Originally Posted by MT's#1Customer!

I think this whole rich vs lean debate will go on for years with newer cars.

As a general rule, most intake and exhaust mods tend to cause some leaning of the a/f and not a rich condition. The only mods that I can think of that will make the car run rich is Long Tube Headers and larger fuel injectors when the MAF is not calibrated for them.

Here is a basic lesson on how the closed-loop fuel system works:

The Mustang's computer is coded with different fuel trims depending on operating conditions. These conditions are based on such things such as engine temperature. For example, when the motor is cold, the car will run rich...on purpose.

The 02 sensors (there are four I believe) are constantly being monitored. The forward bank which is just before the cats is critical to the function of the car. The 02 sensors read the a/f and send a voltage signal that is interpreted by the ECU. The ECU then determines if more or less fuel needs to be added. It is really A LOT more complicated than that, but this is a real simple explanation. As a result, the car is able to compensate quite a bit for the typical bolt-on mods.

Messing with the Mass Air Flow Meter (MAF) can really screw things up big time. If not done right, you can ruin an engine. The ECU gets important data from the MAF.

I guess what I am trying to convinve you of is that the odor that you detect is the result of not having cats and is not from a rich condition. HOWEVER, the car could be running rich also, but not because of not having cats.