B2200 slight bog once warmed up

My 93 b2200 has an interesting issue.

Stock engine with aftermarket turbo (although I am convinced this has nothing to do with the issue because I have done this setup many times before without this issue)

I have a wideband AFR gauge installed

When cold, it runs 100%

As it starts to warm up it it gradually develops a lean spot between 1500-3000RPM which causes less power ( no necessarily a bog, but more of a flat spot in power)

Once fully warm, when you floor it - it'll peg 15-16:1 AFR for a good 3-5 seconds before going rich.

The fuel system is completely stock except for a rising rate regulator and fuel pump, this setup has worked flawlessly for me before.

I am convinced this problem was there before but I just thought the old motor was a dog.

I have looked at the ECU board and do not see leaking caps - I did see what looked like dry solder joints and touched them up.


Has anyone encountered this on a truck before?

I am thinking maybe coolant temp sensor or MAF?

Or ECU - although it looks ok (anyone found a faulty ECU but with good caps?

Bad TPS?

Actually upon further inspection, my O2 sensor is not reading correctly.

I am running a signal modifier which prevents the ECU from seeing the rich condition under boost and it is not working - now I wonder if the truck was in fact working properly without the turbo and it was just a dog...