Trumar616

Hi guys,

My bone stock, 111k mile '98 recently failed its CA emissions test. It passed with good margins in every category except for NOx for the 15mph dyno test, where it was 52ppm over the limit (although for the 25 MPH test, I was well within range). I figured this was a pretty marginal difference that I could fix with a little massaging, and threw a bunch of things at it: fresh air filter, flushed my cooling system, replaced the broken fan shroud, put a bottle of 505 CRF in my gas tank, and patched up a few split/disconnected hoses I noticed that were causing vacuum leaks (which got my A/C blowing stronger to boot, woo).

After all this, I put some premium in the tank, aired up my tires, drove to the smog shop, aaaaaaand failed again by the exact same metric. It didn't help that I had a long wait between arriving at the shop and actually getting on the dyno - close to an hour - but even then, my results only went down by 10 PPM (at 25mph though, my NOx emissions went down by a full 69PPM. Curious. )

So I decided to take a closer look at my exhaust system. Earlier in the year, my engine developed an intermittent misfire which took some time for me to properly diagnose and fix - and I'm beginning to suspect this ****ed up either my cat, my O2 sensors, or both. After doing some research, I bust out my scan tool and get the readings in the attached pic. Now, based off of what the O2 sensor readings are telling me, it looks like my cat has gone bad, right? But when I take temperature readings with a laser thermometer at the upstream and downstream ends of the cat, the downstream end is 100 degrees hotter than the upstream, which indicates that the cat is heating up and catalyzing properly. So which is it? These two readings seem to contradict each other. When I jostle the cat around, I don't hear any loose/broken substrate, and I have no codes related to the cat, O2 sensors, or anything else. I'm not experiencing any loss of power that would indicate the cat is clogged, either - in fact, after all the maintenance I've done, my XJ's been running great, which makes these emissions failures all the more frustrating and confusing.

So, a few questions:
1. Any CA guys out there have recommendations for passing emissions/lowering NOx specifically? I have a little wiggle room in the other emission variables, if it matters.
2. I feel like I'm going crazy - based on the data, does my upstream O2 sensor seem good or not? I'm leaning towards replacing both O2 sensors - I don't know if they've ever been replaced over the vehicle's lifetime. How significant of an impact would fresh sensors have on my emissions/fuel trim?
3. I'm thinking about prying my cat off my exhaust and cleaning it with some degreaser/water before I get tested next. Is this even worth the trouble, or is my cat just toast based off of the O2 sensor readings? Everything with the cat/my emissions is so marginal, it feels like a cleaning could work (instead buying an entirely new CA-compliant cat) - I just want to know if it's even a good idea before I go through the hassle of getting it off.
4. Should I be concerned about my timing advance being so high? Either for emissions or my engine's overall health?

Thanks in advance, I appreciate any help I can get.

exasemech

If the traces you posted are labeled correctly (bank 1 sensor 2 is the downstream sensor) and the vehicle was at least sorta warmed up aka closed loop, your catalyst is likely shot. That downstream voltage, unlike the upstream voltage, should stay pretty constant

Btw100 f difference between inlet and outlet isn't going to tell you much. If you passed HC&CO something is likely working in there to oxidize that stuff, but the NOx " reduction" function could be dead.

"Washing out the catalyst with " degreaser and water " is a waste of time

awg

You must fix all vacuum leaks as the No1 priority, as this will straight out affect an NOS reading

I recently detected some pre-ignition in my vehicle, and when I noticed that despite new plugs I could still detect it on occasion, so I took my XJ for an Italian tuneup

This involved a 90min drive with a lot of hills where I used as much throttle as I could...idea; to burn up as much internal engine carbon as I could,
as my truck does mostly short runs. Mine has more than twice as many miles as yours, so for sure I would do both above, if that does not fix it, then likely the Cat is depleted of the expensive rare elements inside that scrub the gases.

In addition, any exhaust leaks between engine and O2 sensors will cause above