LS swap Random misfire. Need advise

[epaden]

Just a thought... rpm is read off the crank sensor. Check the mounting of the crank sensor . sometimes rust will build up on the block to the point it pushes the crank sensor away from the crankshaft.

[vortecyota]

I wrote a gigantic reply and posted it but it disappeared. Not sure why.

Anyways, I'm gonna make it quick. You're flying blind until you are able to monitor O2 sensor and fuel trim pids. Once in closed loop you should see +\-10% LTFTS, ideally closer to zero. Also look for the upstream O2 sensors to switch from ~100-900 mv consistently. If both of the above check out, disconnect Bank 1 O2 and look for Bank 1 O2 pid to display ~445mv. If Bank 2 pid is the one that flatlines, somehow the O2 connectors are swapped.

The reason the misfire vanishes at heavy throttle is because the PCM has entered power enrichment mode and disabled closed loop operation.

The fact that it runs well in OL implies that the tune is at least close and is NOT causing your misfire.

Incorrect Fuel trim is causing your misfires, there aren't many ways that can happen. Faulty or incorrect O2 sensors (case or isolated gnd) or they are swapped. Can't think of any other scenario that would explain your symptoms.

Let us know!