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High HC's on smog test at 15mph on a 1991 Nissan Sentra

How could you correct a CA smog testing related problem showing high HC's at 15mph on a Nissan Sentra 1.6, 4 cylinder Sedan? The 25mph test passed but not the 15mph. Could there be a simple fix other that oil & filter, plugs and air filter change?

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  • obs62 Dec 31, 2008

    I also had the same issue and very high NO at 15 mph, but ok at 25 mph

  • djishere May 11, 2010

    pcv valve also a good boot on highway before your test will help

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6 Suggested Answers

emissionwiz
  • 61292 Answers

SOURCE: high hc

you may need replace the catalytic converter, but 1st do this, clean the throttle body and idle airbypass valve, change the oil before the retest as well, get it as hot as you can before the test, hold it at a high idle prior to the test, this may squeak you by, as a 20 year smog tech these are the thing I did, if all else fails do the cat conv.

Posted on Oct 15, 2008

  • 194 Answers

SOURCE: 1991 Honda Accord LX failed emission test on smog

not enough info. Typically, CO and HC together can be the result of poor fuel control or misfire, or worn components such as O2 sensor or catalytic converter, but without more data, repair recommendations are likely to waste money and not achieve success.

Posted on Sep 12, 2009

  • 507 Answers

SOURCE: i have a 1992 nissan sentra that failed the 15mph

get oxygen sensor checked,maybe get tune up see if engine is leaning off properly,could be air flow sensor,but needs sensors to be checked

Posted on Oct 30, 2009

  • 834 Answers

SOURCE: failed smog check

How well does the vehicle run?

Assuming that the engine did pass the EGR function test on the emission test results, and the MIL (Malfunction Indicator Light) or check engine light is not on when the engine is running, and if you are not sure when the last time it was that the engine had a complete tune-up, with distributor cap, ignition rotor, spark plugs, spark plug wires, air and fuel filters, then a complete tune-up would certainly help, and if the engine oil and oil filter have not been changed in a while, then they should also be changed before an emission test because long used engine oil traps carbon and it will show up as higher CO on the emission test because the emission analyzer will be able detect the higher CO from the engine oil through the PCV valve.

However, it would seem from those HC readings that there is a vacuum leak, and carefully inspect all of the vacuum lines for any cracks or damage, (especially the vacuum line to the fuel pressure regulator) because cracks in the vacuum lines seem to like to hide underneath the lines.

The emission label under the hood should have the vacuum line routing diagram printed on it, and the vacuum lines should all be checked to be certain that they are all connected correctly.

Here is the firing order diagram for that vehicle to help assist you tune-up the vehicle.


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Posted on Jul 10, 2010

johnjnail
  • 1702 Answers

SOURCE: My 1990 chevy astro van

the smog test looks like the engine is miss firing, that could cause the timing to be out of whack also, try a old fashioned tune up, new spark plugs, wires, and filters also replace the PCV valve. any time you see the NO that high suspect a miss fire.

Posted on Feb 24, 2011

  • 158 Answers

SOURCE: My 1997 Honda Civic failed

Jose, I don't know if this will give you the answer you are looking for because there are many things to consider here. The exhaust gas recirculation valve (EGR) is one of the main things involved in reducing NO emissions. This valve is controled by the power control module. (PCM) This is your vehicles computer. The PCM monitors the vehicles speed and will command the EGR to open when the vehicle attains a certain speed, which is why the ASM test being done is recorded at two different speeds. At 25 MPH your vehicle failed the hydrocarbon (HC) emissions limit and this could be for a number of reasons. When is the last time you car was tuned up? Old spark plugs, wires, clogged air filters, oxygen sensors, leaking injectors (personal experience) or the PCV valve could be contributing to the results of the test. I guess that the easiest way to go is to tune up the engine replacing the spark plugs, wires, air filter and the PCV valve and don't forget to have the oil changed at the same time. Only then should have the vehicle retested and see what the results are. Should it fail again you may have to consider having the EGR or the catalytic converter replaced depending on what portion of the test fails. If the NO fails I would consider the EGR. If the HC fails then I would consider the catalytic converter. Good luck with this.

Posted on Apr 05, 2011

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the smog test looks like the engine is miss firing, that could cause the timing to be out of whack also, try a old fashioned tune up, new spark plugs, wires, and filters also replace the PCV valve. any time you see the NO that high suspect a miss fire.

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I have a 1992 nissan sentra that failed the 15mph part of the smog test because of high Hc's and high NOx so what can i do to fix it.


get oxygen sensor checked,maybe get tune up see if engine is leaning off properly,could be air flow sensor,but needs sensors to be checked

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You did not post the odometer reading on the van but base on the year it should has over 85,000+ miles on it.

The most likely source of the high Hydrocarbon (HC) is clogged or drippy injectors.

Change the fuel filter if it is over 48K miles.

Ran two or more tanks full of Chevron gas will help clean out the fuel system.

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The air fuel ratio can be off is the air filter is clogged or the mass air flow sensor give incorrect data to the computer (ECU).

Change the air filter is it over 20,000 miles or you can see sunlight though it.

Use CRC Mass Air Flow cleaner or electronic part clean to gently clean the mass air flow sensor.

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Change the oil to 20W50 or one grade higher to prevent blow by.

You current motor oil is soak with HC from blow by.

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Make sure the tires are at 32 psi

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Do the smog test is a COLD morning?

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REMOVE excess cargo off the van.

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These are little things that I would do.

If the HC is still high at 25 mph then you will need a professional fuel injector cleaning or replace the injectors.


Please post more data on the third smog test on the comment window.

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1991 Honda Accord LX failed emission test on smog test. High CO and HC on 25mph


not enough info. Typically, CO and HC together can be the result of poor fuel control or misfire, or worn components such as O2 sensor or catalytic converter, but without more data, repair recommendations are likely to waste money and not achieve success.

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High hc


you may need replace the catalytic converter, but 1st do this, clean the throttle body and idle airbypass valve, change the oil before the retest as well, get it as hot as you can before the test, hold it at a high idle prior to the test, this may squeak you by, as a 20 year smog tech these are the thing I did, if all else fails do the cat conv.

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