats survive at 14.7 AFR and high temps at cruise, do the same thing under high load and they melt if it is prolonged--this is a bigger priority. So it is a delicate balance in reducing risk of meltdown, while optimizing the life of the cat by keeping it from being too rich. For a street car this could be done without increasing emissions; but for track use, particularly with forced induction, it is a bigger challenge.

If you look at the factory mapping you can see how they adjusted it for maximum converter life in regard to fuel--very delayed open loop transition (keeping stoich as long as possible), lean AFR in the midrange (13.5) tapering to 12.2 or even 11.8 by redline to keep it cool for prolonged runs. Terrible for power everywhere except 5000-6000 RPM!
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Re: chronic converter failure.

Postby jboemler » Thu Oct 18, 2012 12:23 pm

Mike, would you expect the cat behind the long-tube headers to have better lifetime in hard use, due to being a bit further from the exhaust ports?
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Re: chronic converter failure.

Postby mann » Thu Oct 18, 2012 6:52 pm

jboemler wrote:Mike, would you expect the cat behind the long-tube headers to have better lifetime in hard use, due to being a bit further from the exhaust ports?


Indeed--the further from the head the cooler it will be. Even a few inches makes a difference. From what I can tell the substrate in these should be more resilient as well compared to the stuff in the stock ones.
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Re: chronic converter failure.

Postby jboemler » Fri Oct 19, 2012 1:30 am

Good to know, thanks!
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Re: chronic converter failure.

Postby JeremyJ069 » Mon Oct 29, 2012 8:07 am

wow... subscription obviously does not work!
and here i thought you all were ignoring my silly question.

thanks a lot for all the replies.
so yah, i think i am at my alloted warranty replacements for the year. might try again next year... but im looking into other options.

fallow up question, would changing out the injectors and cleaning up the throttle body [if it is dirty after 20k miles] help out in my mpg woes? granted i do have 8in wheels now instead of the 6.5in. and even with my roll bar im about 5 lbs over stock. so i dont think my mpg is due to those things.
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Re: chronic converter failure.

Postby Brian » Mon Oct 29, 2012 8:14 am

Playing with your injectors and Tb not expected to help the mileage.
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Re: chronic converter failure.

Postby JeremyJ069 » Mon Oct 29, 2012 9:39 am

...nards.
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Re: chronic converter failure.

Postby JeremyJ069 » Tue Nov 06, 2012 6:03 pm

hehehe, so a few days ago my car triggered a CEL. i took my car to a local dealer who said there was a fluctuation in the secondary O2 sensor. later that afternoon they got a call from mazda usa about the warranty replacement request. mazda wanted to send a technician to diagnose the car.

dropped the car off yesterday for 4 hours of the mazda tech running my car with computers hanging out of it. the mazda tech said he could not find any components that were not functioning properly. therefore there was no mechanical issues with the car and that the failing converters were due to driving style. sighting a computer log indicating a "prolonged period of high rpm."

so yah... cats are toast. and im not about to spend $1,900 on some OEM replacements.
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