[Treeve]

G'day Tim,

The ECU tries to hold 14.7:1 all the time in closed loop. However, as your engine's cylinders do not burn perfectly exactly the same each and every ignition event, then there will always be some change in the output. Therefore the ECU aims at 14.7:1 and will constantly overshoot or undershoot a little bit. And it is a little bit - I'm not talking +/- 0.2, it's in the region of +/- 0.01 AFR.

If your car was running at 14.9:1 as you have suggested, then it is running too lean, and there isn't enough fuel to burn cleanly in the cats - if you consider a cat to be a fireplace which can only burn on what comes out of your engine, then it will help to understand. This fire needs fuel (rich) and oxygen (lean). You need this to run cleanly too, so not too rich and not too lean. Now, imagine you can only feed one or the other of these at a time - fuel or oxygen. This means you have to cycle between the two to get your fire to burn efficiently. Therefore you use a narrowband sensor which does this.

Your car running at 14.9:1 is putting heaps of oxygen in, but not enough fuel and hence you are getting low fuel readings, but the lean flame is hot, and hence you aren't breaking down the NOx molecules.

If you go too lean, you get lean misfires and then you get high NOx and high THCs - but normally in spikes which can be picked from the results chart depending on how the load is applied to the engine.

Now, this is an analogy and it doesn't hold up to detailed inspection, but it does get the idea across simply enough I think.

Does this help?

Cheers,

Treeve