get3coffinsready

I have a stock 1994 xj 4.0 aw4 with 113k on the clock that has failed the CA smog test for NOx at 15 and 25.

Vehicle passed smog in 2014, nox was about 20-30 ppm in the test.
2016 test went up to 700-800 ppm.

co2 o2 hc co no outside temp
2014 - 15mph/1690 14.7 0.19 8 0.01 29 80f
2014 - 25mph/1663 14.68 0.14 1 0.01 24

2016-1 15mph/1665 14.7 0.13 6 0.02 897 82f
2016-1 25mph/1633 14.7 0.13 1 0.01 800

2016-2 15mph/1644 14.76 0.03 7 0.02 1138 90f
2016-2 25mph/1623 14.76 0.02 5 0.02 971

The following work has been performed in response to the issue.

-Fixed intake gasket.
-Replaced cat and muffler, cat was bad, muffler had a restriction
did an initial test , still too high
-Replaced the plugs (champion 412)
-Replaced 02 sensor ngk/ntk unknown if existing sensor was bad but with bad cat and intake thought it was a good idea to swap it out
-Cleaned intake
-double checked spark wires
- took for two short drives and a 30 min free way run
- filled the tank with reg, aired up the tires, took the 300lbs of sh** off of the roof rack, coolant is topped of and coolant temp well within normal operating range.
-took the test and failed again, NOx is even higher now.

Stumped, jeep is running better than it ever has and it is failing emissions.

Fuel filter is about due for a change, been in use for 2 years.
Jeep does seem to run better with a full tank so the pump could be bad too.
Oil has low mileage on it but its been 6 months since i changed it.
Engine idle speeds: 700-800 warm; ~900 cold

I have a feeling it is something small and stupid, another user suggested colder plugs, I might try that.

get3coffinsready

pita to diagnose a problem with this engine, always seems to run fine. thinking you would have to punch a hole through the block before you would notice something was off.

mschi772

It may be something in the fuel delivery making it lean (failing pump, dirty injectors) and/or heavy carbon buildup in the cylinders. The latter is easy enough to clean out with a gentle water or Seafoam drip/spray into the intake while running (Google if you're not familiar so you know what you're doing). The former will require some troubleshooting/diagnosis.

Is your exhaust manifold cracked? Sometimes because of fancy physics the cracking will actually draw fresh air IN instead of leaking exhaust out. This gives the O2 sensor the wrong idea of what the engine is doing which leads to an incorrect air/fuel mix which can produce a number of problems/symptoms.

You shouldn't have to mess with non-factory spec plugs. They were chosen by design and worked for 20 years--there's no legitimate reason for you to have to use a different plug now.

What kind of cat did you get? Who makes it? Who sold it?

jmrec100

I went thru the same struggle for months. Someone told me the exhaust manifold. Couldn't believe it. Until FINALLY I checked.
Check your exhaust manifold. Do a smoke test or air test on it. I had the same problem. I epoxied up the cracks. Even had it welded on a used one. Problem is that the exhaust manifolds crack, and with hairline cracks.
I finally bought a new one. Installed. Passed with flying colors.
My air test. I ran my air line in the exhaust pipe, low psi, plugged up the gap in the hose and pipe. Felt for air at the manifold.. Yep. Sure were leaks. One major one I couldn't see. Then a bunch of small hairline cracks.
Check your exhaust manifold. My Cherokee has 280,000 on it. It was time for the exhaust manifold. I'm not the original owner so don't know if it had been replaced before.
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Post back your results when you solve it.

get3coffinsready

picked up a pressure gauge and checked the fuel system. Pump primes the system fine, the regulator seems to work fine, pressure is fine at idle and when reving. After shutting off the engine, for 4.5-5 mins the preussure starts to drop, after 10 minutes, the pressure drops to under 10 pounds. I thought i heard misfire during the test. Going to see about pinching off the lines to isolate and see if their if some key magic for a misfire code.

PNWjeeper

Completely agree, unless you have replaced the exhaust manifold AND engine mounts AND transmission mount all at the same time recently you probably have a crack in the manifold. I know one of the reasons I'm getting 10mpg in my stock jeep is because I have a cracked manifold. Check it out, it may be expensive and time consuming but at least you can legally drive your jeep on the streets of California.

get3coffinsready

got the following codes
22 Coolant sensor signal out of range - May have been disconnected to set timing
23 Incoming air temperature sensor may be bad

89Laredo

Uhhh... What?
You don't set timing on a 4.0.

get3coffinsready

who knows...but both of the sensors would cause the exact condition i am experiencing and the codes are fresh. Feel pretty stupid for not checking the codes now.

MtnGoat

True, but timing can be the cause. Worn dist gear?

get3coffinsready

the air temperature sensor from crown is not threaded correctly, does anyone know who made the intake manifold air temperature sensor for the 4.0 or know of a replacement part that fits?

get3coffinsready

going to try a wells, the quality seems to be higher, they are partnered with ntk/ngk too. If not going ot try cleaning the sensor.

get3coffinsready

Wells IAT was a perfect fit i would say it fit better than the oem. Their coolant sensor looks almost identical the oe as well. Just need to see if it will pass the sniff test.

get3coffinsready

It was the header. Used a smoke machine and a shop vac to blow smoke up the tail pipe when everything was cold. Pulled it out, shined a light through it and it lit up like a Christmas tree.