1990 Miata fails smog on N2O - but good on HC and CO

Looking for a bit of advice to help keep my beloved Miata on the road. It is a

1990 1.6L with 141,000 and running well (apart from the N2O).

I recently failed the new CA dynometer smog test at 25 mpg with 802 ppm on N2O (limit 786), HC measures low at 38 (limit 108) and Co is 0.12 (limit 0.71). After failing I replaced the plug and wires which needed doing anyway and ran G2P fual treatment thru the tank.

On the retest the car passed the pre-test inspection with N2O at 782 ppm, but when we started the real test it started to fail again. I'm guessing N20 starts to rise as the engine or cat gets hot.

Your advise is most welcome.

Reply to
Graham
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Most likely a bad cat converter. CO2 is good so you have good combustion and mixture.

is a 1990 1.6L with 141,000 and running well (apart from the N2O).

N2O (limit 786), HC measures low at 38 (limit 108) and Co is 0.12 (limit

0.71). After failing I replaced the plug and wires which needed doing anyway and ran G2P fual treatment thru the tank.

but when we started the real test it started to fail again. I'm guessing N20 starts to rise as the engine or cat gets hot.

Reply to
Larry Gadbois

Actually, a hot cat is usually more effective.

I had a similar problem, mainly the problem was I had my timing advanced to

14 degrees. Advanced timing gives you more heat and that is a problem with the NOX. Is your timing advanced? You can retard it a bit to help even more (temporary). Also they say your throttle body needs to be as clean as possible (this didn't help on mine though)

Good luck, most of this emissions stuff is being used as a racket to vacuum your wallet.

Chris

92BB&T
Reply to
Chris D'Agnolo

Thanks Larry

Do you know if there is any to check the cat, other than high NO reading?

Reply to
Graham

Thanks Chris

I think the timing is the standard 10` BTDC, but it is a great idea to check, maybe a previous owner changed it. If it is standard can it be retarded to 8`

Graham

Reply to
Graham

Yes, you can retard to 8 btdc (I wouldn't leave it there) but, yes it should help.

Actually, Midas will inspect your cat for free (at least, the local one here did)

Good luck, Chris

92BB&T
Reply to
Chris D'Agnolo

hmmmmm.............. On second thought, I am not sure about the catalytic converter. If the catalytic converter is working properly, the oxygen in the exhaust will combine with the carbon and form carbon dioxide. This reduces the HC and CO emissions and also reduces the available oxygen. I don't know if a failing cat will start to give higher readings on NOx without the other readings be affected.

I think Chris got it right. The engine combustions temperatures are too high.Your EGR valve could be stuck closed. The EGR is used to reduce combustion temperatures. Combustion temperatures exceeding 2500 degrees will creat a large increase in the NOx readings.

Your fuel mixture may be running too lean. Maybe your PCV valve is stuck open or your fuel pump pressure is low. Another possibility is a cooling system problem.

You could try the trick of setting your timing to 8 degrees. Also, using a gasoline that contains 10% ethanol will cool the combustion down some. Auto stores sell a gasoline additive that will do the same thing.

Good luck!

Reply to
Larry Gadbois

High NOx is often the EGR valve. My '94 failed high on Nox when I first got it. Try opening the EGR valve while the caris idling (suck on the vacuum line or use a vacuum pump). If the passages are clear and the diapragm is good, the valve should open and the engine should start running like crap or die. If not, either the valve is bad or the passages are plugged. In my case the intake manifold passages were completely plugged and I had to remove the intake and clean them out mechanically, then I took it to machine shop that had an aluminum-friendly cleaning vat and had them finish the job. Made a huge difference.

John McClary ('94 Miata) jsgmcclary at cox dot net

is a 1990 1.6L with 141,000 and running well (apart from the N2O).

N2O (limit 786), HC measures low at 38 (limit 108) and Co is 0.12 (limit

0.71). After failing I replaced the plug and wires which needed doing anyway and ran G2P fual treatment thru the tank.

but when we started the real test it started to fail again. I'm guessing N20 starts to rise as the engine or cat gets hot.

Reply to
John McClary

Thank you John

Do you know if the EGR value has another name? I have the Madza workshop manual and there is no mention of EGR. Could this be also called a PCV (postive crankcase ventilation).

Graham

Reply to
Graham

Exhaust Gas Recirculation Valve? Definitely not the PCV valve. On my '94 it's mounted at the back of the intake manifold. It has a flying saucer shaped housing about 3" in diameter and has a couple of vacuum ports and an electrical connection on the top. I can't imagine that a '90 wouldn't have one. There should be a small pipe sprouting out of the back or the exhaust manifold, wrapping around the engine and attached to the lower housing of the EGR valve.

John McClary ('94 Miata) jsgmcclary at cox dot net

manual and there is no mention of EGR. Could this be also called a PCV (postive crankcase ventilation).

Reply to
John McClary

On my '94

flying saucer

ports and an

wouldn't have

the exhaust

housing of

Well, I've looked through both the Enthusiasts' Manual and the Chilton's and can't find a thing about an EGR valve in a '90 Miata. There's a whole section on the EGR system in the Chilton's but no instructions/drawings/photos about the EGR valve for a '90 Miata anywhere!

According to the Chilton's, the EGR system "utilizes a control valve, a modulator valve, the engine control unit, a solenoid valve, throttle sensor and a water thermosensor." There's instructions for the 626, the MX6, and the MX3 but no Miata instructions.

Let me go look at my owner's manual and other books and I'll report further. ;)

Iva & Belle.) '90B Classic Red.) #3 winkin' Miata

Reply to
Iva

Thanks John. I found out the EGR was introduced with the 1.8L engine in 94.

Reply to
Graham

Thanks Iva. I just called my local madza dealer and confirmed the 1.6L 90-93 miatas didn't have an EGR value. It can't not be fitted as an upgrade either.

Reply to
Graham

the 1.6L 90-93 miatas didn't have an EGR value. It can't not be fitted as an upgrade either.

Well, I guess *that* explains why no one could find it in the repair manuals!

We're looking at new standards for environmental/anti-pollution inspection this year here in PA ourselves. The newer models get hooked up so the codes can be read from the OBD. But the older ones will be getting a "visual inspection" and a "gas cap inspection". And they'll probably charge the same (exorbitant) price for both procedures!

And, of course, both the Miata and our OTM are 1990 models. Guess the husband's car will be the test case for us since his inspection is up in March and mine is good until April.

Iva & Belle.) '90B Classic Red.) #3 winkin' Miata

Reply to
Iva

I'm glad to say my miata has passed smog. Many thanks to everyone who posted advise.

The fix turned out to be quite easy. When I hooked up a timing light I found the timing to be around 12' BTDC. I adjusted it to 8' BTDC and the N20 reading during the smog test was ~500 ppm from nearly 800 before. Another good idea here is to ensure your smog station places a big cooling fan in front of your car to stop it overheating as many cars cooling systems can not handle being under load while not moving. With N20 the hotter the engine gets to higher the reading.

Making timing adjustments is simple (but awkward), details can be found on

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Reply to
Graham

In CA, timing too far out of spec will cause automatic failure. I don't know what the tolerance is, but I recall a neighbor of mine failed when he was off 2 degrees. Of course that may have been 2 out of 6, for 33% over/under. But nonetheless.

Doug

Reply to
Doug

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