It supposed to go lean when you let off.

 

+1


If you are idling at 20:1, you need to fix the ------- leaks in your exhaust.

 

would'nt exhaust leaks only matter before the wideband and around a foot or two after it?
because running open dp, would give the same results if it mattered after the sensor?
after typing that i think i know where the problem is....im a nog and didnt have a gasket for my hotside to dp, and there is a little leak, the flange is a little warped

 

an exhaust leak can matter after the sensor too. i think about two feet is the limit but i could be way off on distance. i believe i read that in maximum boost

 

The NGK AFX manual explains it best, and deals in optimum sensor placement and distance after turbo/engine and distance before the exhaust exit in "exhaust diameters." Makes sense, really, as different pipe diameters makes the system react differently.

You guys have to view the exhaust stream for what it is, a pulsed flow. Every time an exhaust valve opens: pulse, and the pause inbetween pulses can allow the flow to reverse, of all things. This will bring fresh air back through and to your wideband sensor if the exhaust isn't long enough, and it really shows up at idle and super low rpms. FYI a turbine wheel in the exhaust stream mutes this, but not so much at idle.