I have a 1992 Nissan Maxima GXE, the car failed NOX emissions with a 6.25 (allowable is 3.25). EGR functions, Replaced O2 sensor (still didn’t pass lowered to 5.25). I am looking for another direction to go in. Any ideas would be great.
high NOx: temperature at the combustion chamber too high.
best case: lean mixture caused by a vacuum leak; bad spark plugs; ignition timing off; bad gasoline.
worst case: bad egr valve or catalytic converter.
Please post your results.
I would have the EGR (exhaust gas recycle) system checked to make sure it is functioning properly. This is the primary NOx control for the engine. If this checks out then the next check is the catalytic converter. If the cat is original then it may be failing.
You mention that the EGR functions. Are you sure that the EGR passages are open? Clogging is common.
How have you determined the EGR functions?
Egr tested by: racing engine and verifying movement of diaphram, also if you attach vaccuum to the egr it causes the car to stumble. Also what would be any other signs of a bad catalytic.
I would check for vacuum leaks. BTW is the intake stock with no modifications