My 99 z34 with the buick 3800 series two, the idle is really choppy. I've got bad o2 sensors and a casual misfire but it idles like that all the time. I've looked over my vacuum lines and I dont seem to see anywhere where its leaking. What could cause this? When I first put the motor in the car it did not have an engine light on now that the lights on, I've scanned it with a tool and found the misfire and the o2 sensors. I've driven other vehicles with bad o2 sensors before and none of them idled like this. Jumps between 500 to 900 and it's very choppy. My thought was the plenum gaskets. Anyone have any ideas?

 

If you know for sure the O2 sensor is bad, why not fix that first? Not saying that would necessarily cause the problem just seems odd to try to diagnose engine issues when one of the main engine sensors is broken.

As for a vac leak, just smoke the engine bay around the intake tract. If its sucking in smoke anywhere other than the filter inlet, then you've found the leak.

 

My favorite thing to suggest: Check the MAF sensor..