I have one that is stumping me right now. It's an 81 G20 that we put an 6.0 in a couple years back. Customer brought it back last week reporting no power, backfire and smell of raw fuel.
I drove it and it ran fine for a while, then all of a sudden it started running horribly, backfiring, etc.... When it starts to run bad, The Bank 1 Sensor 1 o2 sensor shows about 46Mv and sticks there. At the same time the ST Fuel trim for bank 1 spikes to 52%. Bank 2 remains normal and LT always remains at 0 (maybe part of the tune, we outsourced the tune). I did try putting an o2 in with no difference.
After the van sit's, It will run good again for a while until it decides to act up again.
[Last edited Jun 20, 2016 20:19:08]