Chrysler Repair: high nitrogen oxides: 95 2.5L 4 cyl, fuel air mixture, nitrogen oxides

Question
We have a 1995 Chrysler Plymouth 2.5L engine with TBI fuel delivery.We took our van to have the Emission tested and it failed the (nitrogen oxides) Test it was high. The engine doesnot have a EGR Valve.Last year we had to replace the cylinder head and gasket.What can we do to lower the emission of  nitrogen oxides???I thought maybe using a hotter spark plug or maybe it's the catalytic converter? We have also replaced the Map sensor,Throttle sensor and the AIR sensor and the single fuel injector.PLEASE HELP!!!!!!

Answer
Hi Vick,
That engine, when used in the cars, did have an egr, but not when used in the minivans. I don't know why but it is probably more marginal on NOX for that reason.
NOX can result from high combustion chamber temperatures due to high engine operating temperature, carbon build-up in the combustion chamber/piston top (presumably not the cause in light of the head replacement recently), too lean fuel/air mixture, too advanced timing, exhaust leaks reducing the cat converter efficiency.
Have you tried for fault codes using the ignition key:"on-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time? Then watch the check engine light, which remain 'on', to see it begin to flash, pause, flash, etc. Count the number of flashes before each pause. Then repeat the process to assure an accurate/repeatable set of flash counts. Tell me the counts in order of appearance and we'll go from there for a code may be a clue to why the high NOX.
One other detail is to be sure when the next test is run that you have driven the van at highway speeds so as to assure that the engine/converter are warmed up when the test starts.
Beforehand you would do well to verify that your timing at idle with the coolant temp sensor plug removed from the sensor is 12 BTDC. Also, how is the coolant temp gauge reading for if that were running higher than normal that would raise combustion chamber temps.
Listen for exhaust leaks in the area BEFORE the converter (a 'pssst' sound).  
If none of those prove out then replacing the Cat may be necessary.
Please let me know what you learn and how the next test goes.
Roland
PS Please 'rate' my answer. Thank you
PPS I have that engine in an '89 LeBaron and have not had difficulty passing smog, but then I do have an egr on it.