I'm at a complete loss here. 396 LT1, GM847 cam, 40# inj, M6, etc. Mail order tune from Ion I think (been too long now). I've got an Autometer wideband installed in my y-pipe.

First thing I noticed when I first fired it up is it's running extremely lean at idle, as in barely running and has the wideband maxed out lean. Plugged in the laptop and checked it out in Datamaster and the PCM is seeing the opposite, thinks it's running extremely rich and pulling fuel, as in STerm BLM's of 98 at idle. It keeps pulling fuel until it almost stalls, jumps back to open loop and richens back up and starts the cycle over again. I replaced an O2 to make sure they weren't fouled and there was no change.

I've cleaned the MAF, checked plug wires, no obvious vacuum leaks, no obvious exhaust leaks.