LS swap Random misfire. Need advise

[12thstreetjunkman]

Try unplugging the o2 sensors and running it down the road, it will stay in open loop. If runs fine you know it's not fuel pressure, or ignition related, but something sensor related.

[Steve l.]

We did not replace the fuel pump. It is showing no lean issues. The PCM was replaced but did not resolve the problem. The first shop has given up. On to the second.

[1991SSP]

What company did the harness... as stated before have you double checked everyone of your grounds... I had a problem on mine and turns out one of my plug on the harness was not seated correctly onto the coil... I have heard of lifter issues causing misfires as well.... your problem sounds like electrical and they can be a pain to track down... seeing that you have replaced the PCM and you are satisfied it's not a fuel pump issue you should definitely start looking at the harness...

[Steve l.]

Try unplugging the o2 sensors and running it down the road, it will stay in open loop. If runs fine you know it's not fuel pressure, or ignition related, but something sensor related.

The second shop unplugged one of the o2 sensors today and truck ran fine. Plugged back in and misfire came back. O2's sensors are new so not sure how they'll be able to fix that.

[Chevydillan]

Do you have home made or do it your self O2 extensions? Its running fine because it's staying in open loop as soon as the temp comes up and it goes into closed loop things start going bad.

[Steve l.]

O2 wiring is as the truck left the factory in 1998. I know it is in open loop when unplugged. The question is why is closed loop causing it to misfire?

[Chevydillan]

I'm going with tune still

[Steve l.]

I've still got what seems to be a closed loop misfire. Everything I have found on the internet indicates replacing the 02 sensors resolved the issue for people with the same problem. I have replaced the old sensors which I knew to be operational, with new o2 sensors and the problem persists. Any thoughts?

[Chevydillan]

Verify the O2 wiring is correct at the ecm?

[Steve l.]

I picked up my truck from the second shop yesterday. They fleeced me for almost $400 without fixing anything. Drove the truck home with the O2s unplugged and it ran pretty well. I’m going to completely rewire the O2 harness and see if that helps. If not, I’ll have it tuned to run without the O2s in open loop all the time.

PS. Anyone in the Waterloo, Iowa area that wants to know the name of the second shop so they can avoid it feel free to send me a PM.