I live in CA. I have a 1989 Jeep YJ, 4.2L, Auto all stock (Carter BBD). Couple weeks ago I went to get my jeep smogged. Unfortunatly, it failed NO at low speed (15mph). It passed NO at 25 mph. All test done in Drive position.
Two years ago it failed the same way. That time the technician put the tranny in 1st gear to get the NO value down. He explained that the higher RPMs would increase the engine compression, therby lowering the NO levels.
The question I have is what causes the high NO levels at the lower speed?
Do I need to replace the O2 sensor, Pulse Air upstream and downstream air valves, checkvalve or whatever???
Please help....

NOx is caused by high combustion temperatures. Normally inert nitrogen in the air is under so much heat with oxygen present that it binds with the oxygen to form oxides of nitrogen. A faulty EGR, too much compression, bad cooling system, and over lean mix will cause high NOx levels. High load at low RPM can also cause excess NOx. I always recommend a good tune-up and de-carbonizing procedure be done on the motor (jiffy lube can do a cheap version of it). A Chilton should have all the procedures needed to test the EGR valve.

Do you have over sized tires? What are the other numbers (CO,HC) at 15 mph?

I worked a few years as a smog tech and there are a few things that can help. I suggest a complete tune up and an oil chage. The oil change is very important if the vehicle is close to passing; your oil will over time pick up hydrocarbons and NO2. Then the PCV system picks up these vapors from the crank case and introduces them into the intake air charge where it is burned. Like I say if the vehicle is close to passing do a tune up and change the oil; it should pass then, barring the EGR valve is not defective.

Gross Polluter is my jeep...Here is my smog inspection
Fuel Cap Functional: FAIL (even though it is 2 years old)
Thermostatic Air Cleaner: MISSING (I need to replace)
Air Injection: FAIL (Tech said they are bad) I probably need to replace the Pulse Air stuff. I
Vacuum Line to Sensors: Disconnected (I disconnected the hose to Thermostatically Controlled Air Cleaner)
Ignition Timing: FAIL (need to check. Need a tuneup)
O2 sensor: PASS
Here are my results:
15mph (1404rpm) - %CO2=12.8; %O2=2.5; HC(PPM)=25; %CO=.01; NO(PPM)=2172
25mph(1452rpm) - %CO2=13.2; %O2=1.6; HC(PPM)=85; %CO=.74; NO(PPM)=785

I have 33" tires.

Guess I have some work to do...Thanks for the help

buy you a jeep that was built the last year, before
inspections start, that will solve that problem.
or move to sunny south cackalackie. <img src="/forums/images/graemlins/smile.gif" alt="" />

It's good to see that the Smog Check system is working. Get that thing fixed before they make you junk it. <img src="/forums/images/graemlins/scared.gif" alt="" />

Repair/replace all the broken stuff and you will pass no problem.

Too much O2 will cause the NOx to go high. Very high. Probably something to do with your air injection or an exhaust leak.

Advanced timing will also cause high NOx.

Set your timing according to hte manufacturers specs, replace your plugs with whatever the dealership parts department supplies, fix the broken stuff, change the oil and you should be good to go.

It sucks when you GP a car.

Clay
(currently employed as a Kaliforia smog tech)