Originally Posted By: gregnash
This is part of the reasoning behind the lower ZDDP% oils of todays cars. The higher ZDDP oils do burn up cats but not something that changing suddenly would kill your cat. If you havent had it changed and it is still original then more than likely it is already dead and needs a good replacement. There are multiple other things that go into not passing smog as well, it really depends. Post up your smog numbers in the 60 forum on MUD and the guys will help out with what you got. It could really be anything from you gas cap venting to much (Both OrangeFJ45 and SpikeStrip have stated this as a cause of not passing smog) to something like your smog pump is clogged but still running or the cat is completely burnt.
smog wise I have definitely done just about everything, gas cap, smog pump went out couple years ago, new air injection rail, cat replaced recently(so not like it has to be done soon), cleaned the archaic o2 sensor in the cat, lean dropped the carb (rebuilt), vacuum lines replaced(all 30ft of it!).
I rebuilt the motor a couple years ago as well. My smog guy has told me that because the head was machined and the cylinders were bored out a bit, the area change could be causing higher(or lower) compression, forgot which one, and that that could be affecting my readings.
Luckily, The Cruiser isn't my main mode of transportation anymore, so smog passing is on the back-burner.