Originally Posted by
peter51
I forgot to add - with multiple misfires you are not really going to find the answer in the O2 sensors. O2 sensors are not the cause of misfires
You need to think about what has been disturbed in the engine bay over the last year. You need to look for any wiring that is rubbing on the chassis or not put back in the P clips - especially the wiring from the Crank and Cam sensors.
Lastly if your mechanic does not have a good USB oscilloscope then I would find someone else. You need to find someone who can check the waveforms coming from the Crank and Cam sensor plus primary and secondary ignition waveforms.
A good operator will have a 4 channel scope and be hooked up fairly quickly. He can capture the input waveforms of the sensors and the outputs from the PCM. He can quickly compare the waveforms against good known waveforms in a worldwide database. He could diagnose your problem very quickly - no more guessing. Ask around and find someone who has this gear.
For an example of how powerful this sort of diagnostics is compared to OBD - see the image below for a capture of a crank and camshaft signal that are slightly out of sync and causing multiple misfires. This problem has nothing to do with O2 - although they were reacting to the misfire.
Any small dropouts in such complex waveforms will create a distorted waveform that cannot be interpreted correctly by the PCM - hence the need to check wiring.
OBD codes can be very misleading when misfires occur and lead you up the garden path. OBD would never be able to correctly identify the misalignment of cam and crank signals as accurately as an oscilloscope image - it would just report multiple misfires in your generation of engine computer.
It would be worthwhile to find someone with the correct equipment if you find no wiring issues. Good luck.
BTW: I have this equipment and am currently chasing signal dropouts on one of my own cars today.
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