Need help with smog failure on 92 Miata

I just failed the Oregon Smog test on my 92 Miata... and would appreciate some help in troubleshooting the underlying problem.

Test results: Total hydrocarbons was about twice the limit and Nitrous Oxide was about 10-20% over limit. THis actually was the second test done in 24 hours and showed a little improvement over the first ( plus I replaced the oxygen sensor after the first test and all the spark plug wires).

Here is some background.

-80 k miles

-original timing belt (will be replaced this year)

-For the past 10 years the car had passed California smog tests with flying colors (similar specs to Oregon's).You could almost breathe the exhaust.

--The car was run recently for a week or so by my wife with a bad spark plug wire... misfiring, hesitation. We replaced one wire and this seemed to solve the problem. After failing the first smog test, I replaced all 4 wires.

-this morning I wiped my finger inside the tail pipe and got some black carbon. I'm assuming this will be there as long as I have the excess hydrocarbons problem (unburnt fuel). I dont recall having seen any deposits like this in past years.

-for the first two smog tests, I was running the vehicle mainly off the battery (loose, worn alternator belt that did not allow the alternator to keep up with lights, fan, etc; the battery would eventually get too low to start). On the last smog test, I had a larger battery hooked up in parallel with the stock trunk battery to ensure plenty of (battery) power. It still failed the test. I replaced the alternator belt yesterday so all is ok with that part of the system. No second battery needed.

-I check the timing yesterday and it was right on 10 degrees BTDC as it should be.

-the engine does not sound like it is missing, but I would like a better way to test that. I did temporairly reduce the rpm to about

600 and it didnt sound that bad.

-I hooked up the strobe light to each of the wires, and noticed that on two of the wires the frequency of flash was about half that of cylinder #1. The other cylinder had the same frequency of flash as #1. Is this expected behavior? I seem to recall that #4 cylinder had the same timing as #1 (10 deg. BTDC) but about half the frequency of flashes. I could redo the test if needed.

-At one point I thought I detected some misfiring on each of the cylinders as the strobe would 'miss a flash' ... perhaps several in 10 seconds. There was not a consistent pattern here, although some cylinders seemed to have more 'misses ' than others. Then, for a while, I didnt see any of the cylinders misfire. I will look at this more closely as this alone might explain the 'unburnt fuel' getting through and failing the hydrocarbon test.

-The sparkplugs all look clean ( were replaced last fall). No soot.

-Oil was changed perhaps 2000 miles ago, along with the filter. What smog specs are affected if you have 'old' oil in the crankcase?

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-Questions:

--Is is possible that the catalytic converter is toast now? I have heard that they can be burnt out with excessive fuel coming through. How can I test this? I dont want to replace it without first solving the underlying problem.

-There was a posting on how to retrieve and read the 'engine' diagnostic codes which I will look into soon... however there are no 'engine' warning lights lit. How useful will these be?

--If I just run the car for a week or so and hope that the catalytic converter comes (back?) online , might this help? I will clean out the end of the exhaust pipe and check for new deposits over time.

-I plan on doing as much of the troubleshooting and repair as I can, and would appreciate any suggestions as to what to do next.

thanks paul

Reply to
paul witort
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Sounds like your catalytic converter is shot. It's about due, with 80K miles, and it's a relatively inexpensive replacement.

Don't sweat the sooty tailpipe. Early Miatas run rich to start off with, so the soot is to be expected.

Mike "Red '97" Verive snipped-for-privacy@mverive.com

Reply to
MiataMan

Thanks Mike,

Here is what I did since my last posting:

-replaced the pvc valve (old one seemed ok, but for $3 why not.

-messed around with the difference in strobe light frequency and sort of convinced myself that this was normal operation. If I dont hear from someone else on this miata board who has used a strobe light on their wires (surely there must be someone :) , then I will try the same test on some other known good miata.

-tested compression- 160-170 in all 4 cylinders.

-cleaned the end of the exhaust pipe and went for a 6 mile ride... came back and there was soot in the pipe.

MIke---- you write: Don't sweat the sooty tailpipe. Early Miatas run rich to start off with, so the soot is to be expected.

Question to Mike: does the soot every leave the pipe? like on a long trip? ...if not, it looks like I cant use this to determine when the high hydrocarbons problem is solved..

------

-broke the coolant temp sensor clean off. This looks like it only affects the analog gauge on the dash (is this right, or does the computer use the reading too?) ; since the car seems to run all right, Ill shop around for a reasonably costing replacement (any suggestions?); Schucks wants about $30...ouch.

-I hooked up a spark plug to each wire, one at a time and observed the spark while the engine was running. All sparks looked similar at idle and robust. No apparent problem there. The voltage on those wires can get really high if you (accidentally) get the loose plug away from a ground. It will leap an inch or more, which is probably over 100,000 volts! Some other posting said to definitely not run the engine unless the spark plug wires are grounded... I saw why firsthand... this current is going to find its way to ground if it has to zap the coil in the process :)

-When doing the compression tests , I disconnected the wiring harness connector to the coil assemblie(s) so that the spark would not be generated during the test. I saw in another posting that someone burned out their coils when they didnt ground the wires...cost of the dual coil replacement is $200+ ...Ouch! Ouch!

-I have not dumped the engine codes that should indicated if the ox sensor is doing its job, etc. That should be done today.

If no other troubleshooting is recomended by anyone, I will run some fuel injector cleaner (B12) through a tank of gas, run another tank of gas with no additives and then head back to the smog station.

Question--- is there a suggested way to test for vacuum leaks (causing lean mixture and unburnt HC). Where should I be looking?

thanks,

Paul

Reply to
paul witort

"Broke the Coolant Temp Sensor clean off." This may be the answer to your black tailpipe. The ECU [computer] uses the input form the CTS to determine whether or not the car has reached operating temperature. Without this input, the ECU will think the car is cold and enrichen the fuel mixture [open loop operation].

Reply to
James Combs

Advanced timing can increase the NOX reading and while I did catch that you have the car set at the stock spec, you could retard this slightly (probably

8 btdc) to get you by on that issue.

Good luck and keep us posted on your results (new post)

Chris

Reply to
Chris D'Agnolo

Paul,

The soot in the pipe is only that which doesn't get blown out (otherwise the tailpipe would quickly get plugged with carbon).

Unfortunately, I don't know of any way to judge hydrocarbon emissions by the thickness of soot.

Fix what you know is broken (like the coolant temperature sensor), get the timing right, use fresh plugs/wires/filters, get the engine nice and warm (idling while waiting for the emissions check doesn't count), then if you don't pass start replacing parts which are due to be replaced anyway, starting with the best bang for the buck items like catalytic converter.

Good luck, and keep us posted!

Mike "Red '97" Verive snipped-for-privacy@mverive.com

Reply to
MiataMan

My first guess would be a bad catalytic converter -- a lean mix would get you excess NOx and a rich one would get you excess HC, both together indicates a bad cat.

The other possibility is that some cylinders are running lean and some rich. This is less likely as it should cause a diagnostic code (O2 sensor), even on a pre-OBD-II Miata, I think. Also in that case you probably also have a bad cat. 80K is a pretty good run for one, particularly if you've had misfire problems.

No. Since you've already done the wires, you might want to do the plugs. However, I'd think that if you were missing on one cylinder you'd know it, so it could be an artefact of your tools.

If it's dead, it doesn't come back.

Reply to
Matthew Russotto

Broken Coolant Sensor is DEFINITELY your problem. Fix it or risk your Catalytic Converter overheating and self destructing.

Reply to
DanO

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