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