Slightly high NOx

I just failed my e-test for NOx only. I tested 3.66 and the limit is 3.5.

Can an exhaust leak raise the NOx levels? I know I have a slight leak that goes away as the engine warms up, and I have the gaskets to replace, it's just that my down pipe is pretty tweaked after Adventures in Moab. I know if I start unbolting the exhaust, it wont be pretty trying to get it back together. I will get around to it, but I have 10 days for the retest, and I'd just as soon hold off doing the exhaust gasket unless I have to to bring down the NOx readings.

Thanks

Just get a new Cat on there, and you will pass !!!!

Mine had a small crack in it too.Replaced cat didn't help, install new header pipe and gasket got the numbers to 0.1.Run SEA FOAM in it and you'll see just how big the leak is.

An exhaust leak would introduce oxygen into the stream and the O2 sensor would read lean. The PCM would respond by commanding a richer mixture. Nox is low when the mix is rich. Typical causes of Nox are high cylinder temps, lean mix, faulty EGR, fuel with high ethanol content, overly advanced timing, excessive carbon build up in combustion chamber causing high compression. You could have a vacuum leak or bad O2 sensor causing a lean condition, overheating engine, etc. A good dose of upper cylinder cleaner followed by an Italian tune up and new O2 probably will get you to pass if everything else is in sound condition. Also, use a good fuel with low ethanol content before the sniffer test.

Just had the same problem, I replaced the cat conv and it passed, converter was hollow inside.

If the exhaust is beat to hell, how is the CAT not beat to hell?

Most of the exhaust is good. I had it all run in 2.5" from the downpipe back, with a small hi-flow cat and Hushpower II muffler. It's all tucked pretty high and tight, aside from the stock downpipe. Some rock jumped up and hit the downpipe to the point where it was actually touching the oil pan.

I got under with a big prybar and tweaked it to scootch it away. That seemed to help a bit. I did the full tune-up and oil change, and I'll get the retest this week.

We shall see.

Thanks.

Are these the specs, year?

96, 4.0, AX-15, 231, D30/D44

Is the engine running lean? is everything else perfect OBD-II wise?

Here is one more easy way to lower N0x... Fill it up with 91 octane premium. High Octane burns slower, and it will lower NOx. Might make the HydroCarbons go up since it burns slower, but this may be perfect for you. Also, drain the oil, fill it up with 20w50 Castrol & put a new Wix 51515R filter on the old 4.0 !! A new air filter is always good luck too... Best of luck !!!

Mike -

Running a little lean, as evidenced by plugs. That's just how they came out firewall to rad, top to bottom.

No other codes, running strong. HC reads .7627, limit 1.2; CO 9.7669, limit 15.

SteveT said:

Mike -

Running a little lean, as evidenced by plugs. That's just how they came out firewall to rad, top to bottom.

No other codes, running strong. HC reads .7627, limit 1.2; CO 9.7669, limit 15.

Click to expand...

OK, now we have some data and clues!!!!! You need a new Cat Converter!!! (That is the highest probability answer).

And yes, the exhaust leak may be adding to the problem (depends on the leak size, location....., and needs to be fixed, so fix both, or the easiest-cheapest first I guess?

But the data says the Cat converter is well used, getting too old...

Ecomike said:

OK, now we have some data and clues!!!!! You need a new Cat Converter!!! (That is the highest probability answer).

And yes, the exhaust leak may be adding to the problem (depends on the leak size, location....., and needs to be fixed, so fix both, or the easiest-cheapest first I guess?

But the data says the Cat converter is well used, getting too old...

Click to expand...

^^^^^^^ agree with this

So, first quick fix was a tune up: plugs, rotor, cap, wires, oil change. One tank of Premium run through with Amsoil fuel system treatment.

Every reading got worse, which casts some doubt on the accuracy of the test centers. Same center, 20 degrees cooler.

HC 1.1152 test, 1.2 limit
CO 13.0149 test, 15.0 limit
NOx 3.8250 test, 3.5 limit


Gotta keep him on the road, so off for a cat converter. I had originally had a generic, "Hi Flow" bullet cat. I got four years use out of it, which from what I understand is about 3 years, 10 months longer than normal. :laugh3:

I therefore ponied up the coin for an OE style performance cat (2.5" in/out) to stay legal.

HC .2207 test
CO 2.1163 test
NOx .6096 test

The old cat was still intact (you could see through it, still) but the Jeep feels much stronger now, and has lost a just off idle stumble.

The big bucks hurt, but I think I got what I paid for.

Thanks for the ideas/suggestions

Steve