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Thread: Failed Emissions Test - High NOx

  1. #1
    Carb and Points!
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    Failed Emissions Test - High NOx

    My old '92 Lumina (3.1L V6, Auto, 201k miles) failed its biennial emissions test. The results are below.



    HC and CO numbers are "normal"; NOx is sky high. However, it failed by under 20 points. My experience has been most all cars if they're going to fail, fail at 15MPH.

    Suspecting a lean mixture, I hooked up my scan tool and monitored the BLM's at both test speeds (road and engine) and the mixture is way rich:

    BLM at 15mph = 108
    BLM at 25mph = 112
    BLM at 65mph = 115

    What doesn't add up in my mind is High NOx and a rich mixture.

    Next steps would be to check EGR function and passages followed by the cooling system. Hopefully, I'll find the problem someplace in these two places and not the converter; the replacement converter is only 4 years old (21k miles).

    The O2 sensor is original to the car.

    Another observation: when I first hooked up the scan tool, I noticed right away that the TPS is reading 0.62V at idle. I believe that this should be closer to 0.50V. The TPS on this car doesn't have any adjustment.

    Any comments, etc. are welcomed.

  2. #2
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    you should probably change the O2 sensor right after verifying that you don't have a carbon plug in the intake manifold the size of your pinky finger... the LH0 loves to do that. then verify that the EGR solenoids themselves are functional(power them with jumper wires with the engine running, should cause a stumble/stall if they're flowing the amount of EGR gasses that they're supposed to).

    then I would look elsewhere. an old O2 sensor can cause all kinds of interesting issues to pop up, even if it gives a reasonable looking signal.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  3. #3
    Carb and Points!
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    Robert, Thank you for the feedback. I've already been following your diag path but in a different order.

    I've replaced the O2 sensor first (I had one on the shelf) with no change.

    I hooked up a pressure gauge to confirm the fuel system is holding pressure which might indicate a leaking injector. The system holds pressure longer than 30 minutes. Pressure starts to a a few pounds after this time.

    The next step is to make sure the EGR is opening and the passages are clear. My scan tool doesn't support EGR demand - thanks for confirming that I can jumper the jumper the valve directly from the battery.

  4. #4
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    Rich mixture is indicated when the O2 content in the exhaust is extremely low compared to O2 in free air. NOx is created when combustion temperatures are very high. If high combustion temperatures are causing free oxygen to react with nitrogen then it will not be read by the O2 sensor. The 3.1 timing maps are generally very advanced at the load and speed where your car failed and require EGR to reduce NOx. Checking EGR is important.

  5. #5
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    I removed the EGR and jumpered the terminals between 12v and ground. All the solenoids/pintles open. I confirmed that the EGR valve passages are clear with compressed air with the pintles opened.

    I left the jumpers in place and re-installed the EGR. With the engine running at idle I connected the jumpers between 12v and ground. The engine would stumble and want to stall when the EGR terminals were grounded (one by one). So the EGR passages are clear.

    Signs are pointing to a bad converter.

    What's bothersome is that the O2 sensor is reading a rich mixture. I'd like to correct this before installing a new converter. I need to go back to my notes and confirm fuel pressures, etc. and re-read the FSM for specs that are unclear.

    Thanks for all the help that's been provided.

    Just a data point; I replaced the plugs (when I replaced the O2 sensor) and they appear as they always have in the past.


    Plugs - 27,393 miles; in order - 1 thru 6 (5/22/16)


    Typical plug (42,369 miles) with new replacement (8/20/11)

  6. #6
    Carb and Points!
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    Quote Originally Posted by paulo57509 View Post
    <snip>

    What's bothersome is that the O2 sensor is reading a rich mixture. I'd like to correct this before installing a new converter. I need to go back to my notes and confirm fuel pressures, etc. and re-read the FSM for specs that are unclear.

    <snip>
    Saturday afternoon I removed the plenum in order to get to the injectors and fuel rail.

    I measured the resistance of each injector and got the following values. Temperature of the injector bodies at the time of measurement was 95.5F.

    #1 - 12.4 ohms
    #2 - 12.3 ohms
    #3 - 12.4 ohms
    #4 - 12.4 ohms
    #5 - 12.3 ohms
    #6 - 12.2 ohms

    I don't have a spec to compare these values to.

    I then ran an injector balance test, the injectors pulsed 50 times for this test. The initial fuel pressure was 43 psi. The results are below.

    #1 - 37.0 psi; pressure drop = 6.0 psi
    #2 - 37.5 psi; pressure drop = 5.5 psi
    #3 - 37.0 psi; pressure drop = 6.0 psi
    #4 - 38.0 psi; pressure drop = 5.0 psi
    #5 - 37.5 psi; pressure drop = 5.5 psi
    #6 - 37.5 psi; pressure drop = 5.5 psi

    Pressure Drop Difference: 0.5 psi/1.0 psi (min/max)

    I pulled the injectors/fuel rail from the manifold and looked into the injector holes in the manifold. I think I found the reason for the O2 sensor reading a rich mixture. The backs of the intake valve heads have a coating of those hard deposits (carbon?). My thinking is the deposits get fuel soaked and add fuel to the mixture that the O2 sensor picks up.

    This link (http://www.crypton.co.za/Tto%20know/...h_mixture.html) states carbon on the intake valve can cause high NOx; I'm assuming due to hot spots?....the statement really doesn't elaborate. I'm thinking this is true if the deposits are on the face....but on the neck side of the valve head?

    Anyone have any luck with removing these kinds of deposits short of taking the valves out of the head and taking a wire brush to it? Short of removing the heads, I'm thinking of trying that CRC intake cleaner for direct injection engines.

    I did find the exhaust passage from the EGR to the plenum restricted but it was still opened. I had to use a burr to remove the carbon build up. Other than these couple of things, I really couldn't find anything out of the ordinary.

    This evening I pulled the ECM and read the stock AXKK EPROM. I just got done burning a "test only" chip. I cut the values in the Main Spark Advance Table by 50% across the board. Perhaps this will get the NOx numbers below the threshold fail limit.


  7. #7
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    Just to close this thread; I got the car to pass.


  8. #8
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    Congratulations. Was the timing change responsible, or were additional steps required?

  9. #9
    Carb and Points!
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    I did multiple things at once so I really can't say what really affected the change the most. At least with the tune:

    1. I went ahead and reduced the timing advance by 50%.

    2. I turned the cooling fans on earlier by reducing their on and off temps by 20-degrees C.

    By and large, the change in timing is what probably did the trick.

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