Lean at idle
 

I have a 1996 1.8 turbo Miata. For some reason, my wideband has recently been reading 17-18 afr at idle at ~850 rpm. The car idles fine, not bogging down at all. It's weird because if the car is idling at 1100 rpm(cold) then the afrs would be closer to 14.7. Aswell, when I start the car, it will be around 14.7 for about 2 seconds then read super lean. When I drive the car, the afrs are all to the correct. I doubt that the car is actually running lean because I did not touch the tune at all. Is the sensor just reading lean because there is not enough exhaust pressure? I recently installed a test pipe and a manzo exhuast; and this is why im thinking that. My afr is around ~16 inches from the turbo outlet, and i know it should be around 11 inches. I've checked for exhaust leaks and i've only found pinhole at my welds with the stick welder :). Could these be the source of the problem? I've also tried to increase the fuel and it would read rich then go back to super lean.

I couldn't decipher what the issue is from that brick wall of text, but try actually tuning your warmup enrichment, MAT and VE. I doubt O2 sensor placement is that minutely important here, mine is 16" downstream with a flex pipe and i get consistent readings.

So recalibrate your oxygen sensor and check your power and ground offset.