I have a 90 yj 4.2 auto that has a howell fi kit ca smog legal. it has failed smog a half dozen times due to high nox. howell sent a new throttle body and it still is having issues. hoping someone can tell me how much vacuum you have at port A at an idle and 1500 rpm
already put new cat/muffler, distributor, radiator, gas cap, ecm, plugs wires cap rotor,
too much in it to stop now! help
Last edited by huntingheep; 08-20-2013 at 07:15 PM..
High nox is a lean burn. You either have a exhaust manifold crack/leak or an issue with the O2 sensor itself. Check your manifold for cracks by the collecting tubes and double check all the manifold bolts torque. Check the egr tube too.
I'd also bet your leaking at the manifold. Mine did the same. You may be money ahead to call around and find a shop that can do a smoke test, they basically pump a fog machine into the engine and you'll see anywhere it's leaking. Found an entire end piece was cracked on my intake this way that I couldn't see with it on there. I think the local guy charged me $45 to do it.
Originally Posted by George Orwell
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damn go to a different smog shop. they should tell you why its not passing not how to rebuild your whole jeep. my friend has a shop and the only thing i've ever changed to pass was a rebuilt carb, gas cap, 02 sensor, or egr diaphram. All things that eventually fail with age.
smog shop says its egr system including the cat, that controls nox gas, its on its 4th egr and I put a new cat on a year and a half ago, no egr tubes are clogged, vaccum is run correct for the howell diagram
Lean burn is the cause for high nox. The question is why. Fuel pressure
being regulated low by the control systems or running low because your fuel pressure is low. Check your fuel pressure.
The cylinder temp too high. The EGR it's responsible for lowering the chamber temp by porting the exhaust into the intake to lower the combustion temp.
Timing being too far advanced. The timing will cause the combustion to run too hot if it's advanced too far. Again I am not familiar with the hesco but the advance on the standard setup is controlled by the map sensor vacuum. If it is setup already you need ported vacuum for it.
The cat is the final stop for nox that is left. Start by checking your fuel pressure. You can try hesco