Hi y'all! Have changed all cat converters, injectors, radiator, spark plugs, oil, fuel pressure regulator and filter, and STILL WONT PASS EMISSIONS~high nox 174,000 miles on vehicle. runs good. does not burn oil. 13 smog checks throughout all these changes and "fixes" and still high nox. Any ideas before I junk this???? Help me love my jeep!!!

 

Have you cleaned the throttle body?

I cleaned that and my MAF on my Mustang after failing the snif test. Just that alone got it to pass.

good luck!

 

If it is a v8, you don't state, open the throttle body use a light and look down the throat, if you see any oil deposits you could be getting some oil slipping past the plenum gasket and being burned causing the NOS to be high but not enough to be noticed as using oil. This was happing to another member on the east coast had to repair the plenum gasket to solve this problem.. This is very common on this engine. If not a v8 sorry I have no idea..

 

Im not sure if the 99's used EGR in either engine but I'm not quite sure if they did in and not sure if there was a cali emissions either. If equipped with EGR that is what it's sole purpose was, to clean/lower NOx emissions. Other things that can contribute are, Lean running condition, carbon build up in the cylinders, engine running hot. Those are the main causes.

NOx is the result of excessive or high combustion temps, High compression engines usually produce more NOx, so excessive carbon build up can increase this. Lean running conditions cause excessive combustion temps too, a hot running engine can also cause higher combustion temps. EGR system recirculates exhaust gasses back through the engine, since exhaust gas is basically "dead air" it will not burn again therefore lowering combustion temps, its basically counterproductive (like most emissions) for peak engine performance/power but the EPA/.gov says its good.