View Full Version : High NOx, CA Smog

November 20th, 2017, 18:50
I'm having trouble passing smog. The specs on the jeep are as follows.

1994 Cherokee, stock 4.0 with about 260,500 miles. Motor has not been rebuilt as far as I know.
I replaced the catalytic converter, spark plugs and wires 2 years ago. I have put maybe 1,000-1,500 miles on the jeep in the last 2 years.

The first test I didn't do anything to the jeep other than put some fresh gas in it. I failed due to high NOx on the 15mph and 25mph test.

For the second test I replaced the 02 sensor, put a new muffler on, fixed a couple vacuum leaks and aired up the tires.

The second test I passed the 15mph test but on the 25mph test my NOx went up.

I'm not sure what to replace next, any suggestions?


November 20th, 2017, 21:17
Based on the info you gave I would ask what temp is the car running at? Normal, or does it run a bit on the warm side?

At that mileage could be carbon build up (especially if it isn't driven much). When you say you fixed some vacuum leaks, how did you test it afterward? Are you certain you got all the leaks fixed? Whats your fuel pressure at the rail?

November 21st, 2017, 13:20
So to understand these tests a little more requires some chemistry.

High NOX usually only happens if there is unburnt oxygen left in the exhaust stream.

You have a few ways this happens.

1. miss fire, however this will also cause a fail on hydrocarbons if it is ignition based, which you don't have. The other issue could be a complete injector failure, however this would be noticeably rough running. You didn't mention that there was a performance problem assuming this is not the cause.

2. Lean burn, (Most Likely). This happens when injectors are plugged partially, and the cylinders are not getting the anticipated amount of fuel. Leaving some oxygen in the exhaust that wasn't burnt do to not being enough fuel to burn all the oxygen.

3. Bad Cat, since you replaced it, i'd say not likely. However since you're not driving it much here is what can happen. If the car is not driven much (several short trips or extended idle) the cat can become coated in soot and not perform its job until cleaned out. To clean it out, run the jeep at +60MPH in 3rd gear for automatics or 4th for manuals for a good 20-30 minutes. This will cause the motor to run fast and cause EGTs to rise and burn the crap out of the cat.

Running the motor fast and for an extended period of time serves two functions. It will also increase the injector pulse width, and run more fuel through them. This will help clean any plugging occurring if it is just varnish and sludge from extended sitting. Adding some injector cleaner, the good stuff like bg44k or equivalent to the tank before hand will further help.

Here is my recommended approach, fill tank with good quality gas with added injector cleaner. Run at freeway speed without overdrive for at least 30 minutes and before shutting it down drive straight to the test station and have them test it immediately following that good "hot" drive...

75% of the time that will fix this stuff in my experience, especially with cars that have had some extended sitting.

My .02$


November 21st, 2017, 15:54
Another thing that can cause high NOX would be an exhaust leak upstream of the cat, something like a cracked header or a blown gasket.

November 22nd, 2017, 11:13
You can try a carbon cleaning treatment , carbon in the combustion chamber can cause high nox .

November 23rd, 2017, 12:23
Try filling up with premium and adding some octane booster. The higher octane burns cooler causing combustion chamber temps to go down resulting in less nox.

November 23rd, 2017, 13:16
Another thing that can cause high NOX would be an exhaust leak upstream of the cat, something like a cracked header or a blown gasket.

I would check this first.

November 26th, 2017, 18:51
Hows the smog check coming ?

November 28th, 2017, 06:06
id have to guess its going a bit lean during the test. you can see on the report passing the 15mph you had a reading of 41ppm on HC. On your failed 25mph you had 14ppm.

how is your air filter aside from that it may just need the "Italian" tune up as posted before.

November 29th, 2017, 05:34
Cylinder temps are too high. A vacuum leak can cause this condition. Make sure that your tank doesn't have any fuel injection cleaner, octane booster, or "heat" in it.

Sent from my LG-LS777 using Tapatalk