I don't know how old this article is, but Vincent Ciulla responded to a writer about his 95 Mazda (Mazda Millenia Fails Inspection) that had high HC and NOx levels, failing California's emission test. Anyway, Mr. Ciulla is a little misguided in his explanation of the emission component, CO (Carbon monoxide). He stated that HC is formed when there is more gasoline than air in combustion (true) and CO is formed when there is more air than gasoline in combustion (false). High CO is always a sign of incomplete combustion, incomplete burning of all the gasoline (HC) molecules, indicating an over-rich mixture. The richer the mixture, the higher the CO (until it gets too rich that it won't burn then you have no CO produced in that combustion chamber). The leaner it becomes, CO will always go lower; I've seen vehicles that had 0.0% CO but 2000ppm HC due to an ultra-lean fuel mixture which caused misfiring, thus unburned gasoline (HC) going out the tailpipe. If both HC & CO are high, you have a rich fuel condition. If HC is high, but CO is low, you either have too lean a mixture or an improper burn (misfire, detonation, etc.).
For example, if HC is 1500ppm and CO is 0.3%, it is NOT a fuel mixture problem (unless a single cylinder or so has an intake leak at the intake runner affecting that cylinder or adjacent one, which could cause a lean misfire on that cylinder(s) causing the high HC). With the high HC and high NOx under load (which is when NOx is typically formed), I suspect possibly that if this vehicle has an EGR passage that feeds cylinders individually, one or more of the runners could be restricted, causing insufficient EGR flow to a couple of cylinders, causing the NOx to increase under load. Just because manually opening the valve causes the engine to stall @ idle doesn't mean that all cylinders are receiving equal amounts of exhaust gas. I have seen this in many Honda engines and it could possibly be a problem with this Mazda. (Sorry, but I don't recall how the EGR gas is fed to the cylinders in this engine.)
Aloha,
Gary Fuchikami
Hawaii
Answer:
I think I may have over-simplified my explanation of HC, CO and NOx. Hydrocarbons (HC) is a result of too lean a fuel mixture and Carbon Monoxide (CO) is a result of incomplete combustion. When fuel is burned in an engine you have carbon atoms and oxygen atoms. When combustion occurs you want to have 2 oxygen atoms join with one carbon atom to create Carbon Dioxide (CO2). That is the ideal we shoot for.
If there are too few oxygen atoms, there will be carbon atoms with no oxygen atoms resulting in a high HC reading. The next possibility is if there are just enough oxygen atoms to join with a carbon atom, one oxygen to one carbon, you will get a high CO reading. The fuel has burned but not burned completely.
So, ideally, we want to combine 2 oxygen atoms with each carbon atom to produce CO2. Mechanics use the CO2 reading of an emissions analyzer to determine the efficiency of engine combustion. The more CO2 that is put out, the more efficient the engine is.
Now while Honda uses individual EGR runners on some of it's engines, Mazda does not. Honda has had a lot of problems with these EGR runners and issued a TSB on it. I have to believe that Mazda Engineers have designed the EGR system and intake manifold on this engine so that each cylinder gets the required amount of EGR to lower NOx emissions, assuming the EGR system is working correctly.
Another thing you have to keep in mind also is that I do not have the vehicle here in the shop. I can only give my best educated guess based on the information provided. There are a multitude of other factors that affect exhaust emissions and any number of them may apply to this vehicle.
