My 99 LS is running rich. I have a cheap narrow band AF gauge and it shows it running rich most of the time. When I let off the throttle it wants to crack and pop. I checked the plugs and none of them are excessively black. I changed out the coolant temperature sensor with one I had from a previous motor and still have the same result. What else should I check?

 

Beside the cheap narrow band gauge any mods? Car acting up? Timing?

 

Header, exhaust. Engine is stock. I haven't changed anything for two years.