1990 Catalytic converter failed

Mac

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Dec 2, 2011, 6:51:40 PM12/2/11

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Have a 1990 Miata. Failed emissions test. Too much NOx ppm. Failed
before and I replaced the cat. Is there something that would make the
cat fail besides it being a cheap one. Not sure how long I am going
to keep the car so don't want to spend a ton of cash on a new one.
Any ideas on what causes them to fail early and a good cheap
replacement one. Thanks.

Arye

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Dec 4, 2011, 3:29:02 PM12/4/11

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The one and only time that my 1990 Miata failed emissions (at about
120,000 miles) it turned out to be the dreaded timing pulley, which had
been installed incorrectly after a previous timing-belt replacement. YMMV.

Christopher Muto

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Dec 4, 2011, 5:36:07 PM12/4/11

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On 12/2/2011 6:51 PM, Mac wrote:

> Not sure how long I am going
> to keep the car so don't want to spend a ton of cash on a new one.
> Any ideas on what causes them to fail early and a good cheap
> replacement one. Thanks.

why it fails after 21 years? probably somebody can answer that better
then me, but besides age (mine failed because the heat shield started to
rattle at 10 years and I could not stand listening to it anymore) but a
car that is burning oil can clog up a cat pretty quickly and make it
useless at doing its job. the point being that before replacing the cat
you may want to make sure the engine is running smoothly and not
throwing up error messages from various sensors...

i replaced the cat in my '95 in april last year. i found lots of really
cheap cats on the market but they were the in-line type that required
cutting the flange off the header and the cat-back sections of the
exhaust. i didn't want to do that. i found a direct fit replacement
made by magnaflow, model 23696, that is just like the oem mazda cat
(complete with flanges) but it lacks the bolts that come standard with
the oem cat. I paid $85 delivered from performancepeddlar.com (after
submitting a low ball offer they came back with this low price). this
was at least $200 less than the discounted price of the oem part from
finishlineperformance.com and so worth the effort for me to find my own
bolts to install it. but a couple of notes: i am not certain that the
magnaflow 23696 is also for your '90, so check it out. you will need
new gaskets and bolts to install it. i do not think it meets california
requirements but it has passed new york state inspection multiple times.
and i don't know why but i could not get it installed completely
silent (i periodically get a rattle from it - wonder if it is
internal?), but it isn't horrible, and god knows that a miata with 140k
is a symphony of noises anyway. ;)

let us know how it works out for you.

Christopher Muto

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Dec 4, 2011, 6:26:52 PM12/4/11

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the '90 cat is a different animal than the '95... so much for the
suggestion for the magnaflow. finislineperfromance.com sells one made
by bosal for your '90 for $149.95. they used to offer free shipping if
you spend over $150 so add an oil filter or something to your order.

XS11E

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Dec 5, 2011, 12:30:41 PM12/5/11

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Mac <modm...@yahoo.com> wrote:

> Have a 1990 Miata. Failed emissions test. Too much NOx ppm.
> Failed before and I replaced the cat.

It should meet emissions with the cat removed.

> Is there something that would make the cat fail besides it being a
> cheap one.

Yes, Cats fail from over use, if they have a continual stream of
unburned fuel they'll overheat and fail. Your NOx PPM might be due to
mis-firing plugs or similar, that causes the cat to have to burn up the
unburned fuel and eventually fail. Have the rest of the car checked
before replacing the cat.

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Mac

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Dec 16, 2011, 5:06:29 PM12/16/11

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Replaced the cat and car passes. Before nox ppm, 2433, allowed 1512,
new cat 178. not sure what is caused the cat to fail but $100 every
year or 2 is not bad. I drive this car everyday and will continue to
until it cost more than a car payment to keep it running. Thanks for
all the advice.

charlie

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Dec 23, 2011, 1:00:24 AM12/23/11

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A 1990 is well up in years and usually mileage. Oil consumption may be a
factor.

Besides and beyond, plugs, wires, points, distributor cap, and rotor
should be considered likely suspects. (assuming a 90 has such things.)

At least it's not a 99 with suspect intermittent coil packs.

Another question has to do with mixture. If you are still getting good
mileage, it's unlikely that the mixture is a problem.

Mac

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Jan 2, 2012, 5:24:40 PM1/2/12

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Thanks again for all the help. It does have high mileage but still
gets around 30mpg. Going to drive it until is won't run anymore.

JC

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Sep 25, 2014, 6:17:59 PM9/25/14

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replying to Mac , JC wrote:

2 other problems I didn't read here is that your timing may likely be
advanced. Get your timing checked & corrected with a timing light. A second
problem may be bad valve oil seals that allow too much oil to be turned,
causing the catalytic converter to get ruined. These 2 things were the problem
for my 1990 Miata. By the way, if you live in California, you need to ensure
that it says California legal and that it's for 50 states. 49 States means
that it's not legal in California.

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