I have a 1990 miata that seems to run rich There are no codes and the car runs nice but sometimes after a high speed run the cat stinks probably from being overworked. I have replaced the cat and oxygen sensor but still the same problem. The car failed on an emissions test. The tailpipe is sooty.. Any ideas???

 

when was the last time that it had a complete tune up? high mileage?

 

It has new plugs, ign wires are good ,air filter new. it doesn't miss 230,000km , compression good

 

Have your fuel pressure tested, could be fuel pressure regulator with leaking diaphram or no vacuum to the regulator.

 

My mistake for not checking the emmissions before posting . I will end this thread as the car is ok .Emissions are very low with the new cat