Moderator: volvite
A good O2 sensor should produce an oscillating waveform at idle that makes voltage transitions from near minimum (0.1 v) to near maximum (0.9v). Making the fuel mixture artificially rich by feeding propane into the intake manifold should cause the sensor to respond almost immediately (within 100 milliseconds) and go to maximum (0.9v) output. Creating a lean mixture by opening a vacuum line should cause the sensor output to drop to its minimum (0.1v) value. If the sensor does not flip-flop back and forth quickly enough, it may indicate a need for replacement.
I had to replace the fuel sending unit since it was right over the mileage limit Nissan extended. About $100 and half a day so it was child's play compared to this cat issue.deermjd wrote:Good info! Thanks for doing all that research. Sorry to hear you sold your Pathfinder. Was it having any problems other that the cats?
You've got that right. Even if you don't want to cut them out and have to spend ~$500 on cats is better than ruining a motor! Cut the back cats off if you don't want to spend that cash. I would make damn sure anything was bad before I replaced it. There are some cat inspection tips on one of the links I posted.disallow wrote:On the Cats, not sure if anyone has mentioned, but RacerZX had an issue on his V6... IIRC there is a potential to get pieces of the catalyst upstream and into the cylinders. If that happened, you could have major engine damage.
Not sure where you live, but if its not in the cards to replace them, I would find someone handy with a welder and cut them out. You'd then have to live with the constant SES light, but at least you wouldn't damage them.
This all assumes, of course, that it is your Cats that are bad, and not just the O2 sensors.
"Chunks" may not be able to get up there, part small bits and particles from the catalyst media certainly can. The issue is similar to the QR25DE in the 02-04 Altimas and Sentras, which, like are VQ40DE's, do not use a conventional EGR system (EGR valve, pipe, solenoid valve, etc.). The accomplish EGR by partially opening the exhaust valve during the intake stroke to suck exhaust back into the combustion chamber, which is made possible with the variable valve timing. The small particulate from the catalyst is very abrasive, which scores the cylinder walls. At least that's what the guys on Nissan's tech hotline were telling their technicians a few years back.RacerZX wrote:There's no way for cat chunks to get up into the cylinders, but the back pressure caused by a failed cat can cause detonation which can be a bad thing.
Have you tried just whacking the cat with a rubber mallet? If you hear rocks inside instead of a solid thunk, that's bad...