mrtoofast4u

I have tried twice to get my 94 ZJ to pass smog and it failed both times. The NO (ppm) is too high. I have already changed the EGR valve and cleaned the spark plugs and changed the oil. Does anyone have any ideas what it could be?

Bustedback

Run some Seafoam through the intake to clear out the build up of carbon in the combustion chambers, make sure the EGR passages are not clogged, and run a 70/30 water to anti-freeze mixture to help keep the cylinder heads cooler. NOX emissions is caused by high combustion chamber temps, so if the EGR is working and the cylinder head temps are down, you should have no problem.

Fuman Shoo

What state do you live in?

Here in California, as most of you all know, our emissions requirements are amongst the highest in the country if not THE highest. However, if you don't have any major problems with your jeep, you can pass emissions with a rather comfortable margin.

I like you, had problems passing emissions, despite having no major mechanical issues with my 1993 Grand Cherokee (V6). However after I BARELY passed emissions year before last, I still faced this years slightly more strict emissions and my test last month failed badly. But after a few fixes, I passed with my best smog test EVER!

Here's what I did:
I bought a new gas cap. ($10)
Installed a new O2 sensor. ($45)
Bought and installed a new catalytic convertor. ($175)
Reset the ECU. (free: do-it-yourself)

Here were my test results:

15 mph test:

%CO2=15.1
%O2 0.1

Hydrocarbons (PPM):
Max is 61
Average is 30
Mine measured 22

CO%:
Max 0.62
Average 0.04
Measured at 0.04

NO% (PPM):
Max 415
Average 188
Measured at 75 (after replacing cat) <---previously was 885, then 612 (after replacing o2 sensor)

25 mph Test:

%CO2: 15.1
%O2: 0.0

Hydrocarbons (PPM):
Max 52
Average 18
Measured 13

CO%
Max 0.75
Average 0.05
Measured 0.02

NO (PPM)
Max 527
Average 220
Measured 74 (after replacing cat)...<--- previously was 647, then 527 (after replacing o2 sensor)