Expert: Sean Halliday - 3/7/2010

Question
I have owned a 1990 mazda miata for the last 5 years and always I have  problem passing emissions testing for high NOX. I have tried new parts over the years, catalytic converter,O2 censor, always new plugs, wires,pcv valve,air cleaner and after a couple of attempts and new parts I barely pass on NOX.All other readings are within proper ranges always.I checked for vacum leaks. It is the same every year put new parts in and just barely pass. Very frustrating. Car always runs good no stalling,oil on plugs etc. One thing that does happen that might be an indication of a problem is when I have the car at exactly 2200 rpm and just barely giving it gas it lunges up and down is the only way to explain it. If I am giving it gas this never happens (TPS sensor? would this affect NOX?). There is no EGR valve on a 1990 miata. Your help would be greatly appreciated since I am all out of ideas and can't afford to take it into a shop.

Answer
The tps will not effect the nox reading but the cat will.If you have replaced the cat then it should be ok but you do not say what mileage on this car so i would look at the head gasket for possible leakage.This is tested by a leak down test.If you are burning coolant,the readings can be way off.If you are not in California,the cat will be the regular type,but the California epa requires a specific cat for this state.