We just purchased a 1994 Grand Cherokee with the 5.2 engine 160K. Went to get the emissions test done and it failed for high Nox. I found the old test from 2008 in the car the car only has 2,000 more miles now, so it looks like it sat for a while.
I did a vaccum test to see if it was good after the failed test we had 16 inches of vaccum it was steady no jumping around.
Old readings 2008 (passed)
HC .9684 limit 2.00
CO 8.8775 limit 20.00
NOx 2.6876 limit 4.0
My readings are
HC 1.2983 limit 2.00
CO 18.5306 limit 20
NOx 4.1738 limit is 4.00
We have not yet done anything to the Jeep since we bought it. I was looking at giving it an oil change this weekend and a tuneup (plugs, cap and rotor) I will also be checking the EGR to make sure it is working correctly.
1) Did you fail at high speed or low speed, or do you just have a single-speed test (if so, what speed(s) - this is instructive.)
2) If you have a two-speed test, what were the results for the other speed range? Also instructive.
3) High NOx is typically a function of failed EGR or clogged passages, but that is also accompanied (typically) by a massive reduction
in HC and CO (increased combustion temperatures = more compleat combustion. HC and CO are "partially-burned fuel.") Ergo, you may want to check your catalytic converter as well.
Quick-and-dirty EGR test - find your EGR valve. Remove the hose from it and cap it off. Attach a hand vacuum pump to the valve vacuum nipple. Start engine and let it idle. Apply vacuum to the valve - the engine should stumble almost immediately. This checks operation
With the engine OFF (stalled or shutdown,) apply 15-18" vacuum to the EGR valve and hold. The gage should not move appreciably for at least two full minutes - if it does, you have a vacuum leak (which will interfere with EGR operation.)
This doesn't change the fact that I think the EGR is a stupid idea and there are far
more efficient ways to solve the problem (EGR causes more problems than it cures...) but the Air Police won't get out of my hair and let me do it. I could probably delete everything except the PCV/CCV, play with the tuning a bit, add a water/MeOH fog, and my 22-year-old XJ with the AMC 6-242 would run cleaner than a 2010 KK with a four-cylinder engine. Yes, that includes getting rid of the catalytic converter, and yes, that means that it would still have lower
emissions than the new KK!