hey. my 1993 cherokee has failed calif state smog test twice now. it is stock 4.0l with 254,325 miles. after 1st failure i did tune,throttle body/fuel injector service,air filter,etc. and also ran can of b-12 chemtool thru tank but failed again. o/2 sensor was changed 4-5 yrs ago and cat looks like it was changed once already. smog test numbers are as follows : hc max is 63, meas 86. co max is 0.66, meas 0.11. nox max is 517, meas 553. this is dyno test at 15mph. test at 25mph is as follows: hc max is 38, meas 64. co max is 0.80, meas 0.07. nox max is 539, meas 496. smog tech was no help as to what may be causing problems. any ideas from jeep nation would be very helpful..thanks

 

How long did you wait between running the Chemtool and taking the second test? IF you didn't wait long enough the engine could have still been coughing up junk that the chemicals were cleaning out.

 

i drove it about 30 miles at frwy speeds before i took it in.

 

Not even close to long enough. I think it needs to be run for a week after the chemical is all used up.
Not saying that could be the only issue, but it may have played a major role.

 

Also, make sure you bring in your rig when it's piping hot...like after a nice drive at highway speed.

 

Would hurt to replace that o2 an just cause the cat looks decent doesnt mean its good the hc means you were or still are burning oil nox is fuel or a missfire can guarentee new o2 an cat she will pass been there twice now

 

NOX is caused by high cumbustion chamber temps, not a misfire. Burning oil will not cause the vehicle to fail the smog test unless there is a constant blue cloud coming out of the tail pipe. For the NOX issue, drain the coolant from the engine and replace with a 70/30 water/anti-freeze mix. The higher water ratio will cool the cylinder head faster and better and could bring the NOX emissions down. You may need a catalytic converter to bring the HC emissions down.

 

Here we go again...

Take a picture of your report, or scan your report, and post it. Feel free to redact personal information, the last six characters of your VIN, address, and the like. But, leave the technical stuff.

If you can't take a picture (a decent pic - cell camera pix suck for this) or scan, then post at least your numbers (all of them!) and mileage.

Why all numbers? Because even the ones that pass are instructive.

Posting the report is de rigeur for getting effective assistance with smog issues, else we're taking some serious WAGs, and no-one gets anywhere...

 

Miss fire cause nox an higher combustion temp an yes a engine passing oil will fail smog because hc is un burnt oil/fuel an if that cat is coated with oil it will fail for sure

 

An if ubwanna bring the combustion temp down fork out for the higher octane fuel why would you add more water will bring the boiling point down? Hmmm

 

i will get the v.i.r. scanned in the am saturday and post asap. thanks for the replies so far

 

After you burn off the tank with the Chemtool junk, fill it with quality gas - regular. Premium will only make it worse.

The addatives in the gas evaporate in a few days, so get it tested soon.
Just before the test - add 1/2 Pint of DENATURED ALCOHOL (paint stores) to the tank and drive it a short distsnce. It will help drop the HC down.
You are close to passing, the slight drop in HC may be enough.

IF it still fails, a new cat will get it down low enough.

NOx is causued by high combustion temperature, not cooling system temp.

 

this is somewhat true, high engine coolant temps will cause the nox to be higher, that's why we have fans in front of our dyno....to keep the engine from overheating and to prevent a "false test"

 

Also from what the OP posted, those numbers are pretty close to passing, co is low and hc/nox a tad high......my money is on a bad converter

 

Rockis4us what city are you in? Maybe I can help. I wrestled with NOX too many times. I finally got it down to 2ppm. Yeah, really. I tested the EGR solenoid with a vacuum guage and found it was not opening/closing.

I replaced the EGR solenoid $125, gently cracked open the EGR transducer and cleaned with dish soap and tooth brush, cleaned out EGR valve, changed the 02 sensor for good measure. I also am using the Champion Copper Plus spark plugs.

I've been driving my 89' since 96'. Been through a lot over the years.
Always carry an extra CPS, coil/ignition control module, and fan belt.

Jason