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Thread: High NOx gas FINALLY fixed! Passed Emissions Test!

  1. #1
    Join Date
    Jan 2002
    Atlanta, Georgia USA

    Thumbs up High NOx gas FINALLY fixed! Passed Emissions Test!

    Hi All,

    Just wanted to give you an update on my situation and share some good news! My '92 Wrangler passed it's emissions test today! I left the 180 thermostat in. Here's the test results:

    Original attempt, FAILED (1/6/03):
    25/25 Test 50/15 Test
    Reading Allowed Reading Allowed
    HC ppm 41 159 16 163
    CO % 0.06 1.78 0.02 1.38
    NOx ppm 872 1014 1310 FAIL 1110
    RPM 2127 2500 max. 1335 2500 max.
    CO+CO2 % 15.3 6.0 min. 15.2 6.0 min.

    Second attempt, re-test PASSED (1/7/03):
    25/25 Test 50/15 Test
    Reading Allowed Reading Allowed
    HC ppm 39 159 19 163
    CO % 0.06 1.78 0.01 1.38
    NOx ppm 780 1014 945 1110
    RPM 2089 2500 max. 1272 2500 max.
    CO+CO2 % 15.3 6.0 min. 15.3 6.0 min.

    (Note: This is a 2003 Georgia State Emissions test on a 1992 Jeep Wrangler 4.0L, 167,096 miles, 5 speed, with no EGR system (didn't have one from the factory), new stock air filter, new OEM Champion plugs (gapped at .035), new OEM Mopar plug wires, new distributor cap & rotor, 1 year-old OEM style catalytic converter, new 180 degree thermostat, Poweraid throttle body spacer, 6 month-old upgraded 3-row radiator, Borla Headers, Borla cat-back exhaust and 31 x 10.50 tires)

    My problem since last year (2002) has been high NOx gasses. After spending $550+ last year with no resolution to the problem, I managed to get a certificate of passage from the shop so that I could get my tag/registration for 2002. I was pretty much guaranteed by the shop that passed me in 2002 that when tested again, the Jeep wouldn't pass and would still be high on NOx readings. So, long story short and a big dent in my bank account, this still left the problem unsolved...

    Fast forward to this year and the 2003 results above. You will notice that due to what I did to the Jeep, the NOx gasses were reduced from 872 to 780 in the '25/25 test' and from 1310 to 945 in the '50/15' test. These new lower NOx readings are well into the passing range.

    Here's what I did to get it to lower the NOx readings and pass.

    (BTW, a guy at my local NAPA suggested I try this because he has a friend that does it all the time on his fleet of taxi cabs with high mileage engines and they all pass emissions)

    OK, I disconnected the rear crank case vent tube on the intake manifold side and plugged that end of the tube. Now, with the open fitting on the intake manifold, I took a 2 foot section of 1/8in. tube, connected one end to the fitting on the intake manifold and then ran the open end of the tube back out of sight (down behind the charcoal canister on the drivers side). This allowed more fresh air directly into the intake manifold besides what was being pulled in through the air filter, etc. I suppose the engine was running a bit more lean at this point due to the greater influx of air.

    Hopefully this info. will help many others in similar situations. My 1 year+ searching for info. about this problem basically led me nowhere and only left me with a nearly $600 dent in my wallet.

    - Chris
    Last edited by csilvestri; 04-25-2016 at 12:29 PM.
    1992 4.0L Wrangler, 5-Spd./Borla Headers, Cat-Back, K&N FIPK 57-1521/3.5 In. Lift/31x10.50 BFG AT K/O


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