I imagine this is an easy one, what does it mean when a truck is "running rich"?

 

It means the fuel-to-air ratio contains too much fuel. Too much fuel is a bad thing. If you filled the cylinders with straight gas, the truck would never run. Running lean means not enough fuel mixed with the air. Our truck control this automatically with the computer by regulating the operation of the fuel injectors. In the days of carbs, this was controlled by adjusting the jets.

 

The ideal air/fuel mixture is 14.7 parts air/1 part fuel. If you are looking at data from the O2 sensors it is whenever the sensor reads 600mv or more. The symptom's are; smell of fuel in the exhaust, carbon build-up on sparkplugs, black soot on tailpipe, poor gas mileage.