Miata flunked emissions test...what now?

Hey all, I'm having trouble figuring out what to do about my '91 Miata. I picked up this car over the summer and it was pretty poorly maintained. The original owner alerted me that it it was overheating and not shifting into gear--sure enough, I fixed the fan switch and the clutch hydraulics and it worked...for about 20 miles before the timing belt broke, on my way to get it emissions tested! I fixed that, front cam and crank seals, put in new plugs (champions, made sure to gap them within spec) and new plug wires.

Then, finally, I took it in for an emissions test and made it there. Here were the results:

"25/25 test" HC ppm: 152 / 160 allowed (squeaked by) CO %: 0.75 / 0.90 NOx ppm: 1927 / 1179 (fail!) RPM: 1580 max (3000 allowed) <--irrelevant? CO+CO2 %: 14.6 / 6.0 min

"50/15 test" HC ppm: 147 / 160 CO %: 0.66 / 0.93 NOx ppm: 2118 / 1304 (fail!) RPM: 1592 (3000 allowed) <--irrelevant? CO+CO2 %: 14.7 / 6.0 min

The guy at the station said "don't take my word for it, but I think it's the cat". Now, I have another used cat that I might have a shop weld in for me, but I'm not real keen on buying one...call me cheap, but I'm trying to keep this car as low-budget as possible. If that's what it takes then that's what I'll do, but I was wondering if anyone has some other ideas or things to check for? I've got to change something by the end of the week to get retested for free!

Thanks!

The high HC and NOx can be poor running/misfire.

If it seems to run well I would run through a tankful of Chevron Supreme, new plugs (colder range if available), and retard timing to minimum specs. Seafoam the intake for extra insurance. Could be enough to get it to pass.

Make sure there is absolutely no exhaust leaks before the catalyst

Ditch the Champion plugs. It's a Miata, not a lawnmower. Go with NGK or Nippondenso.

Move to Michigan, no testing

Where are you located?

Pretty much how my original 323 GTX (same 1.6L engine as the Miata) failed last year in CA. I did most of what Cone Junky suggested first - new plugs, Chevron supreme, & Seafoam. Some improvement, but still failed...new cat and it passed.

The 2010 Challenge GTX has a finger-sized crack in the exhaust manifold, but still passed CA emissions last year. Go figure.

Im guessing if you bought champions you bought parts store plug wires. Miatae seem to like good wires. Dealer wires are NGK blues and do the job well.

The wires also tend to loosen up a bit. Make sure that they're well connected at both ends.

Adrian_Thompson wrote: Move to Michigan, no testing

No sunshine either.

I THINK - and I'm not an emissions specialist - that high NOx is usually a cat.

I'm located in Atlanta. A few miles over and I wouldn't have to worry about it!

I actually do have nice plug wires, got a set of NGKs...I had the champion plugs only because I got them for free, and I was convinced they were better than the old NGK double-electrode plugs that were on there. I don't know how many miles were on them, but they'd seen a few.

So it doesn't sound like it has a misfire (I've dealt with a few of them)....maaaaybe it has once or twice when it's warming up now that it's a lot colder outside, but I don't think it did during the test, at least. I've also run a bottle of Techron through the fuel system since failing the test, I was thinking perhaps it's running lean because of dirty injectors...my understanding is, high HC = rich, high NOx = lean...but both? I don't get it.

What tests can I do? I don't mind changing spark plugs again (as cheap as I am), perhaps I'll try that just in case.

In reply to jsymonds:

I should have said, high NOx meant running lean only if the cat was functioning properly. I didn't think the cat had anything to do with HCs, though.

Also, I'll probably bring it by my local meineke for a free estimate on the exhaust--hopefully they'll tell me if I've got some pre-cat holes in the exhaust. I'd do it myself, but it's cold and I'm lazy, and I don't have a lift available :P

no real help here.. but move to NC only '96 and newer get tested for smog.. and even then not in all counties

Back when I used to have to do emissions testing, I'd go in with about 1/2 tank of fuel. I'd add in a few gallons of alcohol or methanol. As long as the engine is in good condition, it would usually pass with flying colors. Leave the station and fill up the tank with normal gasoline. It sounds like your car probably has a weak spark. A new, hot coil and some plug wires makes a big difference. I also agree with dumping the Champion plugs and going with NGK.

If you were in Maryland you'd pop by the MVA and get a nice historic vehicle tag with no inspection or emissions, ever. This was the main reason to get a '90 NA last year. Historic Miatas. Ha.

But Georgia is 25 years so you're boned.

Yeah, going to have to wait a few more years for those historic plates...but the Merkur's getting to be the right age!

How likely is it that the coil is my problem? In case I've been vague: I REALLY want to keep this car cheap!

Regarding alcohol, I've read that basically you're adding oxygenated fuel so you might burn some unburnt hydrocarbons, but it will burn hotter and probably make some more NOx, which is my primary failure right now. Does that sound right? I wish I could get this tested half a dozen times for free, and collect some real data!

Regardless I think I am probably looking at getting a new (used) cat thrown in there, even if the spark is weak I think that would lead to less NOx and more HC, not the other way around. Perhaps I've got a pre-cat exhaust leak, which can make that go through the roof...

Kinda makes me wish I had more than three days to figure it out!

One of our Miata club members just failed on idle HCs in MD. New NGK plugs (stock, nothing fancy), wires, reset timing to 10 (it had been 15!), and a nice, LONG drive with lots of zoom zoom got him by, no problem.

HCs went from 242ppm to 87ppm.

A lot of people have said to put in good plugs, you seem to be looking for a more expensive fix. Not sure how many more tries you get to pass, or what it costs to try, but the cost of new NGK plugs, setting idle/timing, and a long drive is, what, $8?

Go to the parts store before you do anything else, and for less than $10( Advance Auto Parts), buy a can of "Guaranteed to Pass" by CRC. Dump the can into the gas, drive a little bit to mix it up good, then get it retested. If it fails, supposedly CRC will double your money back! I've used the stuff the last two years to get my Forester through inspection.

meow.

If you want to try, I have seen numbers get better with spirited driving- but not to the degree you are looking for. In all honestly- ALL of the consituents are high, and if nothing else is wrong in the combustion- plugs, injectors, misfires, etc- all point toward the O2 sensor (air fuel control feedback) and the catalyst. So if everything is new, meow.

Eric

Put a small container of methyl hydrate in the gas tank - drive around for a day - pass emissions.

Its the cat. Its your time and money but I suggest not wasting your time and money with used cats or other BS. Just buy a new cat from Flyin Miata or some other vendor and go happily on your way.

Same thing happened to me earlier this year. The FM cat is quite nice and higher flowing.

minimac wrote: Go to the parts store before you do anything else, and for less than $10( Advance Auto Parts), buy a can of "Guaranteed to Pass" by CRC. Dump the can into the gas, drive a little bit to mix it up good, then get it retested. If it fails, supposedly CRC will double your money back! I've used the stuff the last two years to get my Forester through inspection.

This worked for my 97 SER more than one. +1 on retarding the timing.

amg_rx7 wrote: Its the cat. Its your time and money but I suggest not wasting your time and money with used cats or other BS. Just buy a new cat from Flyin Miata or some other vendor and go happily on your way. Same thing happened to me earlier this year. The FM cat is quite nice and higher flowing.

Worked like a charm for me. Of course, I was certain that my cat had failed...

^^ Magic catalyst rock that was rattling around in my muffler...

^^ Stock cat vs. Flyin' Miata cat.