1992 Deville fails NOx emissions test
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HT4100 4.1, 4.5, 4.9 Discussion, 1992 Deville fails NOx emissions test in Cadillac Engine Technical Discussion; My 1992 Deville (4.9L V8) failed NOx emissions testing today. I have had problems getting the car to pass last ...
  1. #1
    Big_Cadillacs is offline Cadillac Owners Member
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    1992 Deville fails NOx emissions test

    My 1992 Deville (4.9L V8) failed NOx emissions testing today. I have had problems getting the car to pass last time for emissions testing and have had several major repairs which were luckily covered by the dealer. It did eventually pass last time but now it failed again. I will provide as much information as I can. I would greatly appreciate any tips. Thanks.

    Here are some of the recent repairs which were done so far.
    Fuel injectors replaced - 2.5 years ago
    Catalytic converter - 2.5 years ago
    Exhaust system - 1 year ago
    thermostat - 2 months ago
    oxygen sensor - 2 months ago
    air filter - 2 months ago

    Here are my emissions testing results
    HC 18.5 ppm Limit - 111.0
    CO 0.02% Limit - 0.61
    NO 1140.9 ppm Limit - 773.0
    CO2 15.24%


    I was reading some of the other posts and it seems like a bad egr valve is a common problem. I did a test that was in my Haynes repair manual which was to push up on the egr diaphragm while the engine was running and the engine idled slower and stumbled indicating that the egr valve is probably fine. My guess is that the egr valve isn't working as well as it should. I don't want to replace it until I know for sure its the problem. It looks like a easy repair.

    The other thing I think might be wrong is the egr vacuum solenoid which also looks like a easy fix.

    I also looked at the spark plugs a few weeks ago and they look fine. They just showed normal wear and probably are good for a few more years.

    The engine seems to run very well and it seems hard to believe anything is wrong with it. The car has 76,000 miles.

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  3. #2
    Dave S Texas's Avatar
    Dave S Texas is offline Cadillac Owners Enthusiast
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    Re: 1992 Deville fails NOx emissions test

    What codes are displayed? The EGR valve can be a cause if it is: stuck Open/obstructed/leaking or the vacuum lines are wrong/passages plugged or carbon build-up in the passages. Other nox failure causes can be: advanced timing/cooling system not performing well(too hot)/fuel mixute too lean/knock-sensor or ESC faulty/contaminated catalytic converter.

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    Edahall's Avatar
    Edahall is offline Cadillac Owners Enthusiast
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    Re: 1992 Deville fails NOx emissions test

    Most likely your problem is either a failed EGR solenoid or bad vacuum hose that goes to the solenoid. You can test the EGR solenoid by going into the onboard diagnostics and manually opening up the solenoid. You might need a GM service manual to explain how to do it.

  5. #4
    Big_Cadillacs is offline Cadillac Owners Member
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    Re: 1992 Deville fails NOx emissions test

    Thanks for the quick responses.

    I did run the diagnostic check and didn't get any codes. I will try checking the vacuum hoses and removing the egr valve to get a closer look. I will post what I find over the next few days. If the egr system seems fine I will look at some of the other areas you guys suggested.

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    HUF's Avatar
    HUF
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    Re: 1992 Deville fails NOx emissions test

    It is strange nobody mentioned two EGR tubes inside the trottle body (under the plates). Clean the carbon build up inside those two tubes with coat hanger. I had the same emission results. Everything was fine as soon as I cleaned those tubes.

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    carnut is offline Cadillac Owners Master
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    Re: 1992 Deville fails NOx emissions test

    I agree. Clean the tubes. MANY, MANY posts related to this very thing. Do a 'search"

  8. #7
    Big_Cadillacs is offline Cadillac Owners Member
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    Re: 1992 Deville fails NOx emissions test

    I did look at the egr tubes under the throttle body plates. There was some carbon build up in them and I did clean them out. It's hard to believe that little bit of build up could cause the car to fail emissions tests. I'm going to get the car tested again within a week and see if that fixed the problem. I will post the results of my second test.

    I did remove the egr valve to take a close look at it. It seems fine. It looks clean with no build up on it. The diaphragm looks good too. The vacuum hoses also look good and have no cracks or leaks. Since I'm not getting any diagnostic codes I can probably assume the egr system is fine.

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    Edahall's Avatar
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    Re: 1992 Deville fails NOx emissions test

    The little carbon found in the tubes could be an indication that your EGR is not working.

    The best way to test whether the EGR is working is to go into the onboard diagnostics to manually open the EGR valve. I know how to do it on my Allante and Eldorado but since I don't own an Deville, it would be slightly different.

    Do you have the service manual for this car? The service manual explains how to do it.

  10. #9
    Big_Cadillacs is offline Cadillac Owners Member
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    Smile Re: 1992 Deville fails NOx emissions test

    I do not have a service manual but I will keep that in mind in the future.

    Now the good news is that the car passed the emissions tests today. I did two things. First I cleaned out some carbon build up in the egr tubes. I also decided to drive the car for about 20 minutes to make sure the catalytic converter was hot enough. The testing station is about a 10 minute drive from my house so maybe that wasn't enough time to fully warm up. My original NOx reading was 1140.9 ppm. This time it was 765.8 ppm.

    Thanks everyone for all your tips!

  11. #10
    HUF's Avatar
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    Re: 1992 Deville fails NOx emissions test

    Quote Originally Posted by Big_Cadillacs View Post
    I do not have a service manual but I will keep that in mind in the future.

    Now the good news is that the car passed the emissions tests today. I did two things. First I cleaned out some carbon build up in the egr tubes. I also decided to drive the car for about 20 minutes to make sure the catalytic converter was hot enough. The testing station is about a 10 minute drive from my house so maybe that wasn't enough time to fully warm up. My original NOx reading was 1140.9 ppm. This time it was 765.8 ppm.

    Thanks everyone for all your tips!
    Those EGR tubes do not need to be plugged completely to mess up emission. Glad it worked. Life's good again!

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