Failed ca emission test for high nox

[BARanger]

My 96 explore 4.0 failed the smog check for high nox at 15 mph and passed at 25 mph. It was almost double the limit the egr sensor was just replaced due to check engine light and the egr was cleaned. the cat is only a few years old does anyone have any ideas what could be causeing this

 

[calikrawler]

TTT, same failed nox only at low rpm test, passed others flying colors

I Have a 1995 ford ranger Xtra cab, failed calif smog for High NOX at the 15 mph test over 600 limit around 500.
15 mph HC Low, CO Low, NOX High Fail
25 mph HC Low, CO Low, NOX Low Pass with flying colors
Nox has gone up slightly since last time barely got it to pass 2 years ago due to the 15 mph NOX levels. 02 AND CO2 are low for both recent tests, maybe my bald 33s aren't letting the egr kick on early enough to decrease nox at 15?? I will try to post data, also has an odd problem once warmed up and ecu goes to closed loop C/L , when releasing the throtthe (return to idle) there's multiple surges (goes into open loop then closed loop, O/L, C/L, open closed loop from around 1500 down to 1000 before it gets to idle and returns to closed loop, have replaced 02s precat, thermostat, coolant/Block flush quite a bit of rusty water..., spark plugs & wires, cleaned throttle body, Maf, removed k&n,first cleaned Iac no change, replaced Iac no change, replaced intake air temp sensor, new battery, have run injector cleaner many times no change in emissions runs smoothly, seems like it takes a few more cranks to start sometimes other times fires right up clogged injecotr?
Please help me pass smog so someone else can enjoy this truck.

 

[calikrawler]

data

heres the test results
-First 2 years ago just scraped by did the usual tune-ups oil, A/F, plugs, gas, Did the Front o2s after this test as the left front went bad n set the CEL changed both pre cat, Post cat looks normal not sluggish like the fronts
-second is the first recent test that failed- oil change, coolant flush, A/F, fuuel cleaner(yes Ran the tank empty before refilling for test) Got Cats Nice and Hot on the freeway Everytime Left Idling while waiting,
-third is the most recent failure, new plugs, wires, cleaned maf + throttle body, cooling block flush, t-stat using stock 192 may back down to 180 to help lower temps, These little things seem to have cut my already small HC And CO emmsions in half to almost nothing, but Nox Is Still way high at 15mph!

NO CEL

 

[alwaysFlOoReD]

I am not trained in this stuff and only know what I've read. My first thought was high ignition temps and some searching seems to bear that out;
http://www.doityourself.com/forum/p...165410-failed-smog-test-nox-too-high-why.html
Maybe someone with better insight will speak up.
Good luck,

Richard

 

[adsm08]

NOx is created by high combustion temps. If you have a good cat and working EGR, which you probably do since y ou pass the 25 MPH test then something else is causing it. I would start with a good induction service, run some SeaFoam through the intake as carbon deposits may be the cause.

 

[calikrawler]

Thank you for the replies to this Dead Thread!

Yes Nox Is due to high combustion temps I believe ~+2300°F causes Nox Formation, usually the egr Lowers the temp by introducing exhaust gas. I picked the smog techs brain a bit he admitted my Cats Read good considering HC and CO were so low. And egr Does function( was just replaced 3 years ago when I bought the truck) because the 25 mph test (higher rpm) nox was 50/600. I have induced seam foam thru the brake line more than once, to no effect, I Just added a big bottle of Concentrated Techron to the tank @fill up, just in case it's a dirty injector (probly not but i know rangers are notorious for them) . I'm thinking maybe the post cat 02 it seems ok (scan tool) but the others were bad so maybe. Also maybe the DPFE (sensor for the egr) is old and not metering correctly?
LTFT, is in the 10's is this indicating a Vacuum Leak? Also one bank of 02 seems to be higher than the other bank so one side looks lean the other rich Vacuum leak like the intake manifolds?

 

[calikrawler]

tested egr system

So did the step by step to check egr components, all the voltages were at spec, noticed the large hose for the dpfe was cracking on both ends, re placed it and checked the small hose . I noticed some corrosion on the hose connector, it's the stock aluminum dpfe with 155k, I think they switched to plastic due to condesation build up in the metal sensor.
I did the road test and if just easing to 15 mph the egr vacuum is minimal less than 1Hg, if I give it some load tho to get to 15 the egr Will kick on up to 7Hg. Figure the sensor just isn't sensitive enough, Ordered a new dpfe from rockauto. Will post back with results. I believe the intake gaskets are old n leaking but if it passed at 25 mph no prob, I don't think it's my main issue, we will see tho